Document MGJ70LzXJB4Nq0rXL47967qoy

778 CHAPTER 71 1962 Guide And Data Book fig. 28 .... Piping for Single-Stage System with LowPressure Receiver and liquid Ammonia Gradation any accumulated liquid. The amount of pipe inside the shell should be kept to a minimum so as not to superheat more than 5 F deg the gas (without liquid) entering the compressor. The flow of high-pressure hot gas through the internal pipe should be such that the gaa will not condense while flowing through it. Pressurized Return Systems 'Another method of transferring liquid from the suction trap to the high side is to have the main suction trap drain into an auxiliary suction trap located below it. When the auxiliary suction trap is nearly filled with ammonia liquid it is valved off. High-pressure gas is then directed into the auxiliary bap to increase the pressure sufficiently so that it be returned either by gravity or by a low-head ammonia pump to the high-pressure receiver. LIQUID AMMONIA RECIRCULATION SYSTEMS In a liquid ammnnin recirculating system, a pump is used to circulate the Ammonia, from a low-pressure receiver to the evaporators. The low-pressure receiver is a shell for storage of refrigerant at low pressure and used to supply evaporators with refrigerant, either by gravity or by a low head pump. It also talrwn the suction from the evaporators and separates the gas from the liquid. The amount of liquid fed into the evaporator is usually several times the amount that is actually evaporated in the evaporator and, therefore, there is always liquid present in the suction return to the low-pressure receiver. Frequently three times the amount that is evaporated is circulated through the evaporator. In general, the nixing of the liquid ammonia pump is based on the gallons of ammonia required to be circulated versus a head differential between suction and discharge of approxi mately 20 psi. This is satisfactory for most single-story ingtftJIftticinq if there is a static lift on the pump discharge, the differential would have to be increased accordingly. The sixing of the low-pressure receiver is based on the cross-sectional area required to obtain liquid and gas sep aration and also on the volume between the normal liquid level and the alarm liquid level in the low-pressure receiver. This volume should be sufficient to contain the maximum fluctuation in liquid due to the various load conditions. The liquid at the discharge of the pump is in the sub cooled region. A total pressure drop of approximately 5 psi in the piping can be tolerated. The remaining pressure is ex pended through the control valve, coil, and suction return line. It is important that the pressure drop and heat pickup in the liquid supply line be low enough to prevent any flash ing in the liquid supply line. A relief valve is required from the liquid main back to the low-pressure receiver, so that when the liquid line solenoid valves at the various evaporators are closed, either for frosting or for temperature control reasons, the exce can be relieved back to the tow-pressure receiver. This pm^, relief valve is generally set at about 40 psi differential. P The suction header between the evaporators and the lowpressure receiver should be pitched in. per ft, if possible, in order to keep the excess liquid flowing back to the tow-pres! sure receiver. It is important to design tire header so that traps are avoided. Liquid Recirculation in Single-Stage System The piping of a typical angle-stage system with a low- pressure receiver and liquid ammonia recirculating type feed is shown on fig. 28. Some of the details shown in previous figures are omitted, but references are made to them in follow ing text. Liquid Recirculation and Evaporator Piping The typical piping of an evaporator designed for liquid ammonia recirculation and equipped with hot gas defrost ing is shown in Fig. 29. liquid is normally fed through tire liquid line into the vertical header and is distributed to the various coils through orifices in the header. Suction a taken from the suction header through a strainer and in automatic flow stop valve. This valve is wide open when the room thermostat is calling for cooling and regulates the back pressure at 80 psig during the defrost cycle. When the unit is on the defrost cycle, the liquid line sole noid valve is closed and the hot gas solenoid valve is open. The hot gas flows through a coil located underneath tire pan of the unit to keep the condensate from refreesing on tire p*n It is then taken through a check valve into tire liquid header and is distributed to the various coils where defrosting takes plare, The condensed liquid is maintained at a temperature above 32 F and its pressure is relieved back to the lowpressure receiver and from there to the compressors. COMPOUND COMPRESSION SYSTEMS Compound compression systems compress tire gas from tire evaporator to the condenser in several stages. They usually are used to produce temperatures of --15 F and be low, where this cannot be economically accomplished with single-stage compression. The limit of single-stage compression systems is generally between 5 and 10 psig suction pressure. Two-stage systems are used down to about -- 65 F evaporator temperatures. Below this temperature three-stage systems become practical. fig. 29 .... Piping for Evaporator with liquid Ammon*0 Recirculation and Hot Gas Defrost Refrigerant Piping Systems for Ammonia 779 |. Far tucUon trap oad high hood pomp dotuHi mo Fig. 25. 2. fw rotary booster coaprcnw coofiog dotaHs mo Fig. 10. j, hr high-dago compressor cooling mo Fig. 7,9 or 9. 4, te coretensor-reccirer dotaiit too Fig. 11. fig. 30 .... Arrangement for Compound System wHh Vertical Type Intercooler and Suction Trap In the area where either a single- or two-stage compres sion system can be used, it will usually be found that the twostage system operates with less power requirements and, therefore, less operating cost but at increased first cost of When the design compression ratio is approximately 9 to 1, the following items are of importance: 1. The discharge temperature on the high-stage compressor becomes high so that of the oil <-*n result. 2. The volumetric efficiency on the high-stage compressor by""*** low, resulting in higher compressor displacement per too. 3. The refrigerating effect for each pound of ammonia circu lated through the evaporator is at a comparatively low value. Items 1, 2 and 3 can be circumvented by use of a twostage system, with each compressor operating at a low com pression ratio; having some means for cooling the discharge gas from the low-stage compressor and subcooling the liquid from the temperature corresponding to high-stage discharge pressure down to a temperature corresponding nearly to. in termediate pressure. The two-stage system consists of one or more compressors operating at the low suction pressure and discharging at an intermediate pressure and also with one or more compressors. operating at the intermediate suction pressure and discharg ing to the condenser. Gas and Liquid Intercoolers The prime reason for nsing an intercooler in a compound system is to cool the discharge gas between stages to prevent overheating the higher-stage compressor. This is accomplished by bubbling the discharge gas from the low-stage compressor through a bath of liquid refrigerant at intermediate pressure corresponding temperature. There is no heat rejected bom the system in the intercooler. The heat removed from the discharge gas is absorbed by the evaporation of a portion of the liquid in the bath and is eventually high-stage compressor to the condenser. through t-he For plant operating cost economy, the liquid for the evapo- fetors on the tow-stage compressor is subcooled after leaving the condenser so that the refrigerating effect per pound of ammonia is increased. This towers the tow-stage compressor displacement per ton and also reduces its operating brake horsepower. Two types of intercoolers will be illustrated for compound compression systems. In Fig. 30, a vertical coil type inter cooler is shown. A liquid level is maintained in the inter cooler by a float which controls the solenoid valve which feeds liquid into the shell tide of the intercooler. Gas from the first-stage compressor enters the lower head of the inter cooler, is distributed by a perforated plate, and is cooled to the saturation temperature corresponding to intermediate pressure. High-pressure liquid from the receiver flows through a coil immersed in the lower part of the intercooler where it is sub cooled to a temperature approaching that temperature corre sponding to intermediate temperature before it flows to the evaporator. This liquid is kept at a high pressure which re duces the required size of the liquid control valve at the evaporators. A horizontal flash type intercooler is illustrated in Fig. 31. A float switch maintain* a level in the intercooler by con trolling the flow of ammonia liquid from the higb-pressure re ceiver and injecting it into the intercooler where it flashes to the intermediate pressure. After the intercooler has sufficient liquid in it, the excess will flow over a weir and be fed. into the low-pressure receiver where the level is maintained by another float switch. Discharge gas from the low-stage compressor is piped to an internal slotted pipe in the intercooler through which it is distributed up through the liquid and is cooled to the saturation temperature corresponding to intermediate pressure. Two-Stage System Piping Fig. 30 illustrates a two-stage compound compression sys tem utilizing a vertical coil type intercooler with a vertical suction trap in the suction line from the evaporators to inter cept any liquid present. A rotary low-stage compressor takes a suction from the vertical suction trap and discharges into the vertical coil type intercooler. A valval bypass line is provided around the rotary compressor and intercooler so that the plant suction pressure can be towered with the high-stage compressor prior to start ing the low stage so as not to overload the motor on the lowstage compressor. A check valve is provided in the discharge line of the rotary compressor to minimise the back flow of gas from the intercooler to the compressor and into the suction trap and evaporators upon a shut down due to a power failure or for any other reason. likewise, a'check valve is used from the gas space at the top of the intercooler to the top of the dis charge line from the rotary compressor to keep the inter mediate pressure in the intercooler from backing liquid up the discharge line and through the check valve and into the com pressor. Suction gas from high-temperature evaporators can be taken into the intercooler where any liquid present can be separated before going to the high-stage compressor. An equalizer header is provided on the intercooler for mounting of float switches. The lowest float switch maintains toe normal liquid level in the intercooler by controlling the flow of high-pressure liquid from toe receiver to the inter cooler. If the intercooler should receive excess liquid from the high temperature evaporators, the liquid level will rise to the set ting of the middle float switch. This switch opens a solenoid"" valve in a drain line from the intercooler at intermediate pres sure to the suction trap where it can be pumped to the highpressure receiver. The highest float switch is for an alarm and sometimes for stopping the compression system in cases of aw*** liquid. The high-stage compressor takes its suction from toe top of the intercooler and discharges its gas into the condenser.