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Commercial and Public Buildings 4. Because Figures 1 and 2 are based on a saturated evaporator temperature, they may indicate slightly higher refrigerant flow rates than are actually in effect when suction vapor is superheated above the conditions mentioned. The written piece is truly fruitful for me personally; continue posting these types of articles. Prev Next.

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ASHRAE members who receive the ASHRAE Handbook as part of their membership can download the PDF of the ASHRAE Handbook—​Refrigeration (I-P. Eligible members who chose the PDF as part of their member benefit can download their electronic copy of the ASHRAE Handbook—Refrigeration from. Latest ASHRAE Handbook Refrigeration SI Edition pdf. The latest AND FLORAL APPLICATIONS; Download ASHRAE Handbook. Its members worldwide are individuals who share ideas, identify Chapters in the ASHRAE Handbook are updated through the needs, support. Views KB Size. Report. seoauditing.ru ASHRAE HANDBOOK HVAC Applications SI Edition ASHRAE Handbook Fundamentals seoauditing.ru

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Service Water Heating Snow Melting and Freeze Protection Evaporative Cooling Fire and Smoke Control Radiant Heating and Cooling Seismic- and Wind-Resistant Design Electrical Considerations Room Air Distribution Integrated Building Design HVAC Security Ultraviolet Air and Surface Treatment Smart Building Systems Moisture Management in Buildings Codes and Standard.

By Ahmed Shawky Last updated May 7, Ahmed Shawky posts 6 comments. Plumbing and Firefighting design Engineer. You might also like More from author. Prev Next. Sign in. Welcome, Login to your account. Forget password? Remember me. Sign in Recover your password. Edition 1. Number of Pages ISBN Its more than 1, pages cover basic principles such as thermodynamics, psychrometrics, and heat transfer, and provide practical guidance on building envelope, indoor environmental quality, load calculations, duct and piping system design, refrigerants, energy resources, sustainability, and more.

Thermal Comfort TC 2. Air Contaminants TC 2. Odors TC 2. Indoor Environmental Modeling TC 4. Climatic Design Information TC 4. Fenestration TC 4. Ventilation and Infiltration TC 4. Space Air Diffusion TC 5. Duct Design TC 5. Pipe Sizing TC 6.

Insulation for Mechanical Systems TC 1. Airflow Around Buildings TC 4. Combustion and Fuels TC 6. Refrigerants TC 3. Thermophysical Properties of Refrigerants TC 3. Sorbents and Desiccants TC 8. Physical Properties of Materials TC 1. Energy Resources TC 2. Sustainability TC 2. Measurement and Instruments TC 1. Abbreviations and Symbols TC 1. Units and Conversions TC 1. Reviews User-contributed reviews Add a review and share your thoughts with other readers.

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The refrigerant and oil bleed from the cooler by gravity. The bleed sometimes drains into the suction line so oil can be returned to the 1. Refrigerant Feed Devices For further information on refrigerant feed devices, see Chapter The pilot-operated low-side float control Figure 9 is sometimes selected for flooded systems using halocarbon refrigerants. Except for small capacities, direct-acting low-side float valves are impractical for these refrigerants.

The displacer float controlling a pneumatic valve works well for low-side liquid level control; it allows the cooler level to be adjusted within the instrument without disturbing the piping. High-side float valves are practical only in single-evaporator systems, because distribution problems result when multiple evaporators are used. Float chambers should be located as near the liquid connection on the cooler as possible because a long length of liquid line, even if insulated, can pick up room heat and give an artificial liquid level in the float chamber.

Equalizer lines to the float chamber must be amply sized to minimize the effect of heat transmission.

The float chamber and its equalizing lines must be insulated. Each flooded cooler system must have a way of keeping oil concentration in the evaporator low, both to minimize the bleedoff needed to keep oil concentration in the cooler low and to reduce system losses from large stills.

At low temperatures, periodic warm-up of the evaporator allows recovery of oil accumulation in the chiller. If continuous operation is required, dual chillers may be needed to deoil an oil-laden evaporator, or an oil-free compressor may be used. When the valve does open, excessive superheat causes it to overfeed until the bulb senses liquid downstream from the interchanger. Therefore, the remote bulb should be positioned between the cooler and the interchanger.

Figure 11 shows a typical piping arrangement that has been successful in packaged water chillers with DX coolers. With this arrangement, automatic recycling pumpdown is needed on the lag compressor to prevent leakage through compressor valves, allowing migration to the cold evaporator circuit. It also prevents liquid from slugging the compressor at start-up. On larger systems, the limited size of thermostatic expansion valves may require use of a pilot-operated liquid valve controlled by a small thermostatic expansion valve Figure The equalizing connection and bulb of the pilot thermostatic expansion valve should be treated as a direct-acting thermal expansion valve.

A small solenoid valve in the pilot line shuts off the high side from the low during shutdown. However, the main liquid valve does not open and close instantaneously.

The most common ways of arranging DX coils are shown in Figures 13 and The method shown in Figure 14 provides the superheat needed to operate the thermostatic expansion valve and is effective for heat transfer because leaving air contacts the coldest evaporator surface. Figure 10 shows typical piping connections for a multicircuit direct-expansion DX chiller. Each circuit contains its own thermostatic expansion and solenoid valves.

One solenoid valve can be wired to close at reduced system capacity. The thermostatic expansion valve bulbs should be located between the cooler and the liquid-suction interchanger, if used. Locating the bulb downstream from the interchanger can cause excessive cycling of the thermostatic expansion valve because the flow of high-pressure liquid through the interchanger ceases when the thermostatic expansion valve closes; Fig.

Direct-expansion air coils can be located in any position as long as proper refrigerant distribution and continuous oil removal facilities are provided. Figure 13 shows top-feed, free-draining piping with a vertical up-airflow coil. In Figure 14, which illustrates a horizontal-airflow coil, suction is taken off the bottom header connection, providing free oil draining.

Many coils are supplied with connections at each end of the suction header so that a free-draining connection can be used regardless of which side of the coil is up; the other end is then capped. In Figure 15, a refrigerant upfeed coil is used with a vertical downflow air arrangement. Here, the coil design must provide sufficient gas velocity to entrain oil at lowest loadings and to carry it into the suction line.

Thermostatic expansion valve operation and application are described in Chapter Thermostatic expansion valves should be sized carefully to avoid undersizing at full load and oversizing at partial load. The refrigerant pressure drops through the system distributor, coil, condenser, and refrigerant lines, including liquid lifts must be properly evaluated to determine the correct pressure drop available across the valve on which to base the selection.

Variations in condensing pressure greatly affect the pressure available across the valve, and hence its capacity. Oversized thermostatic expansion valves result in cycling that alternates flooding and starving the coil.

This occurs because the valve attempts to throttle at a capacity below its capability, which causes periodic flooding of the liquid back to the compressor and wide temperature variations in the air leaving the coil. Reduced compressor capacity further aggravates this problem. Systems having multiple coils can use solenoid valves located in the liquid line feeding each evaporator or group of evaporators to close them off individually as compressor capacity is reduced.

For information on defrosting, see Chapter Flooded Evaporators Fig. A small temperature differential is advantageous in low-temperature applications. In a flooded evaporator, the coil is kept full of refrigerant when cooling is required. The refrigerant level is generally controlled through a high- or low-side float control.

Figure 16 represents a typical arrangement showing a low-side float control, oil return line, and heat interchanger. Circulation of refrigerant through the evaporator depends on gravity and a thermosiphon effect. A mixture of liquid refrigerant and vapor returns to the surge tank, and the vapor flows into the suction line. A baffle installed in the surge tank helps prevent foam and liquid from entering the suction line. A liquid refrigerant circulating pump Figure 17 provides a more positive way of obtaining a high circulation rate.

Taking the suction line off the top of the surge tank causes difficulties if no special provisions are made for oil return. For this reason, the oil return lines in Figure 16 should be installed. These lines are connected near the bottom of the float chamber and also just Fig. They extend to a lower point on the suction line to allow gravity flow. Included in this oil return line is 1 a solenoid valve that is open only while the compressor is running and 2 a metering valve that is adjusted to allow a constant but small-volume return to the suction line.

A liquid-line sight glass may be installed downstream from the metering valve to serve as a convenient check on liquid being returned. Oil can be returned satisfactorily by taking a bleed of refrigerant and oil from the pump discharge Figure 17 and feeding it to the heated oil receiver.

If a low-side float is used, a jet ejector can be used to remove oil from the quiescent float chamber. Although a low pressure drop is desired, oversized hot-gas lines can reduce gas velocities to a point where the refrigerant will not transport oil. Therefore, when using multiple compressors with capacity control, hot-gas risers must transport oil at all possible loadings. Minimum capacities for oil entrainment in hot-gas line risers are shown in Table On multiple-compressor installations, the lowest possible system loading should be calculated and a riser size selected to give at least the minimum capacity indicated in the table for successful oil transport.

In some installations with multiple compressors and with capacity control, a vertical hot-gas line, sized to transport oil at minimum load, has excessive pressure drop at maximum load. When this problem exists, either a double riser or a single riser with an oil separator can be used. Double Hot-Gas Risers. A double hot-gas riser can be used the same way it is used in a suction line.

Figure 18 shows the double riser principle applied to a hot-gas line. Its operating principle and sizing technique are described in the section on Double Suction Risers. Single Riser and Oil Separator. As an alternative, an oil separator in the discharge line just before the riser allows sizing the riser for a low pressure drop.

Any oil draining back down the riser accumulates in the oil separator. With large multiple compressors, separator capacity may dictate the use of individual units for each compressor located between the discharge line and the main discharge header.

Horizontal lines should be level or pitched downward in the direction of gas flow to facilitate travel of oil through the system and back to the compressor. Whenever the condenser is located above the compressor, the hot-gas line should be trapped near the compressor before rising to the condenser, especially if the hot-gas riser is long. This minimizes the possibility of refrigerant, condensed in the line during off cycles, draining back to the head of the compressor.

Also, any oil traveling up the pipe wall will not drain back to the compressor head. The loop in the hot-gas line Figure 19 serves as a reservoir and traps liquid resulting from condensation in the line during shutdown, thus preventing gravity drainage of liquid and oil back to the compressor head.

A small high-pressure float drainer should be installed at the bottom of the trap to drain any significant amount of refrigerant condensate to a low-side component such as a suction accumulator or low-pressure receiver.

This float prevents excessive build-up of liquid in the trap and possible liquid hammer when the compressor is restarted. For multiple-compressor arrangements, each discharge line should have a check valve to prevent gas from active compressors from condensing on heads of idle compressors.

The check valve prevents refrigerant from boiling off in the condenser or receiver and condensing on the compressor heads during off cycles. This check valve should be a piston type, which closes by gravity when the compressor stops running.

A spring-loaded check may incur chatter vibration , particularly on slow-speed reciprocating compressors. For compressors equipped with water-cooled oil coolers, a water solenoid and water-regulating valve should be installed in the water line so that the regulating valve maintains adequate cooling during operation, and the solenoid stops flow during the off cycle to prevent localized condensing of the refrigerant.

Hot-Gas Discharge Mufflers. Mufflers can be installed in hot-gas lines to dampen discharge gas pulsations, reducing vibration and noise. Mufflers should be installed in a horizontal or downflow portion of the hot-gas line immediately after it leaves the compressor.

Because gas velocity through the muffler is substantially lower than that through the hot-gas line, the muffler may form an oil trap. The muffler should be installed to allow oil to flow through it and not be trapped. The parameters associated with sizing the defrost gas line are related to allowable pressure drop and refrigerant flow rate during defrost. Engineers use an estimated two times the evaporator load for effective refrigerant flow rate to determine line sizing requirements.

Pressure drop is not as critical during the defrost cycle, and many engineers use velocity as the criterion for determining line size. The effective condensing temperature and average temperature of the gas must be determined. The velocity determined at saturated conditions gives a conservative line size.

Controlled testing Stoecker showed that, in small coils with R, the defrost flow rate tends to be higher as the condensing temperature increases. The flow rate is on the order of two to three times the normal evaporator flow rate, which supports the estimated two times used by practicing engineers. The check valve should be selected for minimum opening pressure i.

When determining condensate drop leg height, allowance must be made to overcome both the pressure drop across this check valve and the refrigerant pressure drop through the condenser.

In some water-cooled condenser systems, the condenser also serves as a receiver if the total refrigerant charge does not exceed its storage capacity. When an evaporator is fed with a thermal expansion valve, hand expansion valve, or low-pressure float, the operating charge in the evaporator varies considerably depending on the loading.

During low load, the evaporator requires a larger charge because boiling is not as intense. When load increases, the operating charge in the evaporator decreases, and the receiver must store excess refrigerant. Connections for Through-Type Receiver. When a throughtype receiver is used, liquid must always flow from condenser to receiver.

Pressure in the receiver must be lower than that in the condenser outlet. The receiver and its associated piping provide free flow of liquid from the condenser to the receiver by equalizing pressures between the two so that the receiver cannot build up a higher pressure than the condenser. If a vent is not used, piping between condenser and receiver condensate line is sized so that liquid flows in one direction and gas flows in the opposite direction.

Sizing the condensate line for 0. Figure 20 illustrates this configuration. Piping between the condenser and the receiver can be equipped with a separate vent equalizer line to allow receiver and condenser pressures to equalize.

This external vent line can be piped either with or without a check valve in the vent line see Figures 22 and If there is no check valve, prevent discharge gas from discharging directly into the vent line; this should prevent a gas velocity pressure component from being introduced on top of the liquid in the receiver.

When the piping configuration is unknown, install a Fig. The condensate line should be sized so that velocity does not exceed 0. The vent line flow is from receiver to condenser when receiver temperature is higher than condensing temperature. Flow is from condenser to receiver when air temperature around the receiver is below condensing temperature.

Flow rate depends on this temperature difference as well as on the receiver surface area. Vent size can be calculated from this flow rate. Connections for Surge-Type Receiver.

The purpose of a surgetype receiver is to allow liquid to flow to the expansion valve without exposure to refrigerant in the receiver, so that it can remain subcooled.

The receiver volume is available for liquid that is to be removed from the system. Figure 21 shows an example of connections for a surgetype receiver. Height h must be adequate for a liquid pressure at least as large as the pressure loss through the condenser, liquid line, and vent line at the maximum temperature difference between the receiver ambient and the condensing temperature. Condenser pressure drop at the greatest expected heat rejection should be obtained from the manufacturer.

The minimum value of h can then be calculated to determine whether the available height will allow the surgetype receiver. Multiple Condensers. Two or more condensers connected in series or in parallel can be used in a single refrigeration system.

If connected in series, the pressure losses through each condenser must be added. Condensers are more often arranged in parallel. Pressure loss through any one of the parallel circuits is always equal to that through any of the others, even if it results in filling much of one circuit with liquid while gas passes through another. Figure 22 shows a basic arrangement for parallel condensers with a through-type receiver. Condensate drop legs must be long enough to allow liquid levels in them to adjust to equalize pressure losses between condensers at all operating conditions.

Drop legs should be to mm higher than calculated to ensure that liquid outlets drain freely. This height provides a liquid pressure to offset the largest condenser pressure loss. The liquid seal prevents gas blow-by between condensers. Large single condensers with multiple coil circuits should be piped as though the independent circuits were parallel condensers. For example, if the left condenser in Figure 22 has 14 kPa more pressure drop than the right condenser, the liquid level on the left is about 1.

If the condensate lines do not have enough vertical height for this level difference, liquid will back up into the condenser until pressure drop is the same through both circuits. Enough surface may be covered to reduce condenser capacity significantly.

Condensate drop legs should be sized based on 0. The main condensate lines should be based on 0. Figure 23 shows a piping arrangement for parallel condensers with a surge-type receiver. When the system is operating at reduced load, flow paths through the circuits may not be symmetrical. Small pressure differences are not unusual; therefore, the liquid line junction should be about to mm below the bottom of the condensers.

The exact amount can be calculated from pressure loss through each path at all possible operating conditions. When condensers are water-cooled, a single automatic water valve for the condensers in one refrigeration system should be used. Individual valves for each condenser in a single system cannot maintain the same pressure and corresponding pressure drops.

With evaporative condensers Figure 24 , pressure loss may be high. If parallel condensers are alike and all are operated, the differences may be small, and condenser outlets need not be more than to mm above the liquid line junction. If fans on one condenser are not operated while the fans on another condenser are, ASHRAE Handbook—Refrigeration SI then the liquid level in the one condenser must be high enough to compensate for the pressure drop through the operating condenser.

When the available level difference between condenser outlets and the liquid-line junction is sufficient, the receiver may be vented to the condenser inlets Figure In this case, the surge-type receiver can be used. The level difference must then be at least equal to the greatest loss through any condenser circuit plus the greatest vent line loss when the receiver ambient is greater than the condensing temperature.

Air-Cooled Condensers Refrigerant pressure drop through air-cooled condensers must be obtained from the supplier for the particular unit at the specified load. If refrigerant pressure drop is low enough and the arrangement is practical, parallel condensers can be connected to allow for capacity reduction to zero on one condenser without causing liquid back-up in active condensers Figure Multiple condensers with high pressure drops can be connected as shown in Figure 26, provided that 1 the ambient at the receiver is equal to or lower than the inlet air temperature to the condenser; 2 capacity control affects all units equally; 3 all units operate when one operates, unless valved off at both inlet and outlet; and 4 all units are of equal size.

A single condenser can also be connected with an equalizer line to the hot-gas inlet if the vertical drop leg is sufficient to balance refrigerant pressure drop through the condenser and liquid line to the receiver. If unit sizes are unequal, additional liquid height H, equivalent to the difference in full-load pressure drop, is required.

Usually, condensers of equal size are used in parallel applications. If the receiver cannot be located in an ambient temperature below the inlet air temperature for all operating conditions, sufficient extra height of drop leg H is required to overcome the 1. Subcooling by the liquid leg tends to condense vapor in the receiver to reach a balance between rate of condensation, at an intermediate saturation pressure, and heat gain from ambient to the receiver.

A relatively large liquid leg is required to balance a small temperature difference; therefore, this method is probably limited to marginal cases. Liquid leaving the receiver is nonetheless saturated, and any subcooling to prevent flashing in the liquid line must be obtained downstream of the receiver.

If the temperature of the receiver ambient is above the condensing pressure only at part-load conditions, it may be acceptable to back liquid into the condensing surface, sacrificing the operating economy of lower part-load pressure for a lower liquid leg requirement.

The receiver must be adequately sized to contain a minimum of the backed-up liquid so that the condenser can be fully drained when full load is required. If a low-ambient control system of backing liquid into the condenser is used, consult the system supplier for proper piping. They are used for one or more of the following functions: Fig. Efficiency of the thermodynamic cycle of certain halocarbon refrigerants can be increased when the suction gas is superheated by removing heat from the liquid.

This increased efficiency must be evaluated against the effect of pressure drop through the suction side of the exchanger, which forces the compressor to operate at a lower suction pressure. Liquid-suction heat exchangers are most beneficial at low suction temperatures. The heat exchanger can be located wherever convenient. The heat exchanger should be located near the condenser or receiver to achieve subcooling before pressure drop occurs. Many heat pumps incorporating reversals of the refrigerant cycle include a suctionline accumulator and liquid-suction heat exchanger arrangement to trap liquid floodbacks and vaporize them slowly between cycle reversals.

If an evaporator design makes a deliberate slight overfeed of refrigerant necessary, either to improve evaporator performance or to return oil out of the evaporator, a liquid-suction heat exchanger is needed to evaporate the refrigerant. A flooded water cooler usually incorporates an oil-rich liquid bleed from the shell into the suction line for returning oil. The liquid-suction heat exchanger boils liquid refrigerant out of the mixture in the suction line. Exchangers used for this purpose should be placed in a horizontal run near the evaporator.

Several types of liquid-suction heat exchangers are used. Liquid and Suction Line Soldered Together. The simplest form of heat exchanger is obtained by strapping or soldering the suction and liquid lines together to obtain counterflow and then insulating the lines as a unit.

To maximize capacity, the liquid line should always be on the bottom of the suction line, because liquid in a suction line runs along the bottom Figure This arrangement is limited by the amount of suction line available. These units are usually installed so that the suction outlet drains the shell. When the units are used to evaporate liquid refrigerant 1. Liquid refrigerant can run along the bottom of the heat exchanger shell, having little contact with the warm liquid coil, and drain into the compressor.

By installing the heat exchanger at a slight angle to the horizontal Figure 29 with gas entering at the bottom and leaving at the top, any liquid returning in the line is trapped in the shell and held in contact with the warm liquid coil, where most of it is vaporized. An oil return line, with a metering valve and solenoid valve open only when the compressor is running , is required to return oil that collects in the trapped shell.

Concentric Tube-in-Tube Heat Exchangers. The tube-intube heat exchanger is not as efficient as the shell-and-finned-coil type. It is, however, quite suitable for cleaning up small amounts of excessive liquid refrigerant returning in the suction line. Figure 30 shows typical construction with available pipe and fittings. Plate Heat Exchangers. Plate heat exchangers provide highefficiency heat transfer. They are very compact, have low pressure drop, and are lightweight devices.

They are good for use as liquid subcoolers. For air-conditioning applications, heat exchangers are recommended for liquid subcooling or for clearing up excess liquid in the suction line. For refrigeration applications, heat exchangers are recommended to increase cycle efficiency, as well as for liquid subcooling and removing small amounts of excess liquid in the suction line.

Excessive superheating of the suction gas should be avoided. Two-Stage Subcoolers To take full advantage of the two-stage system, the refrigerant liquid should be cooled to near the interstage temperature to reduce the amount of flash gas handled by the low-stage compressor.

The net result is a reduction in total system power requirements. The amount of gain from cooling to near interstage conditions varies among refrigerants.

Figure 31 illustrates an open or flash-type cooler. This is the simplest and least costly type, which has the advantage of cooling liquid to the saturation temperature of the interstage pressure.

One disadvantage is that the pressure of cooled liquid is reduced to interstage pressure, leaving less pressure available for liquid transport. Although the liquid temperature is reduced, the pressure drops correspondingly, and the expansion device controlling flow to the cooler must be large enough to pass all the liquid refrigerant flow.

Failure of this valve could allow a large flow of liquid to the upper-stage compressor suction, which could seriously damage the compressor. Liquid from a flash cooler is saturated, and liquid from a cascade condenser usually has little subcooling. In both cases, the liquid temperature is usually lower than the temperature of the surroundings. Thus, it is important to avoid heat input and pressure losses that would cause flash gas to form in the liquid line to the expansion device or to recirculating pumps.

Cold liquid lines should be insulated, because expansion devices are usually designed to feed liquid, not vapor. Figure 32 shows the closed or heat exchanger type of subcooler. It should have sufficient heat transfer surface to transfer heat from the liquid to the evaporating refrigerant with a small final temperature difference.

Pressure drop should be small, so that full pressure is available for feeding liquid to the expansion device at the lowtemperature evaporator. The subcooler liquid control valve should be sized to supply only the quantity of refrigerant required for the subcooling. This prevents a tremendous quantity of liquid from flowing to the upper-stage suction in the event of a valve failure. Discharge Line Oil Separators Oil is always in circulation in systems using halocarbon refrigerants.

Refrigerant piping is designed to ensure that this oil passes Fig. When the compressor starts up with a violent foaming action, oil is thrown out at an accelerated rate, and the separator immediately returns a large portion of this oil to the crankcase. Normally, the system should be designed with pumpdown control or crankcase heaters to minimize liquid absorption in the crankcase. Oil separators reduce the amount of bleedoff from the flooded cooler needed for operation.

The oil separator is usually supplied with the compressor unit assembly directly from the compressor manufacturer. The oil separator can be an integral part of the total system oil management system. This is true if the condenser is in a warm location, such as on a roof. During the off cycle, the oil separator cools down and acts as a condenser for refrigerant that evaporates in warmer parts of the system.

A cool oil separator may condense discharge gas and, on compressor start-up, automatically drain it into the compressor crankcase. To minimize this possibility, the drain connection from the oil separator can be connected into the suction line. This line should be equipped with a shutoff valve, a fine filter, hand throttling and solenoid valves, and a sight glass. The throttling valve should be adjusted so that flow through this line is only a little greater than would normally be expected to return oil through the suction line.

If it sticks open, hot gas will continuously bypass to the compressor crankcase. If the valve sticks closed, no oil is returned to the compressor. To minimize this problem, the separator can be Fig. A separate external float trap can then be located in the oil drain line from the separator preceded by a filter. Shutoff valves should isolate the filter and trap.

The filter and traps are also easy to service without stopping the system. The discharge line pipe size into and out of the oil separator should be the full size determined for the discharge line. For separators that have internal oil float mechanisms, allow enough room to remove the oil float assembly for servicing.

To minimize entrance of condensed refrigerant from the low side, a thermostat may be installed and wired to control the solenoid in the oil return line from the separator. The thermostat sensing element should be located on the oil separator shell below the oil level and set high enough so that the solenoid valve will not open until the separator temperature is higher than the condensing temperature.

A superheat-controlled expansion valve can perform the same function. If a discharge line check valve is used, it should be downstream of the oil separator. Surge Drums or Accumulators A surge drum is required on the suction side of almost all flooded evaporators to prevent liquid slopover to the compressor. Exceptions include shell-and-tube coolers and similar shell-type evaporators, which provide ample surge space above the liquid level or contain eliminators to separate gas and liquid.

A horizontal surge drum is sometimes used where headroom is limited. The drum can be designed with baffles or eliminators to separate liquid from the suction gas. More often, sufficient separation space is allowed above the liquid level for this purpose. Usually, the design is vertical, with a separation height above the liquid level of to mm and with the shell diameter sized to keep suction gas velocity low enough to allow liquid droplets to separate. Because these vessels are also oil traps, it is necessary to provide oil bleed.

Compressor Floodback Protection Certain systems periodically flood the compressor with excessive amounts of liquid refrigerant. When periodic floodback through the suction line cannot be controlled, the compressor must be protected against it.

Figure 29 illustrates an arrangement that handles moderate liquid floodback, disposing of liquid by a combination of boiling off in the exchanger and limited bleedoff into the suction line. This device, however, does not have sufficient trapping volume for most heat pump applications or hot-gas defrost systems using reversal of the refrigerant cycle. The refrigerant charging connection should be located between the receiver outlet valve and liquid-line drier so that all refrigerant added to the system passes through the drier.

Reliable moisture indicators can be installed in refrigerant liquid lines to provide a positive indication of when the drier cartridge should be replaced. Strainers Fig. The arrangement shown in Figure 33 has been used successfully in reverse-cycle heat pump applications using halocarbon refrigerants.

It consists of a suction-line accumulator with enough volume to hold the maximum expected floodback and a large enough diameter to separate liquid from suction gas. Trapped liquid is slowly bled off through a properly sized and controlled drain line into the suction line, where it is boiled off in a liquid-suction heat exchanger between cycle reversals. Refrigerant Driers and Moisture Indicators The effect of moisture in refrigeration systems is discussed in Chapters 6 and 7.

Using a permanent refrigerant drier is recommended on all systems and with all refrigerants. It is especially important on low-temperature systems to prevent ice from forming at expansion devices. A full-flow drier is always recommended in hermetic compressor systems to keep the system dry and prevent decomposition products from getting into the evaporator in the event of a motor burnout.

Replaceable-element filter-driers are preferred for large systems because the drying element can be replaced without breaking any refrigerant connections. The drier is usually located in the liquid line near the liquid receiver. It may be mounted horizontally or vertically with the flange at the bottom, but it should never be mounted vertically with the flange on top because any loose material would then fall into the line when the drying element was removed.

Strainers should be used in both liquid and suction lines to protect automatic valves and the compressor from foreign material, such as pipe welding scale, rust, and metal chips. The strainer should be mounted in a horizontal line, oriented so that the screen can be replaced without loose particles falling into the system.

A liquid-line strainer should be installed before each automatic valve to prevent particles from lodging on the valve seats.

Where multiple expansion valves with internal strainers are used at one location, a single main liquid-line strainer will protect all of these. The liquid-line strainer can be located anywhere in the line between the condenser or receiver and the automatic valves, preferably near the valves for maximum protection. Strainers should trap the particle size that could affect valve operation. With pilot-operated valves, a very fine strainer should be installed in the pilot line ahead of the valve.

Filter-driers dry the refrigerant and filter out particles far smaller than those trapped by mesh strainers. No other strainer is needed in the liquid line if a good filter-drier is used. Refrigeration compressors are usually equipped with a built-in suction strainer, which is adequate for the usual system with copper piping. The suction line should be piped at the compressor so that the built-in strainer is accessible for servicing. Both liquid- and suction-line strainers should be adequately sized to ensure sufficient foreign material storage capacity without excessive pressure drop.

In steel piping systems, an external suction-line strainer is recommended in addition to the compressor strainer. Liquid Indicators Every refrigeration system should have a way to check for sufficient refrigerant charge. Common devices used are liquid-line sight glass, mechanical or electronic indicators, and an external gage glass with equalizing connections and shutoff valves. A properly installed sight glass shows bubbling when the charge is insufficient. Liquid indicators should be located in the liquid line as close as possible to the receiver outlet, or to the condenser outlet if no receiver is used Figure The sight glass is best installed in a vertical section of line, far enough downstream from any valve that the resulting disturbance does not appear in the glass.

If the sight glass is installed too far away from the receiver, the line pressure drop may be sufficient to cause flashing and bubbles in the Halocarbon Refrigeration Systems Fig. When sight glasses are installed near the evaporator, often no amount of system overcharging will give a solid liquid condition at the sight glass because of pressure drop in the liquid line or lift.

Subcooling is required here. An additional sight glass near the evaporator may be needed to check the refrigerant condition at that point. Sight glasses should be installed full size in the main liquid line. In very large liquid lines, this may not be possible; the glass can then be installed in a bypass or saddle mount that is arranged so that any gas in the liquid line will tend to move to it. A sight glass with double ports for back lighting and seal caps, which provide added protection against leakage, is preferred.

Moisture-liquid indicators large enough to be installed directly in the liquid line serve the dual purpose of liquid-line sight glass and moisture indicator. Oil Receivers Oil receivers serve as reservoirs for replenishing crankcase oil pumped by the compressors and provide the means to remove refrigerant dissolved in the oil.

Outlets are arranged to prevent oil from draining below the heater level to avoid heater burnout and to prevent scale and dirt from being returned to the compressor. Purge Units Noncondensable gas separation using a purge unit is useful on most large refrigeration systems where suction pressure may fall below atmospheric pressure see Figure 30 of Chapter 2.

This ensures that the valve will not pass water during off cycles. These valves are usually sized to pass the design quantity of water at about a to kPa difference between design condensing pressure and valve shutoff pressure. Chapter 11 has further information. Water Bypass In cooling tower applications, a simple bypass with a manual or automatic valve responsive to pressure change can also be used to maintain condensing pressure.

Figure 36 shows an automatic threeway valve arrangement. The valve divides water flow between the condenser and the bypass line to maintain the desired condensing pressure. This maintains a balanced flow of water on the tower and pump. Evaporative Condensers Among the methods used for condensing pressure control with evaporative condensers are 1 cycling the spray pump motor; 2 cycling both fan and spray pump motors; 3 throttling the spray water; 4 bypassing air around duct and dampers; 5 throttling air via dampers, on either inlet or discharge; and 6 combinations of these methods.

In water pump cycling, a pressure control at the gas inlet starts and stops the pump in response to pressure changes. The pump sprays water over the condenser coils. As pressure drops, the pump stops and the unit becomes an air-cooled condenser.

Constant pressure is difficult to maintain with coils of prime surface tubing because as soon as the pump stops, the pressure goes up and the pump starts again. This occurs because these coils have insufficient capacity when operating as an air-cooled condenser.

The problem is not as acute with extended-surface coils. Shortcycling results in excessive deposits of mineral and scale on the tubes, decreasing the life of the water pump. One method of controlling pressure is using cycle fans and pumps. This minimizes water-side scaling. In colder climates, an indoor water sump with a remote spray pump s is required. With water-cooled condensers, pressure controls are used both to maintain condensing pressure and to conserve water.

On cooling tower applications, they are used only where it is necessary to maintain condensing temperatures. Upon rising pressure 1. When pressure drops below the setting of the modulating control valve, it opens, allowing discharge gas to enter the liquid drain line. This restricts liquid refrigerant drainage and causes the condenser to flood enough to maintain the condenser and receiver pressure at the control valve setting. A pressure difference must be available across the valve to open it.

Although the condenser imposes sufficient pressure drop at full load, pressure drop may practically disappear at partial loading. Therefore, a positive restriction must be placed parallel with the condenser and the control valve. Systems using this type of control require extra refrigerant charge. In multiple-fan air-cooled condensers, it is common to cycle fans off down to one fan and then to apply air throttling to that section or modulate the fan motor speed.

Consult the manufacturer before using this method, because not all condensers are properly circuited for it.

Using ambient temperature change rather than condensing pressure to modulate air-cooled condenser capacity prevents rapid cycling of condenser capacity. A disadvantage of this method is that the condensing pressure is not closely controlled. However, because most microchannel condensers are made up of many individual heat exchangers, there is an opportunity to mechanically isolate portions of the condenser to reduce the usable surface area. This type of control scheme can be used instead of holding back excess refrigerant to flood portions of the condenser.

Any one of the following control methods accomplishes this. Automatic Pumpdown Control Direct-Expansion Air-Cooling Systems The most effective way to keep liquid out of the crankcase during system shutdown is to operate the compressor on automatic pumpdown control.

The recommended arrangement involves the following devices and provisions: Fig. One drawback of dampers is formation of ice on dampers and linkages.

Figure 38 incorporates an air bypass arrangement for controlling pressure. A modulating motor, acting in response to a modulating pressure control, positions dampers so that the mixture of recirculated and cold inlet air maintains the desired pressure. In extremely cold weather, most of the air is recirculated. Air-Cooled Condensers Methods for condensing pressure control with air-cooled condensers include 1 cycling fan motor, 2 air throttling or bypassing, 3 coil flooding, and 4 fan motor speed control.

The first two methods are described in the section on Evaporative Condensers. If the cut-in setting is any higher, liquid refrigerant can accumulate and condense in the crankcase at a pressure corresponding to the ambient temperature.

Then, crankcase pressure would not rise high enough to reach the cut-in point, and effective automatic pumpdown would not be obtained. Halocarbon Refrigeration Systems Crankcase Oil Heater Direct-Expansion Systems A crankcase oil heater with or without single nonrecycling pumpout at the end of each operating cycle does not keep liquid refrigerant out of the crankcase as effectively as automatic pumpdown control, but many compressors equalize too quickly after stopping automatic pumpdown control.

Crankcase oil heaters maintain the crankcase oil at a temperature higher than that of other parts of the system, minimizing absorption of the refrigerant by the oil.

Operation with the single pumpout arrangement is as follows. Whenever the temperature control device opens the circuit, or the manual control switch is opened for shutdown purposes, the crankcase heater is energized, and the compressor keeps running until it cuts off on the low-pressure switch. Because the crankcase heater remains energized during the complete off cycle, it is important that a continuous live circuit be available to the heater during the off time.

The compressor cannot start again until the temperature control device or manual control switch closes, regardless of the position of the low-pressure switch. A crankcase heater is the best solution, with a solenoid valve in the liquid line that closes when the compressor stops. Effect of Short Operating Cycle With reciprocating compressors, oil leaves the crankcase at an accelerated rate immediately after starting. Therefore, each start should be followed by a long enough operating period to allow the oil level to recover.

Controllers used for compressors should not produce short-cycling of the compressor. In the control sequence, the unloading bypass valve is energized on demand of the control calling for compressor operation, equalizing pressures across the compressor.

After an adequate delay, a timing relay closes a pair of normally open contacts to start the compressor.

After a further time delay, a pair of normally closed timing relay contacts opens, deenergizing the bypass valve. In using these arrangements, hot gas should not be bypassed until after the last unloading step. Hot-gas bypass should 1 give acceptable regulation throughout the range of loads, 2 not cause excessive superheating of the suction gas, 3 not cause any refrigerant overfeed to the compressor, and 4 maintain an oil return to the compressor.

Hot-gas bypass for capacity control is an artificial loading device that maintains a minimum evaporating pressure during continuous compressor operation, regardless of evaporator load. This is usually done by an automatic or manual pressure-reducing valve that establishes a constant pressure on the downstream side.

Four common methods of using hot-gas bypass are shown in Figure Figure 39A illustrates the simplest type; it will dangerously overheat the compressor if used for protracted periods of time. Figure 39B shows the use of hot-gas bypass to the exit of the evaporator. The expansion valve bulb should be placed at least 1.

In Figure 39D, the hot-gas bypass enters after the evaporator thermostatic expansion valve bulb. Another thermostatic expansion valve supplies liquid directly to the bypass line for desuperheating. It is always important to install the hot-gas bypass far enough back in the system to maintain sufficient gas velocities in suction risers and other components to ensure oil return at any evaporator loading. Figure 39C shows the most satisfactory hot-gas bypass arrangement.

Here, the bypass is connected into the low side between the expansion valve and entrance to the evaporator. If a distributor is used, gas enters between the expansion valve and distributor. Refrigerant distributors are commercially available with side inlet connections that can be used for hot-gas bypass duty to a certain extent.

Pressure drop through the distributor tubes must be evaluated to determine how much gas can be bypassed. This arrangement provides good oil return. Solenoid valves should be placed before the constant-pressure bypass valve and before the thermal expansion valve used for liquid injection desuperheating, so that these devices cannot function until they are required. Control valves for hot gas should be close to the main discharge line because the line preceding the valve usually fills with liquid when closed.

The hot-gas bypass line should be sized so that its pressure loss is only a small percentage of the pressure drop across the valve. Usually, it is the same size as the valve connections.

When sizing the valve, consult a control valve manufacturer to determine the minimum compressor capacity that must be offset, refrigerant used, condensing pressure, and suction pressure. When unloading Figure 39C , pressure control requirements increase considerably because the only heat delivered to the condenser is that caused by the motor power delivered to the compressor. Discharge pressure should be kept high enough that the hot-gas bypass valve can deliver gas at the required rate. The condenser pressure control must be capable of meeting this condition.

However, if a leak does occur, the consequence are reduced if the system charge has been minimized. There are many ways to reduce charge, but most require significant system modifications; consequently, charge reduction is usually performed during system remodeling or replacement.

One of the best opportunities to reduce refrigerant charge exists in the distribution piping that feeds liquid to the evaporator from the receiver and returns the suction gas to the compressor. Systems serving numerous evaporators across a facility e. For systems that use single circuiting, in which each evaporator or small group of adjacent evaporators has its own liquid and suction line piped back to the compressor, charge can be significantly reduced by zoning the loads.

For loads operating at similar evaporator pressures, one suction and liquid line can run from the machinery room and branch out closer to the load to feed multiple evaporators loop piping. Expansion, solenoid, and evaporator pressure regulating valves must be next to the heat load in these systems, but benefits beyond reduced charge include cheaper installation cost and less physical space required to run the lines.

Note that using hot-gas defrost with this type of piping scheme is typically not preferred, because it requires a third branched line that must also be field installed. In the liquid feed lines, subcooling the liquid can further reduce charge. Subcooling is typically chosen for its energy benefits and is also often used to protect liquid from flashing before it reaches the expansion valve, so the fact that the refrigerant charge can be reduced is often considered a secondary benefit.

The other factor affecting the amount of refrigerant in the distribution piping is the equipment location. Minimizing the distance between the receiver and the evaporators also reduces the refrigerant charge in the liquid piping.

For this reason, some users install compressor systems throughout their facilities instead of centralizing them in a compressor room. Distributed systems typically use quieter scroll compressors, along with special noise-reducing enclosures to allow installations in more exposed and occupied areas. Replacing or retrofitting a direct system to an indirect or secondary system is another way to reduce refrigerant charge in distribution piping.

This method requires a much more dramatic change to the system, but it is probably the most effective because it can restrict the halocarbon refrigerant to a compact unit composed of a compressor, condenser, and evaporator.

The secondary fluid can then be pumped through air-cooling heat exchangers at the load. In this type of system, only a few evaporators are required and the distribution piping is eliminated, so the chance of refrigerant leaks is dramatically reduced.

Opportunities to reduce charge also exist on the high-pressure side of the system between the compressor and the receiver. In comparison to standard air-cooled condensers, systems that use watercooled condensers operate with a lower charge. If a condensing Halocarbon Refrigeration Systems water source is available, a flat-plate condenser can be mounted near the compressors and used to reject heat from the high-pressure side of the system to the water loop.

Because condenser flooding is no longer required, refrigerant charge can be reduced. Microchannel condensers also have lower charges than standard air-cooled condensers but may require long runs of liquid piping in installations with indoor compressors.

In systems that require flooding, microchannel condensers allow for reduced refrigerant charge because of their smaller internal volume. Alternatively, in low ambient conditions, in conjunction with fan controls, entire banks of some microchannel condensers can be isolated using solenoid valves if the outlet piping is correctly trapped; this approach provides the same benefit as condenser flooding, but requires less refrigerant.

Such conversions require planning and preparation. The most glaring concern is the effect of the new refrigerant on system capacity. Not only should the capacity of the compressor s be considered, but also the capacity of every other component in the system condensers, evaporators, valves, etc. Equipment and component manufacturers often can provide the needed derating factors to adjust capacities appropriately. Before any work begins, it is a good idea to record how the system is performing: data such as highand low-side pressures and temperatures help to suggest how the system should operate after the retrofit.

Thermal expansion valves TXV require attention in any retrofit. At the very least, the superheats need to be adjusted; often, the temperature-sensing bulbs and nozzles must be changed out. The designer should consult with the valve manufacturer to decide what action should be taken, and whether the entire TXV should be replaced. Mineral oils and alkylbenzene oils are often replaced with POE oils to maintain oil miscibility with the new refrigerant. It is always important to follow a thorough change-out procedure to ensure that all traces of the existing oil are removed from the system.

A typical procedure includes among other tasks draining the existing oil; changing out liquid driers, suction filters, and oil filters; and recharging the system with the new oil. The draining and recharging steps may need to be repeated more than once to achieve the desired purity for the new oil.

Elastomeric gasket and seal materials in the system will also react differently to new refrigerants and oils. Swell characteristics of different elastomers can be referenced from Table 9 in Chapter 29 of the ASHRAE Handbook—Fundamentals; however, testing is necessary to know exactly how gaskets and seals will react to mixtures of different refrigerants and oils and what factors other than swell may come into play, such as the overall integrity and functionality of the material.

For this reason, it is common practice to change out all elastomeric gaskets and seals as part of the retrofit procedure. After the system is up and running with the new refrigerant and oil, the performance of the system can be evaluated to determine 1. Refrigerant and oil levels should also be monitored until the correct levels are achieved, and filters should be changed until the system is clean.

Finally, it is always crucial to make the appropriate signage and labeling modifications to prevent anyone from topping off the system with the old refrigerant or oil. Beyond this, the designer must know what temperatures to use to properly size equipment. As the refrigerant continues to condense, its temperature drops until it reaches the bubble-point temperature, at which point it is fully condensed.

The liquid can then be subcooled. Conversely, when the refrigerant starts to boil in the evaporator, it starts at the bubble-point temperature and is not fully evaporated until it reaches the dew-point temperature. The gas can then be superheated.

So, when calculating subcooling at the condenser exit, the bubble-point temperature must represent the saturation point; when calculating superheating at the evaporator exit, the dew-point temperature must represent the saturation point.

Refrigerant manufacturers publish pressure-temperature charts that allow the bubble, mean, and dew-point temperatures to be easily referenced given a specific pressure.

Blended refrigerants essentially separate fractionate during phase changes, so leaky condensers and evaporators create concern: refrigerant composition changes can occur in the system, leading to unpredictable system operation. For this reason, it is necessary to only charge systems with refrigerant in the liquid state unless the entire cylinder will be immediately used.

Furthermore, if a leak occurs and the system is repaired, the refrigerant composition should be checked for significant changes before topping off the system. When calculating temperature differences to check the rated capacity of existing condensers and evaporators, the mean temperatures should be used along with any derating factors provided by the manufacturer. The challenge, however, exists in accurately determining the dew-point temperatures. Simply adding half of the glide to the mean temperature may not be accurate: it is difficult to determine what the actual mean temperature really must be for effective evaporator or condenser operation.

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By Ahmed Shawky Last updated May 7, Ahmed Shawky posts 6 comments. Plumbing and Firefighting design Engineer. You might also like More from author. Prev Next. Sign in. Welcome, Login to your account. Forget password?