Head pressure control on supermarket refrigeration systems is used to maintain a minimum high-side-pressure to low-side-pressure relationship.
Low side pressures are the result of established refrigerated case or walk-in cooler temperatures (and their saturated suction pressures), which remain relatively constant year-round. Therefore, minimum pressure ratio is a result of the minimum high side pressure. The system’s minimum pressure ratio is dictated by the compressor’s capabilities and its minimum operating envelope. The design engineer selects the appropriate head pressure control system to maintain system performance and efficient year-round operation.
Consistent pressure ratios
The greatest influence on thermostatic expansive valve (TEV) capacity is the pressure difference that exists between its inlet and outlet (the pressure differential). If the high side to low side pressure differential falls because of inadequate head pressure control, the resulting reduction in TEV capacity can create a number of problems, including:
- High evaporator superheats with loss of evaporator capacity
- Oil logging in the evaporator and suction piping
- Elevated compressor temperatures and short-cycling
- Poor refrigerant distribution with irregular evaporator frost patterns interfering with air flow and evaporator capacity
- Reduced evaporator pressure (unless controlled by evaporator pressure regulators or compressor unloading)
An additional consideration regarding the minimum design pressure ratio of a refrigerator system is the type of compressors being used. Very low pressure ratios in reciprocating-type compressors can cause valve damage. As pressure ratios decrease, the volume of gas pumped increases, causing the compressor valves to bend or flex beyond their design limits, leading to metal fatigue and breakage.
There are two ways head pressure control can maintain the high to low side pressure ratio: air side control and refrigerant side control.
Air side control
Air side control consists of increasing or decreasing the air movement across the condenser coil. Supermarkets generally use remotely located, air-cooled condensers for heat transfer. This type of heat exchanger usually employs six or eight individual fans to move the air. Head pressure drops with decreased evaporator load and/or lower ambient temperatures. One way to hold head pressure within design parameters is to cycle each fan motor with pressure switches. This works well in geographical areas where ambient temperatures seldom fall below 50 degrees F (10 Degrees C).
Stable head pressures may be more difficult to achieve if ambient temperatures consistently fall below 50 degrees F (10 Degrees C). The entire system becomes increasingly unstable as lower-than-design air temperature is pulled across the condenser. The instability is caused when the fans suddenly start dropping the high side pressure rapidly, overshooting the corresponding liquid temperature. This creates bubbles in the liquid line as the refrigerant pressure lowers below its saturation temperature and boils, thereby cooling to the new saturation pressure. This reduces TEV capacity and ability to properly feed the evaporators.
Air side temperature control can also be obtained through fan speed control. A Variable Frequency Drive (VFD) can control the speed of each fan by changing the electrical frequency supplied to these fans. Variable speed fans can also be used to modulate the fan speed. Both technologies can provide more stable operation to manage air flow while maintaining a minimum high side pressure.
Refrigerant side control
This is advisable when outdoor ambient temperatures are consistently lower than 50 degrees F (10 Degrees C) . This control system may or may not be used in conjunction with air side controls.
Refrigerant side control systems accomplish head pressure control by effectively reducing the size of the condensing surface. This can be done by flooding a portion of the condenser with liquid refrigerant, thus reducing its condensing surface. This is the “flooded condenser method.”
Another control method is splitting the condenser into two or more sections. With the use of valves, discharge gas is diverted only to the section that is large enough to maintain discharge pressures under given ambient conditions. This is called the “split condenser method.” This method is usually used in combination with the flooded condenser method, along with fan cycling or fan speed control.
To maintain head pressure during winter operation, extra refrigerant must be available to partially fill the condenser. Furthermore, with the additional refrigerant being required for winter operation, consideration must be given to the size of the receiver, so it will have enough capacity to hold the extra refrigerant charge during summer conditions.
With the high cost of refrigerants, it is important to design the system to minimize the need for the extra refrigerant charge.
One way to do this is to combine refrigerant side control with air side control. This is accomplished by either cycling the fans or controlling their speed along with refrigerant side control. Both of these air side methods use pressure as the controlling signal to reduce air flow during periods of low outdoor ambient temperatures. The reduced or eliminated air flow across the condenser means that less heat is rejected to the air, and therefore less refrigerant flooding is needed.
Another method for minimizing refrigerant is to split the condenser into two or more circuits, usually within the same tube bundle. This kind of system uses both condenser circuits during summer operation when the load is high. During the winter, the second circuit is deactivated and drained of refrigerant. It is re-activated only during high ambient conditions when the maximum condenser capacity is required to keep condensing pressures as low as possible.
Proper head pressure control saves time and money
Operating cost may be higher or lower in different supermarkets, depending on the preventative maintenance program and regional climatic conditions. However, with the proper control mechanisms in place, engineers can take advantage of lower ambient temperatures to maximize efficiency and improve the supermarket’s bottom line.
Article contributed by Sporlan Division, Supermarket Refrigeration, Parker Hannifin. For more information, contact Sporlan Division technical support at 636-392-3906.
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