As with other pressure valves, it is possible to introduce nitrogen or carbon dioxide in an idle system to check for correct pressure settings.
The usual precautions for working with gases apply here. The standard factory setting for the 0/50 psig range is 30 psi. For the 30/100 psig range, it is 60 psig. Since these valves are adjustable, the setting may be altered to suit the system.
The main purpose of an OBIT valve is to keep the evaporator pressure above some given point at minimum-load conditions. The valves are selected on the basis of the pressure drop at full-load conditions. Nevertheless, they should be adjusted to maintain the minimum allowable evaporator pressure under the actual minimum-load conditions.
These valves can be adjusted by removing the cap and turning the adjustment screw with a hex wrench of the proper size. A clockwise rotation increases the valve setting, while a counterclockwise rotation decreases the setting. To obtain the desired setting, a pressure gage should be utilized on the inlet side of the valve. Thus. the effects of any adjustments can be observed.
When these valves are installed in parallel, each should be adjusted the same amount. If one valve has been adjusted more than the other, the best performance will occur if both are adjusted all the way in before resetting them to an equal amount.
ORITs must be installed upstream of any other suction-line controls or accessories. These valves may be installed in the position most suited to the application. However, these valves should be located so that they do not act as an oil trap or so that solder cannot run into the internal parts during brazing in the suction line. Since these valves are hermetic, they cannot be disassembled to remove solder trapped in the internal parts. Installation of a filter-drier and a strainer may be worth the expense to keep the system clean and operational. Brazing procedures are the same as for other valves of this type. The valve core of the access valve is shipped in an envelope attached to the access valve. If the access valve connection is to be used as a reusable pressure tap to check the valve setting, the OBIT must be brazed in before the core is installed. This protects the synthetic material of the core. If the access valve is to be used as a permanent pressure tap, the core and access valve cap may be discarded.
For any pressure sensitive valve to modulate to a more closed or open position, a change in operating pressure is required. The unit change in the valve stroke for a given change in the operating pressure is called the valve gradient. Every valve has a specific gradient designed into it for the best possible operation. Valve sensitivity and the valve’s capacity rating are functions of the valve gradient. Thus, a relatively sensitive valve is needed when a great change in the evaporating temperature cannot be tolerated. Therefore, the valves have nominal ratings based on the 8-psi evaporator-pressure change, rather than a full stroke.
Evaporator pressure-regulator valves respond only to variations in their inlet pressure (evaporator pressure). Thus, the designation for evaporator pressure-regulating valves is ORI (opens on the rise of the inlet pressure) (see Fig. 11-29).
Pressure at the outlet is exerted on the underside of the bellows and on top of the seat disc. The effective area of the bellows is equal to the area of the port. Thus, the outlet pressure cancels out and the inlet pressure acting on the bottom of the seat disc opposes the adjustable spring force. These two forces are the operating forces of the ORIT. (The “T” added to the valve designation indicates an access valve on the inlet connection.) When the evaporator load changes, the ORIT opens or closes in response to the change in evaporator pressure. An increase in inlet pressure above the valve setting tends to open the valves. If the load drops, less refrigerant is boiled off in the evaporator and evaporator pressure will decrease. The decrease in evaporator pressure tends to move the ORIT to a more closed position. This, in turn, keeps the evaporator pressure up. The result is that the evaporator pressure changes as the load changes. The operation of a valve of this type is improved by an antichatter device built into the valve. Without this device, the OBIT would be susceptible to compressor pulsations that can reduce the life of a bellows. This antichatter feature allows the ORIT to function at low load conditions without chattering or other operating difficulties.
Evaporator pressure-regulating valves offer an efficient means of balancing the system capacity and the load requirements during periods of low loads. They are also able to maintain different evaporator conditions on multi temperature systems. The main function of this valve is to prevent the evaporator pressure from falling below a predetermined value at which the valve has been set.
Control of evaporator pressure by cycling the compressor with a thermostat or some other method is quite adequate on most refrigeration systems. Control of the evaporator pressure also controls the saturation temperature. As the load drops off, the evaporating pressure starts to decrease and the system performance falls off. These valves automatically throttle the vapor flow from the evaporator. This maintains the desired minimum evaporator pressure. As the load increases, the evaporating pressure will increase above the valve setting and the valve will open further.
Since CRO valves are hermetic and cannot be disassembled for inspection and cleaning, they are usually replaced if inoperative. If a CRO fails to open, close properly, or will not adjust, solder or other foreign material is probably lodged in the port. It is sometimes possible to dislodge these materials by turning the adjustment nut all the way in, with the system running. If the CRO develops a refrigerant leak around the spring housing, it probably has been overheated during installation or the bellows has failed due to severe compressor pulsations. In either case, the valve must be replaced.
The standard setting by the factory for CROs in the 0/60 psig range is 30 psig. Since these valves are adjustable, the setting may be altered to suit the specific system requirements. CROs should be adjusted at startup when the pressure in the evaporator is above the desired setting. The final valve setting should be below the maximum suction pressure recommended by the compressor or unit manufacturer.
The main purpose of the CRO is to prevent the compressor motor from overloading due to high suction pressure. Thus, it is important to arrive at the correct pressure setting. The best way to see if the motor is overloaded is to check the current draw at start-up or after a defrost cycle. If overloading is evident, a suction gage should be put on the compressor. The CRO setting may be too high and may have to be adjusted. If the compressor is overloaded and the CRO valve is to be reset, the following procedure should be followed.
The unit should be shut off long enough for the system pressure to equalize. Observe the suction pressure as the unit is started, since this is the pressure the valve is controlling. If the setting is to be decreased, slowly adjust the valve in a counterclockwise direction approximately one-quarter turn for each 1 psi pressure change required. After a few moments of operation, the unit should be cycled off and the system pressure allowed to equalize again. Observe the suction pressure (valve setting) as the unit is started up. If the setting is still too high, the adjustment should be repeated. The proper size hex wrench is used to adjust these valves. A clockwise rotation increases the valve setting, while a counterclockwise rotation decreases the setting.
When CROs are installed in parallel, each should be adjusted the same amount. If one valve has been adjusted more than the other, best performance will occur if both are adjusted all the way in before resetting them an equal amount.
As Fig. 11-27 indicates, the CRO valve is applied in the suction line between the evaporator and the compressor. Normally, the CRO is installed downstream of any other controls or accessories. However, on some applications it may be advisable or necessary to locate other system components, such as an accumulator, downstream of the CRO. This is satisfactory as long as the CRO valve is applied only as a CRO valve. CRO valves are designed for application in the suction line only. They should not be applied in hot-gas bypass lines or any other refrigerant line of a system.
Crankcase pressure-regulating valves are designed to prevent overloading of the compressor motor. They limit the crankcase pressure during and after a defrost cycle or after a normal shutdown period. When properly installed in the suction line, these valves automatically throttle the vapor flow from the evaporator until the compressor can handle the load. They are available in the range of 0 to 60 psig.
Crankcase pressure-regulating valves (CROs) are sometimes called suction pressure-regulating valves. They are sensitive only to their outlet pressure. This would be the compressor crankcase or suction pressure. To indicate this trait, the designation describes the operation as close on rise of outlet pressure (CRO). As shown in Fig. 11-26, the inlet pressure is exerted on the underside of the bellows and on top of the seat disc. Since the effective area of the bellows is equal to the area of the port, the inlet pressure cancels out and does not affect valve operation. The valve outlet
pressure acting on the bottom of the disc exerts a force in the closing direction. This force is opposed by the adjustable spring force. These are the operating forces of the CRO. The CROs pressure setting is determined by the spring force. Thus, by increasing the spring force, the valve setting or the pressure at which the valve will close is increased.
As long as the valve outlet pressure is greater than the valve pressure setting, the valve will remain closed. As the outlet pressure is reduced, the valve will open and pass refrigerant vapor into the compressor. Further reduction of the outlet pressure will allow the valve to open to its rated position, where the rated pressure drop will exist across the
valve port. An increase in the outlet pressure will cause the valve to throttle until the pressure setting is reached.
The operation of a valve of this type is improved by an anti chatter device built into the valve. Without this device, the CRO would be susceptible to compressor pulsations that greatly reduce the life of a bellows. This feature allows the CRO to function at low-load conditions without any chattering or other operational difficulties.
A float valve, either high-side or low-side, can serve as a metering device. The high-side float, located in the liquid line, allows the liquid to flow into the low side when a sufficient amount of refrigerant has been condensed to move the float ball. No liquid remains in the receiver. A charge of refrigerant just sufficient to fill the coils is put into the system on installation. This type of float, formerly used extensively, is now limited to use in certain types of industrial and commercial systems.
The low-side float valve keeps the liquid level constant in the evaporator. It is used in flooded-type evaporators where the medium being cooled flows through tubes in a bath of refrigerant. The low-side float is more critical in operation than the high-side float and must be manufactured more precisely. A malfunction will cause the evaporator to fill during shutdown. This condition will result in serious pounding and probable compressor trouble on start-up.
Needle valves, either diaphragm or packed type, may be used as hand expansion valves. As such, they are usually installed in a bypass line around an automatic- or thermostatic-expansion valve. They are placed in operation when the normal control is out of order or is removed for repairs.
Small-bore capillary tubing is used as a metering device. It is used on everything from the household refrigerator to the heat pump. Essentially, it is a carefully measured length of very small diameter tubing. It creates a predetermined pressure drop in the system. The capillary has no moving parts.
Because a capillary tube cannot stop the flow of refrigerant when the condensing unit stops, such a refrigeration unit will always equalize high-side and low-side pressures on the off cycle. For this reason, it is important that the refrigerant charge be of such a quantity that it can be held on the low side of the system without damage to the compressor. In a charge of several pounds, this “critical charge” of refrigerant may have to be carefully weighed.
An accumulator, or enlarged chamber, is frequently provided on a capillary tube system to prevent slugs of liquid refrigerant from being carried into the suction line.