Document 0gK1y330jjQmX9z3Y1p6YEpVV

558 CHAPTER 38 1959 Guide evaporator, aad this in turn operates through the power ele ment to increase the flow of liquid refrigerant. A flooded evap orator reduces the discharge superheat, and thus tends to reduce the flow of liquid refrigerant. Such an expansion valve is satisfactory for operation with fluctuating loads since this type of control tends to keep the evaporator Ailed with re frigerant at all times. Low-Side Float Folses. A liquid refrigerant control of the low-aide float valve type consists of a ball float located in either a receiver or the evaporator itself on the low-pressure side of the system. A needle valve, operated through a simple lever mechanism attached to the float, permits the passage of more or less refrigerant, as the level to the receiver or the evaporator fluctuates. Such a control must be used'in con junction with a flooded evaporator, and has been applied ex tensively to household refrigerators and, to some extent, in commercial and industrial installations. High-Side Float Valves. A high-side float valve differs from a low-side float valve in that the float is located in a receiver or container on the high-pressure side o! the system. Proper operation again depends upon metering of the refrigerant through a controlled opening, depending upon the level of the liquid refrigerant in the container. Such a control has the disadvantage that the evaporator must be placed directly adjacent to the float container, or some intermediate pressure device must be applied to prevent flashing of the refrigerant upon pressure drop. Capillary Tabes. A capillary tube may be used as a liquid refrigerant expanding device. Such a device consists of an extremely small bore tube (in the order of 0.04 inch in diam eter) of five to twenty feet in length. Although such a restrict ing device operates as a very simple means of expanding the liquid refrigerant, it has the disadvantage that no modifica tions are possible to adjust the rate of expansion under various operating conditions- The bore and length of the tube, as well as the proportions of the rest of the system, are critical. It is for these reasons that its application has been limited to fac tory-assembled domestic and commercial units. Refrigerant How Control The capacity of the refrigeration system must be con trolled in accordance with the load imposed on the system. Except for those cases where the compressor is started and stopped by a thermostat responding to load conditions, some form of control of the refrigerant flow is usually required. Controlling the flow of refrigerant in accordance with the load may be accomplished in a number of ways, some of which are: 1. Solenoid valves are frequently used for control of gas or liquid flow. A solenoid valve is placed in the liquid line ahead of the expansion valve. It is closed whenever the compressor is not in operation and thus leakage into the evaporator is prevented. In most cases it is controlled by a thermostat re sponding to the load. Solenoid liquid valves are widely used for control of refrigerant fiow to individual evaporators in a multiple evaporator system operated by one compressor. The compressor capacity is controlled separately by a controller responding to suction pressure. Solenoid liquid and suction valves are sometimes used to isolate an evaporator completely for defrosting purposes. When solenoid liquid valves are used for controlling refrig erant flow, a pump-down cycle of control is ofteo employed. The thermostat closes the solenoid liquid valve and the com pressor then pumps down until stopped by the low-euction pressure switch. The compressor, cannot restart until the thermostat again opens the solenoid liquid valve. 2. Expansion Valve Modulation. This form of fiow control utilises the throttling characteristics of the expansion valve to starve the coil under reduced-load conditions. This may be accomplished by a device attached to the expansion valve or a pilot control which varies the setting of the expansion valve by changing the pressure in the equaliser line. Either of these are actuated by a controller sensitive to load conditions and the compressor capacity control is from suction pressure. Since short-cycling of the compressor is apt to occur if the flow of refrigerant is reduced too much, some limit to prevent this is required. 3. Evaporator Pressure Control is accomplished by back pressure valves. These are placed in the suction line between the evaporator and the compressor, and maintain the evap orator pressure constant by regulating the amount of vapor drawn from the evaporator. The compressor capacity is con trolled from suction pressure at the compressor. The use of back pressure valves permits a lower pressure at the compres sor than would be permissible at the evaporator, thus mini mizing short-cycling. On a multiple-evaporator system served by one compressor, the evaporators may be maintained at different temperatures. Back pressure valves can be equipped with an air connection for automatically varying the evap orator pressure in accordance with the demands of a pneu matic controller. Means for mechanically readjusting the valves by use of a proportioning electric operator also are available. Condenser Control The majority of refrigeration systems, other than frac tional horsepower, use water-cooled rather than air-cooled condensers. Condenser control is used' for the purpose of conserving water or limiting the condenser pressure, or both. Solenoid water valves on small compressors, or pneu matic or electric valves on larger compressors, are usually controlled simultaneously with starting or stopping of the compressor. A pressure-operated valve will provide pro portional control of water flow in response to condenser pressure. Similar water valves controlled thermostatically by the temperature of water discharged from the condenser are sometimes used. Evaporative condensers may be con trolled by an automatic diamper responding to a condenserpressure controller. Cooling tower fans sometimes are started and stopped by a thermostat in the condensing water or a pressure controller in the condenser. Some form of freeze protection for evaporative condensers and cooling towers is necessary if they are required to operate in cold weather. Refrigeration Control for Air Conditioning When refrigerating equipment is used for space cooling, two major control problems exist: one is control of the temperature and the other, control of the humidity. In some applications the amount of latent heat to be removed is small compared with the sensible heat. In such cases, sufficient debumidification will usually occur without any special provisions. In other cases, such as theaters, where the latent load is relatively high, the air must be cooled below its dew-point temperature, and sometimes rewarmed to return it to the comfort range. Refer to Chapter 43 for general information on the subject of controls and for ap plications relating to air-conditioning systems. REFRIGERATION PIPING The pressure drop which occurs during passage of the. refrigerant through connecting piping is similar in effect to that which occurs through suction and discharge valves of the compressor. Thus, the effect of the pressure drop in the suction line between evaporator and compressor requires that a lower pressure be maintained intide the compressor during suction than is maintained in the evaporator. The pressure drop through the connecting piping between the compressor and condenser requires that a higher pressure be maintained inside the compressor during discharge than in the condenser. These losses result in a greater compres sion ratio, and therefore greater power requirements, as well as a lower volumetric efficiency and higher displace ment requirements. Pressure losses in the liquid line be tween condenser or receiver and the expansion valve may Refrigeration 559 Table 5 .... Dichlorodifluorocnethane* (CCJjFi) liquid lines. Tons Capacity per 100 Ft Equivalent Length line Size, Inches Pressure Drop per 100 Ft EqinVafenf leogtt, Psf 5 10 20 hod HOD HIPS H OD X IPS X IPS HOD 0.88 2.89 4.86 4.86 9.73 9.73 10.5 1.14 3.64 6.81 6.81 12.6 12.6 14.1 1.80 5.56 10.2 10.2 18.5 18.5 21.8 2.58 8.50 15.8 15.8 27.0 27.0 33.0 1 IPS 1H OD 1H IPS IH OD 1H IPS \% OD 21.4 21.4 36.9 36.9 62.0 62.0 28.2 28.2 48.1 48.1 80.2 80.2 41.3 41.3 ' 70.5 , 70.5 114. 114. 60.8 60.8 101. 101. 160. 160. 2 IPS 2H IPS 3 IPS 3H IPS 4 IPS 124. 161. 231.. 328. 230. 297. 426. 607. 364. 469. 676. 972. 539. 704. 1005. 1430. 753. 972. 1385. 1945. Note: Toonae rmlaea above tboae underlined eive velocatiea of 100 fpm or *Refricennt IS. result in some flashing of the liquid refrigerant, unless the liquid is subcooled. In all cases, friction losses should be kept to a minimum, and piping should be selected which will give the smallest loss consistent-with overall economy in the system. Refrigerant liquid lines from the receiver to the expansion valve should preferably be designed with a pressure drop of less than 5 pti, and with 10 psi as the maximum. A velocity of 100 to 250 fpm is recommended to prevent a pressure drop great enough to cause vaporization of the refrigerant ahead of the expansion valve. If the evaporator is to be located at a higher elevation than the condenser or receiver, account should be taken of the pressure drop for each foot of static liquid lift'. Approximate values are 026 pti per foot for ammonia, 057 pti per foot for dichlorodifiuoromethane, 051 psi per foot for monochlorodifluoromethane, and 0.64 pti per foot for monofluorotrichloromethane. Where there is a postibility of vaporization of some of the liquid before reaching the expansion valves, means for subcooling should be provided. Since a reduction of suction pressure at the compressor results in an appreciable reduction in capacity and more power input per ton of refrigeration, great care should be given to the proper sizing of suction lines between the evaporator and the compressor. Although comparatively high velocities, 500 to 5000 fpm, may be used, the optimum value will depend upon the refrigerant and the operatingpressure range. Since return of the oil to the compressor must be considered in the case of the fluorinated hydro carbons and methyl chloride, for these refrigerants the minimum velocity should be 500 fpm for horizontal runs and 1000 fpm for vertical runs. For the former, the **! design velocities range between 1000 and 2000 fpm. Too Table 6 .... Maximum Tons of Compressor Capacity for CGjFj Lines* (Onfy for tenpvfuftmi indicated) Section Uses Based on 105 f Condensing Temperature Discharge line* Inches M Pressure Drop per 100 Ft Equivalent length at 40 F Saturation * 2 345 Condenseig Temperature USE 90 f H OD X IPS H OD H IPS 0.14 0.17 0.25 0.35 0.20 0.24 0.35 0.45 0.28 0.34 0.51 0.65 0.35 0.42 0.62 0.79 0.41 0.49 0.73 0.93 0.45 0.54 0.81 1.03 1.43 1.87 1.15 1.50 H OD H IPS 1H OD IPS 0.55 0.68 1.28 1.43 0.76 0.94 1.80 2.01 1.10 1.35 2.67 2.89 1.34 1.65 3.17 3.54 1.58 1.92 3.76 4.17 1.75 2.12 4.15 4.60 2.97 3.26 5.05 5.29 2.38 2.62 4.05 4.25 m OD IX IPS IX OD lH IPS 2.21 2.70 3.40 4.05 3.12 3.82 4.78 5.75 4.45 5.37 6.79 8.10 5.50 6.38 7.05 7.72 6.19 6.72 7.6S 8.48 9.16 7.35 8.42 9.77 10.8 10.92 8.75 10.12 11.6 12.8 12.5 10.0 2X OD 2 IPS m OD 2M IPS 6.12 8.60 12.1 7.66 10.9 15.3 12.0 17.1 24.0 12.0 17.1 24.0 15.1 19.2 30.1 30.1 17.4 19.2 32.2 24.5 34.61 38.2 34.6 38.2 19.2 20.6 32.2 32.2 15.3 16.5 25.9 25.9 3M OD 3 IPS 3X OD 3H IPS 19.1 20.9 27.8 30.2 27.2 29.4 39.7 43.2 38.2 42.3 55.7 61.0 47.8 51.8 69.8 76.1 55.0 60.0 80.3 87.0 60.7 66.2 88.7 96.0 51.5 54.5 72.0 78.8 39.8 43.8 57.6 63.3 4M OD 4 IPS 5 IPS 6 IPS 38.6 55.2 78.0 97.3 ' 40.7 58.6 83.0 103 71.3 100 141 176 126 183 257 322 111 118 203 366 123 95.8 77.1 130 101.6 81.6 224 171.5 137.8 403 266 214 8 IPS 211 297 422 523 602 664 461 370 10 IPS 352 503 712 887 1024 1130 725 582 12 IPS 550 780 1106 1373 1582 1748 1041 836 *Bdncemat 13. high velocities create noise problems and excessive pressure drops. The total pressure drop in the suction line should be between one and two psi, if the velocity can be kept within the specified limits. Compressor discharge or hot gas lines may be designed with velocities from 1000 to 5000 fpm, except for dense gases such as carbon dioxide, where noise considerations will reduce the upper limit. A pressure drop of 2 to 4 pti is recommended for the discharge lines. Extensive tables are available in the literature for the determination of pressure drops through refrigerant lines with various refrigerants. The capacities listed in Tables 5, 6, and 7 are published in ARl Equipment Standards 520 (1946), of tire Air-Condi tioning and Refrigeration Institute, and are used by per mission. Table 5 shows the tonnage capacity normally allowed for CCLF* liquid lines per foot equivalent length of pipe, and Table 6, the maximum tonnage for suction