Tuesday, December 2, 2014

Other system components

Other system components

This chapter describes the function and operation of a number of components not previously covered in detail.

Solenoid valve

This is an electrically operated magnetic valve. It consists of a coil wound around a sleeve of non-magnetic substance. When energized, the coil carries an electrical current and becomes a magnet.

Inside the sleeve is a movable iron core, which will be attracted by the magnetic field of force to remain suspended in a midway position inside the sleeve. The iron core movement opens the valve. When the coil is de-energized, the iron core returns to its normal position to close the valve.

This type is normally closed. Others are available which are normally open; these close when the coil is energized.

Some solenoid valves have flare fitting unions, whilst others are brazed into position. It is important that the coil is removed when brazed joints are made. For optimum operation the valve should be installed in an upright position and rigidly mounted; the valve should never rely on the system pipework for support.

There are many uses for this type of valve, such as in defrost systems, multiple evaporator systems and pump-down cycles. In effect, it is used wher- ever it is necessary to stop the flow of refrigerant to a specific section of pipework on a particular application.

Crankcase heater

This is a resistance heater. If externally mounted it is located on the underside of the compressor crankcase. It may be mounted internally as an integral part of the compressor; this is standard for larger commercial equipment.

The presence of liquid refrigerant in a compressor crankcase is wholly undesirable. It could cause dilution of the lubricating oil, resulting in poor

lubrication. Liquid refrigerant vaporizing in a crankcase will cause foaming when the compressor operates after an off cycle. Excess oil will then be discharged by the compressor; this could drastically reduce the amount of oil in the crankcase. Non-condensible slugs of oil and liquid refrigerant could enter the compressor cylinders to cause considerable damage to valves, pistons and connecting rods, and even to fracture crankshafts.

Crankcase heaters are essential for compressors installed for low tempera- ture applications, where evaporating temperatures and crankcase temperatures can be extremely low. They are also necessary for remote installations where the compressor is exposed to low ambient temperatures (winter conditions). Whenever the crankcase temperature falls below that of the evaporator, refrig- erant vapour will migrate and condense in the crankcase unless a heater is employed to maintain a temperature in the crankcase above the temperature of the refrigerant vapour.

Because of the tendency of oil to absorb miscible refrigerant, a certain amount of refrigerant will always be present in the crankcase.

Check valve

This is sometimes called a non-return valve. It is a simple device used to ensure that fluid or vapour can only travel in one direction and not back up to another part of the system pipework. Check valves have been mentioned in connection with oil separators (Figure 75), multiple evaporators (Figure 84) and hot gas defrosting (Figures 86a and 86b).

Figure 87 shows the construction of the valve, and the locations in a multiple evaporator system.

Sight glass

Two distinct types are commonly used: the clear liquid indicating, and the moisture indicating (Figure 88). Many designs are available, but the common function is to indicate levels and conditions of fluids in the system.

They should be installed close to the liquid receiver. They may be down- stream or upstream of the filter drier, but are generally upstream.

Where long liquid line runs or high liquid line risers are necessary, it is advisable to install an extra sight glass immediately before the thermostatic expansion valve. This will indicate the presence of bubbles, possibly due to pressure drop in the liquid line, thereby creating a shortage of refrigerant to the evaporator.

imageFilter drier

This is installed in the liquid line of the system after the receiver. Construction is generally in the form of a tube which contains coarse and fine mesh filters. These prevent foreign matter such as dirt, metal filings and carbon sludge circulating with the refrigerant. The tube also contains a drying agent or dessicant which will absorb any moisture in the refrigerant (see Figure 89).

A burn-out drier is specifically intended for installation in both the liquid line and the suction line of a system following the replacement of a hermetic or semi-hermetic compressor in which the motor windings have burnt out (see

imageChapter 6). This type of filter drier has the extra ability to retain acids which could be present in the oil residue entrained in parts of the system.

Oil pressure failure switch

This is generally used with compressors incorporating oil pumps and on multiple compressor systems. The function of the control is to stop the compressor(s) when the oil pressure developed by the pump falls below a specific level, or if the oil pressure fails to reach a maximum safe level within a desired period after starting.

The oil pressure, as measured with a gauge, is the sum of the crankcase pressure (suction) and the pressure developed by the pump. The failure switch should be set to operate at the ‘useful’ pressure, and not at the total pressure. To determine the useful pressure (assuming correct compressor lubrication), subtract the suction pressure from the total pressure. Since the oil pump functions only when the compressor operates, the total pressure will be equal to the crankcase pressure during off cycles.

When the compressor starts, the oil pressure rises to the cut-in point of the switch. The differential switch will open and break the circuit to the heater, and the compressor will operate normally (see Figure 90).

If the useful pressure does not rise to the cut-in point within the time limit (60 to 180 seconds), the differential switch contacts will not open and the

imageheater will stay in circuit. This causes a bimetal strip in the timing relay to warp and open the timing contacts, which will break the circuit to the starter coil and stop the compressor. Similarly, if the useful pressure falls below the cut-in point during operation, the differential switch will close and energize the heater. The timing relay will stop the compressor after the time delay characteristic of the switch.

Controls are available with 60 and 90 seconds delay, but it must be realized that the time is not variable.

Most current production oil pressure failure switches are provided with terminals for the connection to a crankcase heater. Since the terminal arrangements vary, reference should be made to the wiring diagram provided with the switch.