DBVs provide an economical method of compressor capacity control in place of cylinder unloaders or of handling unloading requirements below the last step of cylinder unloading.
On air-conditioning systems, the minimum allowable evaporating temperature that will avoid coil icing depends on evaporator design. The amount of air passing over the coil also determines the allowable evaporator minimum temperature. The refrigerant temperature may be below 32°F (0°C). However, coil icing will not usually occur with high air velocities, since the external-surface temperature of the tube will be above 32°F (0°C). For most air-conditioning systems the minimum evaporating temperature should be 26 to 28°F (?3.3 to ?2.2°C). DBVs are set in the factory. They start to open at an evaporating pressure equivalent to 32°F (0°C) saturation temperature. Therefore, evaporating temperature of 26°F (?3.3°C) is their rated capacity. However, since they are adjustable, these valves can be set to open at a higher evaporating temperature.
On refrigeration systems, discharge-bypass valves are used to prevent the suction pressure from going below the minimum value recommended by the compressor manufacturer. A typical application would be a low temperature compressor designed for operation at a minimum evaporating temperature on Refrigerant 22 of ?40°F (?40°C). The required evaporating temperature at normal load conditions is ?30°F (?34°C). A discharge-bypass valve would be selected that would start to open at the pressure equivalent to ?34°F (?36°C) and bypass enough hot gas at ?40°F (?40°C) to prevent a further decrease in suction pressure. Valve settings are according to manufacturer’s recommendations.
The discharge-bypass valve is applied in a branch line off the discharge line as close to the compressor as possible. The bypassed vapor can enter the low side at one of the following locations:
? To evaporator inlet with distributor
? To evaporator inlet without distributor
? To suction line
Figure 11-38 shows the bypass to evaporator inlet with a distributor. The primary advantage of this method is that the system thermostatic expansion valve will respond to the increased superheat of the vapor leaving the evaporator and will provide the liquid required for desuper heating. The evaporator also serves as an excellent mixing chamber for the bypassed hot gas and the liquid-vapor mixture from the expansion valve. This ensures that dry vapor reaches the compressor suction. Oil return from the evaporator is also improved, since the velocity in the evaporator is kept high by the hot gas.
Discharge-bypass valves (DBV) respond to changes in downstream or suction pressure (see Fig. 11-37). When the evaporating pressure is above the valve setting, the valve remains closed. As the suction pressure drops below the valve setting, the valve responds and begins to open. As with all modulating-type valves, the size of the opening is proportional to the change in the variable being controlled. In this case, the variable is the suction pressure. As the suction pressure drops, the valve opens further until the limit of the valve stroke is reached. However, on normal applications there is not sufficient pressure change to open these valves to the limit of their stroke. The amount of pressure change available from the point at which it is desired to have the valve closed to the point at which it is to be open varies widely with the refrigerant used and the evaporating temperature. For this reason, DBVs are rated on the basis of allowable evaporator temperature change from closed position to rated opening. A 6°F (3.3°C) change is considered normal for most applications and is the basis of capacity ratings.
On many air-conditioning and refrigeration systems it is desirable to limit the minimum evaporating pressure. This is so especially during periods of low load either to prevent coil icing or to avoid operating the compressor at lower suction pressure than it was designed for. Various methods of operation have been designed to achieve the result—integral cylinder unloading, gas engines with variable speed control, or multiple smaller systems. Compressor cylinder unloading is used extensively on larger systems. However, it is too costly on small equipment, usually 10 hp or below. Cycling the compressor with a low pressure-cutout control has had widespread usage, but is being reevaluated for three reasons:
? On-off control on air-conditioning systems is uncomfortable and does a poor job of humidity control.
? Compressor cycling reduces equipment life.
? In most cases, compressor cycling is uneconomical because of peak load demand charges.
One solution to the problem is to bypass a portion of the hot discharge gas directly into the low side. This is done by the modulating-control valve—commonly called a discharge-bypass valve (DBV). This valve, which opens on a decrease in suction pressure, can be set to maintain automatically a desired minimum evaporating pressure, regardless of the decrease in evaporator load.
The nonadjustable ORO head pressure-control valve and the ORD pressure-differential valve offer the most economical system of refrigerant side head-pressure control. Just as the ORI/ORD system simplified this type of control, the ORO/ORD system offers the capability of locating the condenser and receiver on the same elevation (see Fig. 11-36). By making these two valves available either separately or brazed together, there is added flexibility in the piping layout. The operation of the ORO/ORD system is such that a nearly constant receiver pressure is maintained for normal operation. As the temperature of the ORO element decreases, the pressure setting decreases accordingly. However, by running the bypassed hot gas through the ORO the element temperature is adequately maintained so the ORO/ORD system functions well to ambient temperatures of ?40°F (?40°C) and below. This third connection on the ORO also eliminates the need for a tee connection in the liquid-drain line.
Note that in Fig. 11-36 the ORO is located in the liquid-drain line between the condenser and the receiver, while the ORD is located in a hot-gas line bypassing the condenser. Other than the fact that the ORO operates in response to its outlet pressure (receiver pressure), the ORO/ORD operates in the same basic manner as the ORI/ORD system previously explained.
Excessive leak testing or operating pressures may damage these valves and reduce the life of the operating members. For leak detection, an inert dry gas such as nitrogen or carbon dioxide can be added to an idle system to supplement the refrigerant pressure. Remove the cap and adjust the adjustment screw with the proper wrench. Check the manufacturer’s recommended pressures before making adjustments.
Refrigerant and charging procedures require that enough refrigerant be available for flooding the condenser at the lowest expected ambient temperature. There must still be enough charge in the system for proper operation. Ashortage of refrigerant will cause hot gas to enter the liquid line and the expansion valve. Refrigeration will cease.
The receiver must have sufficient capacity to hold at least all of the excess liquid refrigerant in the system. This is because such refrigerant will be returned to the receiver when high-ambient conditions prevail. If the receiver is too small, liquid refrigerant will be held back in the condenser during high-ambient condition. Excessively high discharge pressures will be experienced.
Follow the manufacturer’s recommendations for charging the system. Procedures may vary with different valve manufacturers.
Any of the commonly used brazing alloys for high-side usage are satisfactory. It is very important that the internal parts be protected by wrapping the valve with a wet cloth to keep the body temperature below 250°F (121°C). Also, when using high-temperature solders, the torch tip should be large enough to avoid prolonged heating of the copper connections. Always direct the flame away from the valve body.
The ORD valve is a pressure differential valve. It responds to changes in the pressure difference across the valve (see Fig. 11-34). The valve designation stands for opens on rise of differential pressure. Therefore, the ORD is dependent on some other control valve or action for its operation. In this respect, it is used with either the ORI or ORO for head-pressure control.
As either the ORI or ORO valve starts to throttle the flow of liquid refrigerant from the condenser, a pressure differential is created across the ORD. When the differential reaches 20 psi, the ORD starts to open and bypasses hot gas to the liquid drain-line. As the differential increases, the ORD opens further until its full stroke is reached at a differential of 30 psi. Due to its function in the control of head pressure, the full stroke can be utilized in selecting the ORD. While the capacity of the ORD increases as the pressure differential increases, the rating point at 30 psi is considered a satisfactory maximum value.
The standard pressure setting for the ORD is 20 psig. For systems where the condenser pressure drop is higher than 10 or 12 psi, an ORD with a higher setting can be ordered.
Head-pressure control can be improved with an arrangement such as that shown in Fig. 11-35. In this operation, a constant receiver pressure is maintained for normal system operation. The ORI is adjustable over a nominal range of 100 to 225 psig. Thus, the desired pressure can be maintained for all of the commonly used refrigerants—12, 22, and 502 as a well as the latest alternatives.
The ORI is located in the liquid-drain line between the condenser and the receiver. The ORD is located in a hot-gas line bypassing the condenser. During periods of low ambient temperature, the condensing pressure falls until it approaches the setting of the ORI valve. The ORI then throttles, restricting the flow of liquid from the condenser. This causes refrigerant to back up in the condenser, thus reducing the active condenser surface. This raises the condensing pressure. Since it is really receiver pressure that needs to be maintained, the bypass line with the ORD is required.
The ORD opens after the ORI has offered enough restriction to cause the differential between condensing pressure and receiver pressure to exceed 20 psi. The hot gas flowing through the ORD heats up the cold liquid being passed through the ORI. Thus, the liquid reaches the receiver warm and with sufficient pressure to assure proper expansion-valve operation. As long as sufficient refrigerant charge is in the system, the two valves modulate the flow automatically to maintain proper receiver pressure regardless of outside ambient temperature.
The ORO head pressure-control valve is an outlet pressure–regulating valve that responds to changes in receiver pressure. The valve designation stands for opens on rise of outlet pressure (see Fig. 11-33). The inlet and outlet pressures are exerted on the underside of the seat disc in an opening direction. Since the area of the port is small in relationship to the diaphragm area, the inlet pressure has little direct effect on the operation of the valve. The outlet or receiver pressure is the control pressure. The force on top of the diaphragm that opposes the control pressure is due to the air charge in the element. These two forces are the operating forces of the ORO.
When the outdoor ambient temperature changes, the condensing pressure changes. This causes the receiver pressure to fluctuate accordingly. As the receiver pressure decreases, the ORO throttles the flow of liquid from the condenser. As the receiver pressure increases, the valve modulates in an opening direction to maintain a nearly constant pressure
in the receiver. Since the ambient temperature of the element affects the valve pressure setting, the control pressure may change slightly when the ambient temperature changes. However, the valve and element temperature remain fairly constant.
The ORI head pressure-control valve is an inlet pressure regulating valve. It responds to changes in condensing pressure only. The valve designation stands for opens on rise of inlet pressure. As shown in Fig. 11-32, the outlet 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 outlet pressure cancels out. The inlet pressure acting on the bottom of the seat disc opposes the adjusting spring force. These two forces are the operating forces of the ORI.
When the outdoor ambient temperature changes. the ORI opens or closes in response to the change in condensing pressure. An increase in inlet pressure above the valve setting tends to open the valve. If the ambient temperature drops, the condenser capacity is increased and the condensing pressure drops off. This causes the ORI to start to close or assume a throttling position.
Design of air-conditioning and refrigeration systems using air-cooled condensing units involves two main problems that must be solved if the system is to be operated reliably and economically. These problems are high-ambient and low-ambient operation. If the condensing unit is properly sized, it will operate satisfactorily during extreme-ambient temperatures. However, most units will be required to operate at ambient temperatures below their design dry-bulb temperature during most of the year. Thus, the solution to low-ambient operation is more complex.
Without good head-pressure control during low-ambient operation, the system can have running-cycle and off-cycle problems. Two runningcycle problems are of prime concern:
? The pressure differential across the thermostatic-expansion valve port affects the rate of refrigerant flow. Thus, low head pressure generally causes insufficient refrigerant to be fed to the evaporator.
? Any system using hot gas for defrost or compressor-capacity control must have a normal head pressure to operate properly. In either case, failure to have sufficient head pressure will result in low suction pressure and/or iced evaporator coils.
The primary off-cycle problem is the possible inability to get the system on-the-line if the refrigerant has migrated to the condenser. The evaporator pressure may not build up to the cut-in point of the low pressure control. The compressor cannot start, even though refrigeration is required. Even if the evaporator pressure builds up to the cut-in setting, insufficient flow through the TEV will cause a low suction pressure, which results in compressor cycling.
There are nonadjustable and adjustable methods of head-pressure control by valves. Each method uses two valves designed specifically for this type of application. Low-ambient conditions are encountered during fall winter-spring operation on air-cooled systems, with the resultant drop in condensing pressure. Then, the valve’s purpose is to hold back enough of the condensed liquid refrigerant to make part of the condenser surface inactive. This reduction of active condensing surface raises condensing pressure and sufficient liquid-line pressure for normal system operation.