Maintaining Head Pressure Control
In part 1 of this article, on the fundamentals of head pressure control ("Fundamentals of Head Pressure Control," Nov. 1), we made the case that reducing head pressure lowers the operating expense of the compressor. However, it was also shown that reducing it too much can adversely affect the health and longevity of the system.
We can conclude that the determining factor in deciding what the minimum allowable head pressure should be is the minimum thermostatic expansion valve (TEV) pressure drop (delta-P) required for its capacity to meet the demands of the evaporator load. Once that has been determined, it becomes a simple matter of adjusting the head pressure controls on the system to maintain that minimum.
There are several different methods of maintaining head pressure, and it is important to understand the principles of how each one operates so they can be set correctly. In addition, on systems that utilize more than one method of head pressure control, setting them to operate together is imperative.
The common methods of head pressure control are:
Fan CyclingThe capacity of the condenser mentioned in the first article, rated at 150,000 Btuh (at 110 degrees F and a 10 degree temperature difference [TD]), is based on all of the fans operating. If this is an eight-fan condenser, we can reduce the condenser capacity as needed by cycling off the fans.
Typically, fan motors 1 and 2 would be controlled by a pressure control, fan motors 3 and 4 by another pressure control, etc. This would allow for four stages of fan cycling. A typical pressure control setup is shown in Table 1. Excluding a cold, windy day, where the wind is able to blow through the tube bundle on the condenser, the head pressure would never fall below 160 psig with this control strategy.
While this is a simple method of control, it does have a few disadvantages. For example, on a cool day, when the head pressure reaches 170 psig, pressure control No. 2 will open the control circuit for the contactors powering fan motors No. 3 and 4.
Once fan motors No. 3 and 4 cycle off, the head pressure will gradually start to rise, as the reduction in airflow has reduced the condenser capacity. The corresponding refrigerant saturation temperature will also rise. In the receiver, where liquid and vapor are present, the refrigerant always will be at a saturated condition. (For example, R-404A is 78 degrees at 170 psig.)
For simplicity, we will assume the pressure in the receiver is equal to the head pressure. In an operating system, there would be some pressure loss in the piping and flow controls, resulting in a receiver pressure that would be less than the head pressure.
When the switch in pressure control No. 2 closes at 190 psig, the temperature of the refrigerant in the receiver will be 85 degrees (saturation temperature at 190 psig).
With the second bank of fan motors operating, the head pressure will fall rapidly, as will the corresponding saturation temperature. The temperature reduction of saturated refrigerant in the receiver is accomplished by liquid flashing into vapor.
As a portion of the liquid flashes, it absorbs enough heat from the remaining liquid to lower its temperature to the new saturation condition. Depending on the refrigerant level in the receiver, and how quickly the pressure falls, the ability to provide vapor-free refrigerant to the liquid header may be compromised temporarily.
If there is little or no subcooling in the liquid line, flashing may occur all the way to the TEV inlet. This temporary disruption of vapor-free refrigerant to the TEV results in erratic operation and poor superheat control.
In addition, as the pressure controls cycle the fan motors, and pressure fluctuates between cut-ins and cut-outs, the available CP across the TEV port will vary. In a perfect world, the condition of the refrigerant at the TEV inlet would be constant year-around. Allowing the head pressure to fluctuate up and down every few minutes will result in TEV capacities that proportionally fluctuate.
Condenser FloodingThe ability to maintain constant head pressure (liquid pressure) during varying periods of low ambient operation would be ideal. One method of achieving this is to use condenser flooding valves. In larger systems, two valves are required. (See Figure 1.)
The first, commonly referred to as the condenser holdback valve, is installed at the outlet of the condenser. Its function is to maintain a constant pressure in the condenser.
The ORIT valve is normally closed, and opens on a rise of inlet pressure. If the ORIT valve was set to maintain 180 psig, it would simply remain closed until the condenser pressure increased to that level.
While the ORIT valve is closed, the compressor continues to pump refrigerant into the condenser. As heat is removed from the superheated discharge vapor, it will start to condense into a liquid and the liquid refrigerant will start backing up from the inlet of the closed ORIT valve, flooding a portion of the condenser.
The portion of the condenser that is full of liquid refrigerant (flooded) no longer serves as a condenser. Flooding a portion of the condenser reduces its effective condensing surface, and therefore its capacity. When the appropriate amount of condenser flooding has occurred, the reduced condenser capacity will cause the pressure to increase to 180 psig. At this point, the ORIT will begin to open and allow refrigerant to flow into the receiver.
Pressure regulating valves can control either upstream pressure or downstream pressure, but not both. When the ORIT throttles, maintaining constant condensing pressure by flooding the condenser, it does so at the expense of its outlet pressure (receiver pressure).
The ORIT valve may influence receiver pressure, but it cannot maintain it at a constant level. Without an additional regulating valve, the pressure in the receiver will be erratic during periods of low ambient operation due to the ORIT valve throttling.
A second valve is necessary to maintain constant receiver pressure. It is commonly referred to as the receiver pressurizing valve. (See Figure 1.)
The CROT valve is normally open, and closes on rise in outlet pressure. It is set typically to maintain a pressure approximately 20 psig less than the ORIT valve setpoint. As the ORIT valve throttles, maintaining constant condenser pressure and interrupting the flow of refrigerant to the receiver, it is the CROT valve that maintains a constant pressure in the receiver.
The benefit of condenser flooding is the ability to provide very consistent liquid pressure in the receiver during periods of low ambient operation. Consistent liquid pressure will result in very stable TEV operation during the winter months.
There are two drawbacks to this method of head pressure control.
First, extra refrigerant is required to accomplish condenser flooding. The approximate percentage of required condenser flooding can be calculated (as with Sporlan's Bulletin 90-31).
During extremely low ambient conditions, it may be necessary to flood upwards of 85 percent of the condenser. Depending on the size of the condenser, this may require several hundred pounds of extra refrigerant. In today's marketplace, this can become quite expensive.
Second, receivers should be sized such that they are at 80 percent of their capacity while containing the entire system charge.
If extra charge is needed in the system for condenser flooding, a larger receiver will be required. The extra refrigerant added to flood the condenser during periods of low ambient will be in the receiver during the warmer months.
In systems where the additional refrigerant charge hasn't been considered in the receiver sizing, the technician will have to remove refrigerant every spring to prevent high discharge pressures at design ambient, only to add it back in the fall when it will be required for flooding.
Condenser SplittingReducing the amount of extra charge for condenser flooding can be accomplished by splitting the condenser into two identical circuits: one for summer-winter operation, and the other for summer operation only. The summer condenser will be cycled off as needed during periods of low ambient. This requires the addition of a condenser splitting valve, a three-way valve that will be installed in the discharge line. (See Figure 2.)
When the valve (for example, the Sporlan 12D13B-SC) is de-energized, the main valve piston is positioned to enable refrigerant to flow from the inlet port equally to the two outlet ports, feeding both condenser halves.
When required, energizing the solenoid coil will shift the main piston, closing off the flow of refrigerant to the port on the bottom of the valve. This removes the summer half of the condenser from the circuit, and now the minimum head pressure can be maintained by flooding the summer-winter half of the condenser.
To prevent the summer condenser from logging refrigerant during low ambient periods, a check valve is installed at its outlet. This will eliminate the possibility of refrigerant backflow into the idle summer condenser.
While a check valve isn't necessary at the outlet of the summer-winter condenser for backflow prevention, it is added so that the delta-P through each condenser is equal. This is necessary to ensure equal refrigerant flow through the two condensers.
During periods of split condenser operation, it is recommended that the refrigerant in the idle condenser be transferred back to the system. This can be accomplished by using the B split condenser valve model, which has a bleed hole in the upper piston. The refrigerant will flow through the piston, into the valve's pilot assembly, and back to the suction header.
If the B version is not used, a dedicated pump-out solenoid valve is required, which vents the idle condenser to the suction header through a restriction such as a cap tube. The pump-out line also must have a check valve installed to prevent backflow.
As an alternative to a three-way split condenser valve, two normally open solenoid valves can be used. (See Figure 3.) In this application there would be a normally open solenoid at the inlet of the summer condenser, which would cycle closed during low ambient conditions. An identical solenoid is installed at the inlet of the summer-winter condenser. It does not require a solenoid coil; this valve is installed to maintain equal pressure drop through the two condensers.
As in the three-way split condenser valve application, a check valve will be necessary for the outlet of each condenser. With this method, a dedicated normally closed pump-out solenoid valve will be required to vent the refrigerant from the idle summer condenser into the suction header.
SummaryAllowing the head pressure in supermarket refrigeration systems to operate at reduced levels during periods of low ambient results in lower compressor motor amperage, increased compressor efficiency, and lower monthly utility bills.
Head pressure has a direct effect on available delta-P across the TEV port, which, in addition to evaporator and liquid refrigerant temperatures, will determine the TEV capacity.
The minimum delta-P required to deliver the necessary TEV capacity to meet the load demand of the evaporator is the limiting factor on how low the head pressure can be allowed to float. Once this is calculated, based on TEV capacity data, the minimum head pressure can be determined and used to establish the setpoints of the head pressure control devices.
While several methods of head pressure control are available, one that allows the head pressure (liquid pressure) to remain constant is desirable. This is best accomplished by condenser flooding valves, which maintain consistent head pressure by flooding a portion of the condenser with liquid refrigerant.
While condenser flooding valves provide the most consistent head pressure, this method requires adding extra refrigerant to the system.
Using a three-way condenser splitting valve (or two normally open solenoid valves) will offer the ability to reduce the condenser capacity by 50 percent during low ambient conditions.
After cycling off the summer condenser, the remaining summer-winter condenser can utilize the flooding method with a minimum of additional refrigerant to maintain constant head pressure.
Dave Demma is a senior supermarket application engineer at Sporlan Valve Co. The company's Web site is www.sporlan.com.
Publication date: 12/06/2004