Document VjZEX4Z9x4g1m2RMjXrrVqDgg

576 CHAPTER 38 I960 Guide adjusted spring, balanced against the bellows, and these two forces operate to maintain a constant pressure in the evapora tor by increasing or decreasing the flow of liquid refrigerant. Such an expansion valve is usually applied to evaporators of the direct-expansion type, but is not satisfactory for fluctuat ing loads such as are encountered in air-coudiliouing instal lations. Thermostatic Expansion Valves. A thermostatic expansion valve controls the flow of liquid refrigerant to the evaporator so as to maintain the entire coil filled with evaporating re frigerant, and to keep a constant superheat in the refrigerant gas leaving the coil. The construction of such a valve is shown m Fig. IS and is similar to that for an automatic expansion valve out incorporates, in addition, a power element responsive to changes in the degree of superheat of the refrigerant gas leaving the coil. This power element consists of a bellows con nected by means of a capillary tube to a feeler bulb fastened to the suction line from the evaporator. The bulb, bellows, and tube are usually charged with the same liquid refrigerant used in the evaporator itself. A starved condition in the evap orator results m a greater superheat in the gas leaving the evaporator, and this in turn operates through the power ele- Fig. 15 .... Typical Thermostatic Expansion Valve 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 filled with re frigerant at all times. Low-Side Float Valves. A liquid refrigerant control of the low-side 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 in 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 of 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 Tubes. 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 th* liquid refrigerant, it has the disadvantage that no modifica tions are possible to a<tiust 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 ana commercial units. Refrigerant Flow 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. Id most cases it is controlled by a thermostat re sponding to the load. Solenoid liquid valves arc widely used for control of refrigerant flow 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 often employed. The thermostat closes the solenoid liquid valve and the com pressor then pumps down until stopped by the low-suction pressure switch. The compressor canoot restart until the thermostat again opens the solenoid liquid valve. 2. Expansion Valve Modulation. This form of flow control utilizes 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 equalizer 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 mising 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 tions! 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 damper responding to a condenser- pressure 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 Refrigeration 577 Table 5 .... DicWorodiftuoromethane* (CGjFi) liquid Lines, Tons Capacity per 100 Ft Equivalent Length Im Sam, Inches Pmw* Drop per 100 Ft Equivalent length, P 3 S 10 20 HOD HOD HIPS HOD H IPS KIPS KOD 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 IHOD IK IPS 1H OD lH 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 ITS 4 IPS 124. 161. 231. 328. 230. 297. 426. 607. 364. 469. 676. 972. 539. 704. 1005. 1430. 753. 972. 1385. 1945. Note'. Tongaze vslaee tbove tbcee underlined jive vekete at 300 1pm or `Kefrizeruot IS. 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 dehumidification 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 rewanned 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 iu the suction line between evaporator and compressor requires that a lower pressure be maintained inside 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 made 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- Toble 6 .... Maximum Tom of Compressor Capacity for CCliFj Lines* (O.-Jy for temperature* inditafoal Uam Size, Inchci Suction liner Bated on 10S F Condensing Temperature Psi Pressure Drop per 100 Ft Equivalent length of 40 F Saturation Discharge liner Condensing Temperature i 2 3 4 5 115 F 90 F H OD 0.14 0.20 0.28 0.35 0.41 0.45 % IPS 0.17 0.24 0.34 0.42 0.49 0.54 H OD 0.25 0.35 0.51 0.62 0.73 0.81 1.43 1.15 H IPS 0.35 0.45 0.65 0.79 0.93 1.03 1.87 1.50 H OD H IPS iM OD l IPS 0.55 0.68 1.26 1.43 0.76 0.94 1.80 2.01 1.10 1.35 2.57 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 IK IPS m OD IK 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 7.68 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 oQ 2K 2 IPS 2H OD 2K 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 32.2 34.6 34.6 19.2 24.5 38.2 38.2 19.2 20.6 32.2 32.2 15.3 16.5 25.9 25.9 3K OD 3 IPS 3K OD 3K 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 4K 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 * KefTiz*rnt U. tween condenser or receiver and the expansion valve may 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 psi, 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 0.26 psi per foot for ammonia, 0.57 psi per foot for dichlorodifluoromethane, 051 psi per foot for monochloro-