Refrigeration table pdf download

If a discharge line check valve is used, it should be downstream of the oil separator. Exceptions pressor. This is true if the eliminators to separate gas and liquid. A horizontal surge drum is condenser is in a warm location, such as on a roof. During the off sometimes used where headroom is limited. A cool rate liquid from the suction gas.

More often, sufficient separation oil separator may condense discharge gas and, on compressor space is allowed above the liquid level for this purpose. Usually, start-up, automatically drain it into the compressor crankcase. To the design is vertical, with a separation height above the liquid minimize this possibility, the drain connection from the oil sepa- level of to mm and with the shell diameter sized to keep rator can be connected into the suction line. This line should be suction gas velocity low enough to allow liquid droplets to sepa- equipped with a shutoff valve, a fine filter, hand throttling and rate.

Because these vessels are also oil traps, it is necessary to pro- solenoid valves, and a sight glass. The throttling valve should be vide oil bleed. If it sticks open, hot gas will continuously bypass to the compressor crankcase. If the valve sticks closed, no oil is returned Compressor Floodback Protection to the compressor.

To minimize this problem, the separator can be 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 mod- erate liquid floodback, disposing of liquid by a combination of boil- ing 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 rever- Fig. A three-valve bypass is usually used, as shown in Figure 34, to provide a way to isolate the drier for servicing. 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 Strainers should be used in both liquid and suction lines to pro- tect 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 loca- Fig.

The Accumulator with Controlled Bleed liquid-line strainer can be located anywhere in the line between the condenser or receiver and the automatic valves, preferably near the For heavier floodback, a larger volume is required in the trap.

Strainers should trap the particle size The arrangement shown in Figure 33 has been used successfully in that could affect valve operation. With pilot-operated valves, a very reverse-cycle heat pump applications using halocarbon refrigerants. It consists of a suction-line accumulator with enough volume to Filter-driers dry the refrigerant and filter out particles far smaller hold the maximum expected floodback and a large enough diameter than those trapped by mesh strainers.

No other strainer is needed in to separate liquid from suction gas. Trapped liquid is slowly bled off the liquid line if a good filter-drier is used. In steel piping systems, an external suction-line Refrigerant Driers and Moisture Indicators strainer is recommended in addition to the compressor strainer. The effect of moisture in refrigeration systems is discussed in Chapters 6 and 7. Using a permanent refrigerant drier is recom- Liquid Indicators mended on all systems and with all refrigerants.

It is especially Every refrigeration system should have a way to check for suf- important on low-temperature systems to prevent ice from forming ficient refrigerant charge. Common devices used are liquid-line at expansion devices. A full-flow drier is always recommended in sight glass, mechanical or electronic indicators, and an external hermetic compressor systems to keep the system dry and prevent gage glass with equalizing connections and shutoff valves.

A decomposition products from getting into the evaporator in the properly installed sight glass shows bubbling when the charge is event of a motor burnout. Replaceable-element filter-driers are preferred for large systems Liquid indicators should be located in the liquid line as close as because the drying element can be replaced without breaking any possible to the receiver outlet, or to the condenser outlet if no refrigerant connections.

The drier is usually located in the liquid receiver is used Figure The sight glass is best installed in a line near the liquid receiver. It may be mounted horizontally or ver- vertical section of line, far enough downstream from any valve tically with the flange at the bottom, but it should never be mounted that the resulting disturbance does not appear in the glass.

If the vertically with the flange on top because any loose material would sight glass is installed too far away from the receiver, the line pres- then fall into the line when the drying element was removed.

Condenser-Water-Regulating Valves When sight glasses are installed near the evaporator, often no The shutoff pressure of the valve must be set slightly higher than amount of system overcharging will give a solid liquid condition at the saturation pressure of the refrigerant at the highest ambient tem- the sight glass because of pressure drop in the liquid line or lift. Sub- perature expected when the system is not in operation. This ensures cooling is required here.

An additional sight glass near the evapora- that the valve will not pass water during off cycles. These valves are tor may be needed to check the refrigerant condition at that point. Chapter 11 has further information.

A sight glass with dou- Water Bypass ble ports for back lighting and seal caps, which provide added pro- In cooling tower applications, a simple bypass with a manual or tection against leakage, is preferred.

Moisture-liquid indicators automatic valve responsive to pressure change can also be used to large enough to be installed directly in the liquid line serve the dual maintain condensing pressure. Figure 36 shows an automatic three- purpose of liquid-line sight glass and moisture indicator.

The valve divides water flow between the condenser and the bypass line to maintain the desired condensing Oil Receivers pressure. This maintains a balanced flow of water on the tower and Oil receivers serve as reservoirs for replenishing crankcase oil pump.

Outlets are arranged to and stops the pump in response to pressure changes. The pump prevent oil from draining below the heater level to avoid heater sprays water over the condenser coils. As pressure drops, the pump burnout and to prevent scale and dirt from being returned to the stops and the unit becomes an air-cooled condenser. Constant pressure is difficult to maintain with coils of prime sur- face tubing because as soon as the pump stops, the pressure goes up Purge Units and the pump starts again.

This occurs because these coils have insufficient capacity when operating as an air-cooled condenser. Noncondensable gas separation using a purge unit is useful on The problem is not as acute with extended-surface coils. Short- most large refrigeration systems where suction pressure may fall cycling results in excessive deposits of mineral and scale on the below atmospheric pressure see Figure 30 of Chapter 2.

This minimizes water-side scaling. With water-cooled condensers, pressure controls are used both to Upon rising pressure maintain condensing pressure and to conserve water. The third method holds condensing pressure up by backing liq- uid refrigerant up in the coil to cut down on effective condensing surface.

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 im- poses sufficient pressure drop at full load, pressure drop may prac- tically 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 pres- sure 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 microchan- nel condensers are made up of many individual heat exchangers, there is an opportunity to mechanically isolate portions of the con- denser to reduce the usable surface area.

This type of control scheme can be used instead of holding back excess refrigerant to flood por- tions 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 pump- down control.

One drawback of dampers is formation of tor fan, so refrigerant flow stops when the fan is out of operation. A modulating motor, acting in response to a modulating overloads , so that the refrigerant solenoid valve closes when the pressure control, positions dampers so that the mixture of recircu- compressor stops.

If the cut-in setting is Air-Cooled Condensers any higher, liquid refrigerant can accumulate and condense in the Methods for condensing pressure control with air-cooled con- crankcase at a pressure corresponding to the ambient temperature.

The first two meth- cut-in point, and effective automatic pumpdown would not be ods are described in the section on Evaporative Condensers. Crankcase Oil Heater Direct-Expansion Systems To prevent overheating, this valve is open only during the starting A crankcase oil heater with or without single nonrecycling period and closed after the compressor is up to full speed and full pumpout at the end of each operating cycle does not keep liquid voltage is applied to the motor terminals.

Crankcase oil heaters main- izing pressures across the compressor. After an adequate delay, a tain the crankcase oil at a temperature higher than that of other parts timing relay closes a pair of normally open contacts to start the com- of the system, minimizing absorption of the refrigerant by the oil. After a further time delay, a pair of normally closed timing Operation with the single pumpout arrangement is as follows.

Because the crankcase heater bypass arrangements can be used in ways that will not overheat the remains energized during the complete off cycle, it is important that compressor. In using these arrangements, hot gas should not be a continuous live circuit be available to the heater during the off bypassed until after the last unloading step. The compressor cannot start again until the temperature con- Hot-gas bypass should 1 give acceptable regulation throughout trol device or manual control switch closes, regardless of the posi- the range of loads, 2 not cause excessive superheating of the suc- tion of the low-pressure switch.

Figure 39A illustrates the simplest type; it will dangerously heater during the compressor off cycle and deenergize it during overheat the compressor if used for protracted periods of time. The expansion valve bulb should be placed at least 1. Another thermostatic expansion overloads , so that the refrigerant flow valve closes when the valve supplies liquid directly to the bypass line for desuperheating.

Automatic pumpdown control is undesirable for direct-expansion Figure 39C shows the most satisfactory hot-gas bypass arrange- water chillers because freezing is possible if excessive cycling ment. Here, the bypass is connected into the low side between the occurs. A crankcase heater is the best solution, with a solenoid valve expansion valve and entrance to the evaporator. If a distributor is in the liquid line that closes when the compressor stops. Refrigerant distributors are commercially available with side inlet Effect of Short Operating Cycle connections that can be used for hot-gas bypass duty to a certain With reciprocating compressors, oil leaves the crankcase at an extent.

Pressure drop through the distributor tubes must be evalu- accelerated rate immediately after starting. Therefore, each start ated to determine how much gas can be bypassed. This arrangement should be followed by a long enough operating period to allow the provides good oil return.

Controllers used for compressors should not Solenoid valves should be placed before the constant-pressure produce short-cycling of the compressor.

Most large reciprocating compressors are equipped with unloaders The hot-gas bypass line should be sized so that its pressure loss that allow the compressor to start with most of its cylinders unloaded. However, it may be necessary to further unload the compressor to Usually, it is the same size as the valve connections. When sizing the 1 reduce starting torque requirements so that the compressor can valve, consult a control valve manufacturer to determine the mini- be started both with low-starting-torque prime movers and on low- mum compressor capacity that must be offset, refrigerant used, con- current taps of reduced voltage starters and 2 allow capacity con- densing pressure, and suction pressure.

Dis- Starting the compressor without load can be done with a manual charge pressure should be kept high enough that the hot-gas bypass or automatic valve in a bypass line between the hot-gas and suction valve can deliver gas at the required rate. The condenser pressure con- lines at the compressor.

However, if a leak does occur, from flashing before it reaches the expansion valve, so the fact that the consequence are reduced if the system charge has been mini- the refrigerant charge can be reduced is often considered a second- mized.

There are many ways to reduce charge, but most require sig- ary benefit. Minimizing the distance One of the best opportunities to reduce refrigerant charge exists in between the receiver and the evaporators also reduces the refrigerant the distribution piping that feeds liquid to the evaporator from the re- charge in the liquid piping. For this reason, some users install com- ceiver and returns the suction gas to the compressor. Systems serving pressor systems throughout their facilities instead of centralizing numerous evaporators across a facility e.

Distributed systems typically use quieter a network of distribution piping, which can contain a large portion of scroll compressors, along with special noise-reducing enclosures to the entire system charge. For systems that use single circuiting, in allow installations in more exposed and occupied areas.

For loads operating tion piping. This method requires a much more dramatic change to at similar evaporator pressures, one suction and liquid line can run the system, but it is probably the most effective because it can from the machinery room and branch out closer to the load to feed restrict the halocarbon refrigerant to a compact unit composed of a multiple evaporators loop piping.

Expansion, solenoid, and evapo- compressor, condenser, and evaporator. The secondary fluid can rator pressure regulating valves must be next to the heat load in these then be pumped through air-cooling heat exchangers at the load.

In systems, but benefits beyond reduced charge include cheaper instal- this type of system, only a few evaporators are required and the dis- lation cost and less physical space required to run the lines. Note that tribution piping is eliminated, so the chance of refrigerant leaks is using hot-gas defrost with this type of piping scheme is typically not dramatically reduced.

In com- In the liquid feed lines, subcooling the liquid can further reduce parison to standard air-cooled condensers, systems that use water- charge.

Refrigerant and oil levels the compressors and used to reject heat from the high-pressure side should also be monitored until the correct levels are achieved, and of the system to the water loop. Typically, bypass lines, variable- filters should be changed until the system is clean. Be- fications to prevent anyone from topping off the system with the old cause condenser flooding is no longer required, refrigerant charge refrigerant or oil. In systems that require flood- ing, microchannel condensers allow for reduced refrigerant charge It is not uncommon to retrofit existing systems from single-com- because of their smaller internal volume.

Alternatively, in low am- ponent azeotropic refrigerants to blended, zeotropic refrigerants. Beyond this, the designer must know what temperatures to use to properly size equipment. Because of the halocarbon phaseout, many users are retrofitting When a zeotropic refrigerant starts to condense in the condenser, existing systems to use newer, more acceptable refrigerants e. Such conver- it does so at a constant pressure ignoring pressure drop at the sions require planning and preparation.

As the refrigerant continues to The most glaring concern is the effect of the new refrigerant on condense, its temperature drops until it reaches the bubble-point system capacity. Not only should the capacity of the compressor s temperature, at which point it is fully condensed. The liquid can be considered, but also the capacity of every other component in the then be subcooled. Conversely, when the refrigerant starts to boil in system condensers, evaporators, valves, etc.

Equipment and com- the evaporator, it starts at the bubble-point temperature and is not ponent manufacturers often can provide the needed derating factors fully evaporated until it reaches the dew-point temperature. The gas to adjust capacities appropriately.

Before any work begins, it is a can then be superheated. So, when calculating subcooling at the good idea to record how the system is performing: data such as high- condenser exit, the bubble-point temperature must represent the sat- and low-side pressures and temperatures help to suggest how the uration point; when calculating superheating at the evaporator exit, system should operate after the retrofit. Any available energy data the dew-point temperature must represent the saturation point.

Thermal expansion valves TXV require attention in any retrofit. At the very least, the superheats need to be adjusted; often, by the varying boiling points of the constituent refrigerants within the temperature-sensing bulbs and nozzles must be changed out. Blended refrigerants essentially separate fractionate The designer should consult with the valve manufacturer to decide during phase changes, so leaky condensers and evaporators create what action should be taken, and whether the entire TXV should be concern: refrigerant composition changes can occur in the system, replaced.

Mineral oils and alkylbenzene oils are often unless the entire cylinder will be immediately used. Furthermore, if replaced with POE oils to maintain oil miscibility with the new a leak occurs and the system is repaired, the refrigerant composition refrigerant. It is always important to follow a thorough change-out should be checked for significant changes before topping off the procedure to ensure that all traces of the existing oil are removed system.

A typical procedure includes among other tasks When calculating temperature differences to check the rated draining the existing oil; changing out liquid driers, suction filters, capacity of existing condensers and evaporators, the mean temper- and oil filters; and recharging the system with the new oil. The atures should be used along with any derating factors provided by draining and recharging steps may need to be repeated more than the manufacturer.

When checking the capacity of existing compres- once to achieve the desired purity for the new oil. Swell characteristics requires, when rating compressor capacities, that dew- of different elastomers can be referenced from Table 9 in Chapter 29 point temperatures be used as the reference temperatures at the cor- of the ASHRAE Handbook—Fundamentals; however, testing responding evaporating and condensing pressures.

The challenge, is necessary to know exactly how gaskets and seals will react to mix- however, exists in accurately determining the dew-point tempera- tures of different refrigerants and oils and what factors other than tures. Simply adding half of the glide to the mean temperature may swell may come into play, such as the overall integrity and function- not be accurate: it is difficult to determine what the actual mean tem- ality of the material.

For this reason, it is common practice to change perature really must be for effective evaporator or condenser oper- out all elastomeric gaskets and seals as part of the retrofit procedure.

Because most published capacity data for heat exchangers are After the system is up and running with the new refrigerant and based on temperature that is assumed to be constant during phase oil, the performance of the system can be evaluated to determine change, supplemental derating factors must be used. Influence of oil-refrigerant relationships on oil return. Performance rating of positive displacement refrigerant com- pressors and compressor units.

Air- Giunta, C. Thomas Midgley, Jr. Bulletin for the History of Alofs, D. Hasan, and H. Sauer, Jr. Influence of oil on pres- Chemistry 31 2 Cambridge Safety standard for refrigeration systems. Scheideman, F. Kazem, and N. Oil Standard Refrigeration piping and heat transfer components. American Society of Mechanical Engi- ity and thermal conductivity of vapor refrigerants.

Atwood, T. Pipe sizing and pressure drop calculations for HFCa. Stoecker, W. International Journal of Refrigeration 7 4 Calm, J. The next generation of refrigerants—Historical review, considerations, and outlook.

Ecolibrium Nov. Timm, M. An improved method for calculating refrigerant line pres- Colebrook, D. Turbulent flow in pipes. Journal of the Institute sure drops. Wile, D. Refrigerant line sizing. Ammonia is followed. See Chapter 24 for information on refrigeration load calculations. The compressor applications.

This type of vapor absorp- and can also be internally compounded to provide multiple compres- tion system remains in use today. Use of ammonia as a refrigerant in vapor compression systems The reciprocating compressor is the most common compressor followed. David Boyle established an ice production plant in Jeffer- used in small, 75 kW or less, single-stage or multistage systems.

The son, TX, in using an improved compressor design, and he later screw compressor is the predominant compressor above 75 kW, in set up the Boyle Ice Machine Co. Various combinations of com- ; Woolrich et al.

With the financial backing of several pressors may be used in multistage systems. Rotary vane and screw breweries, Professor Carl von Linde of Munich, Germany, had 30 ice compressors are frequently used for the low-pressure stage, where machines of his design built between and Dincer ; large volumes of gas must be moved. The high-pressure stage may Schmidt The first commercial production of synthetic ammonia began in When selecting a compressor, consider the following: IIAR n. Worldwide annual production of ammonia is approximately million metric tons, of which 9.

Ammonia is considered a natural refrigerant because it is a com- ditions for either initial or normal operation. The temperature mon, naturally occurring compound, and it naturally breaks down must be pulled down frequently for some applications for a pro- into hydrogen and nitrogen. When a reciprocating compressor requires There is renewed interest in using ammonia for HVAC systems, lubricant cooling, an external heat exchanger using a refrigerant or in part because of the scheduled phaseout and increasing costs of secondary cooling is usually added.

Screw compressor lubricant cooling is covered in detail in the section on Screw Compressors. The preparation of this chapter is assigned to TC The correct electric motor size s for a Controls and Accessories. When the final.

Electric motors should not be overloaded, even when a service factor is indicated. Reciprocating Compressors For screw compressor applications, motors should be sized by add- Piping. Screw compressors have built-in compressors operating in parallel off the same suction main. Suc- unloading mechanisms to prevent motor overload. The motor tion mains should be laid out with the objective of returning only should not be oversized, because an oversized motor has a lower clean, dry gas to the compressor.

This usually requires a suction trap power factor and lower efficiency at design and reduced loads. A dead-end trap nal combustion engines are used when electricity is unavailable, or can usually trap only scale and lubricant. As an alternative, a shell- if the selected energy source is cheaper.

Sometimes they are used in and-coil accumulator with a warm liquid coil may be considered. It is also good practice to connect compressor suction Steam turbine drives for refrigerant compressors are usually branch piping above the centerline of the suction main. In all cases, torsional analysis is consider how pressure drop in the selected piping affects the com- required to determine what coupling must be used to dampen out pressor size required.

First costs and operating costs for compressor any pulsations transmitted from the compressor. For optimum effi- and piping selections should be optimized. Neither 1. Practical suction line friction losses the gear reducer nor the turbine can tolerate a pulsating backlash should not exceed 0. Generally, a slightly oversized discharge line is desirable Advantages of turbines include variable speed for capacity control to hold down discharge pressure and, consequently, discharge and low operating and maintenance costs.

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