Document da4kQ65nGwJV2e5g4J3eVDzEe

758 CHAPTER 70 dicate whether or not a system has excessive moisture in it. They are valuable additions to the system as a positive in dication of when the drier cartridge should be replaced. Strainers Strainers must be used in all refrigerant systems, in both the liquid and suction lines. Their function is to protect all automatic valves and the compressor from foreign material in the system, such as pipe and welding scale, rust, metal chips, etc. The strainer should always be mounted in a horizontal line with the flanged end down so that the screen can be replaced without loose particles falling into the system. The installation of a liquid line strainer before each auto matic valve is recommended to prevent particles from lodging on the valve seats. Where multiple expansion valves with internal strainers are used at one location, a tangle main liquid line strainer will be sufficient to 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. Refrigeration compressors are usually equipped with a built-in suction strainer which is adequate for the usual system with copper piping. Very often the compressor is shipped with a felt sock, or other temporary liner, in the suction strainer. This sock is effective in straining out finer particles thn is possible with the standard suction screen and is left in for the first 50 hr of operation. At the end of that fcim*> it should be re moved and inspected. If it has collected foreign material it should be cleaned and replaced for another 50 hr operation. This procedure should be repeated until it shows cWn, at which time it should be removed permanently. The sock should not be forgotten and left in permanently NyAm it causes a substantial pressure drop. Care should be taken to pipe tire suction line at the com pressor so that the built-in strainer is accessible for servicing. Both the liquid and suction line strainers should be ade quately sized to insure adequate foreign material storage capacity without excessive pressure drop. In steel piping systems an external suction I'm* strainer is recommended in addition to the compressor strainer. Liquid Indicators Every refrigeration system should have a nwma of check ing for sufficient refrigerant charge. The common devices ngd are liquid line sight glass, liquid level test cock on condenser or receiver and an external gage glass with equalizing con nections and shut-off valves. 1962 Guide And Data Book The liquid line right glass is one of the most convenient to install and use. A properly installpH sight gla*a will show bubbling when there is insufficient charge and a solid dear glass when there is sufficient charge. The preferred location for checking sufficient refrigerant charge in the system is in the liquid line as close as prvg^hlc to the receiver outlet, or condenser outlet if no receiver is The sight glass is best installed in a vertical section of the line a sufficient distance downstream from any valve so that the resulting disturbance does not appear in the glass: If it is in stalled too far away from the receiver, there may be pressure drop in the line to cause ftwhing and bubbles in the glass even though there is sufficient charge for a liquid seal at the receiver outlet. It often happens, with sight glaw^ stalled near the evaporator, that no amount of overcharging of the system will give a solid liquid condition at the right glass, because of pressure drop in the liquid line or lift. Subcooling is required here. It may also be desirable to have an additional right gl^ just before the solenoid valve near tire evaporator, to rh^fc on the condition of the refrigerant at this point. Sight glasses should be installed full size in the main liq^ line, not in a bypass line that parallels the main lino A sight glass with double ports and seal caps is preferable The double ports allow a light to be put behind one port so that the state of the refrigerant is easily seen. The serve as an added protection against leakage since they are removed only when checking the refrigerant. Moisture indicators are now appearing on the market large enough to be installed directly in the liquid line, in mnri * thus serving the dual purpose of liquid line right gi*** and moisture indicator. ' Many condensers and receivers are equipped with llguM level test cocks or connections for these devices, right on the shell as indicated in Fig. 43. The test cock is located at a height in the shell just below where the minimum operating liquid level should be. Opening of this cock while the system is Hg. 43 .... Condenser with Liquid Level Test and Purge Cocks* Refrigerant Piping Systems for Refrigerants 12, 22 and 500 759 io operation will show a jet of liquid refrigerant if the system Ma sufficient charge, but only gas if there is an insufficient cjjargC_ if a liquid level test cock is used, a right glass in the finaid hue leaving the condenser or receiver is unnecessary although it may be installed for convenience. Refrigerant Charging Connection The two usual methods of charging the refrigeration sys tem are: I Charging into the liquid line between the receiver shut off Valve and the expansion valve as shown in Pig. 44. The advantages of thismethod are: (a) dumping liquid into the eomut[rrr is prevented, (6) the system can be charged rapidly sice the refrigerant is charged as a liquid and does not have to be evaporated from the drum, and (c) the charging connection is convenient to the liquid test cock or liquid line right glass. To prevent the impurities and moisture in the drum being drawn into the system, the practice of charging through a filter and dehydrator should be adopted. 2. Charging into the suction line. Except for very small systems, thiii method is not practical because of the time re quired to evaporate the refrigerant from the drum and because the danger of dumping raw liquid into the compressor. HEAD PRESSURE CONTROL FOR REFRIGERANT CONDBvISERS Id applying refrigerant condensers, it is well to take ad vantage of lower than design condensing temperatures when ever posable to obtain more refrigeration capacity from the system, lower brake bp requirements per ton of refrigeration, and lower compressor discharge temperatures. At the same time, it is often necessary to take precautions that the condensing pressure does not drop too low during periods of low water or outside air temperatures. This is par ticularly true for applications that have high internal or process loads throughout the year, and also systems having compressor capacity control. The principal problem arising from abnormally low oondenring pressures is the reduction of (he pressure difference available across the expansion valve to the point where it is unable to feed sufficient refrigerant to the evaporator to handle the cooling load. liquid feed devices are rated in accordance with the pres sure difference across the port. A valve selected at a pressure difference of 60 lb, the standard rating point for Refrigerant 12 valves, would have substantially less capacity if the avail able pressure difference were only 20 lb. Water-Cooled Condensers With water-cooled condensers, head pressure controls are used for the dual purpose of maintaining, the condensing . presure and conserving water. On cooling tower applications, they are used only where it is necessary to maintain con densing temperatures. Condenser Water Regulating Valves The control commonly used with water-cooled condensers is the condenser water regulating valve. This valve is usually located in the supply water line to the condenser. It is a throttling type valve and responds to a difference between condensing pressure and a pre-determined shut-off pressure. The higher this difference, the more the valve opens up, and vice versa. A pilot connection to the top of the condenser transmits condensing pressure to the valve which assumes an opening position according to this pressure. The shut-off pressure of the valve must be set slightly higher than (he highest refrigerant pressure expected in the condenser when the system is not in operation. This is to make sure the valve will close and not pass water during off cycles. This pressure will be slightly higher than the saturation pressure of the refrigerant at the highest ambient temperature ex pected around the condenser. Condenser water regulating valves are usually sized to pass the design quantity of water at about a 25 to 30 lb difference between design condensing pressure and valve shut-off pressure. Water Bypass In cooling tower applications a simple bypass *n also be used to maintain condensing temperature, employing a man ual valve or an automatic valve responsive to head pressure change. Hie valve can be either the two-way or three-way type. Evaporative Condensers With evaporative condensers, various methods are em ployed for condensing pressure control: (1) cycling spray pump motor, (2) cycling both fan and spray pump motors, (3) throttling spray water, (4) air bypassing duct and dam pers, and (5) air throttling dampers, either on inlet or dis charge. The water pump cycling method employs a pressure con trol at the gas inlet which starts and stops the pump in re sponse to changes in the head pressure. As the head pressure increases, the pump starts and sprays water over the con denser coils. As the head pressure drops, the pump stops and the unit becomes an air-cooled condenser. The main disadvantage of this system is that it is difficult to maintain a constant head pressure for as soon as the pump stops the pressure goes up and the pump starts again. This short cycling occurs because the coil, not having an extended surface, has insufficient capacity when operating as an air cooled condenser. Short cycling results in excessive deposits of mineral and scale on the water side of the tubes and f-lm decreases the life of the water pump and its motor and starter. An additional disadvantage is that the sprayed water may freeze on the coil tubes when the inlet air temperature is be low 20 F. One method of regulating head pressure is to control the amount of air admitted by the damper (Fig. 45). The modulating head pressure control located on the in coming hot gas line operates the damper motor at the inteC" air duct. As the head pressure begins to rise, the damper opens and allows increased air flow over the condenser; as the head pressure falls, the pressure control causes the motor to dose the damper restricting air flow over the condenser until the desired pressure is restored.