The need for better head pressure control

June 1, 2000
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Various methods of head pressure control can have a rather dramatic effect on the performance of refrigeration systems and their components. Are you familiar with them?

To review briefly, the first part of this article dealt with a system with a capacity of 12,000 Btuh, R-22, 20°F evaporating temperature, 110° condensing temperature, 50-psi system pressure drop (including distributor pressure drop), and 100° liquid temperature, with a net pressure drop across the TX valve of 133 psi, and the selection of a 1-ton externally equalized valve the perfect choice.

However, it’s not a perfect world out there. Part one discussed adding head pressure control through fan cycling controls, fan modulating controls, and dampers. This week we conclude with a discussion of split condensers, head pressure control valves, and compression ratios.

Split condensers, head pressure valves

A split condenser is just that, a condenser that has two or more circuits designed into it.

When ambient temperatures fall and the condenser becomes too efficient due to increased temperature difference, a portion of the condenser is valved off. This allows the remaining portion of the condenser to operate in the prevailing ambient at the wider temperature difference, balancing condensing capacity with the load.

Head pressure control valves perform several functions. The basic head pressure control valve will monitor the pressure of the condenser bypass line at the valve’s B port, and at a predetermined pressure setting, modulate the condenser bypass line open through the B port of the valve.

This increased pressure in the C port of the valve restricts flow at the outlet of the condenser and causes liquid to stay in the tubes, filling it up and decreasing its effective condensing surface area.

Since the condenser has less surface area, the efficiency is decreased to the point that the pressure raises to balance with the bypass port. Discharge gas and liquid can then mix and be introduced to the drain line to the receiver through the R port of the valve.

This serves two functions: to create a smooth adjustment of condensing capacity to lessen the likelihood of liquid line flash gas, and to maintain the system design pressure drop across the TX valve.

When head pressure control valves are used, receiver capacity must be considered. Since the condenser is being filled with liquid, the receiver may starve in the winter and be overcharged in the summer when the condenser is operating normally.

In some cases, the receiver capacity at 80% should be equivalent to the total liquid capacity of the condenser and liquid line to provide a liquid seal in the receiver at all times. There should also be a check valve installed in the condenser drain line between the head pressure control valve and the receiver, to prevent refrigerant migration to the condenser in off periods, when the condenser is colder than the receiver.

Charging procedures for systems with head pressure control valves are relatively simple. In the summer, the system should be charged to a full sight glass. If the receiver has been sized correctly, the equivalent of one-half of the liquid volume of the condenser can be added.

In the winter when the valve is in operation, the system should be charged to no more than a full sight glass. The charge should be checked at every change in season to maintain the best performance.

Some refrigeration systems are allowed to let the head pressure “float,” or follow the ambient to a predetermined point, at which time a head pressure control valve controls the minimum pressure value prescribed by design. Floating head pressure systems may take advantage of lower head pressures to save in energy costs and maintenance from wear on compressors.

Compression ratios

Energy efficiency is achieved by lower compression ratios resulting from decreased high-side to low-side pressure differences.

Reviewing the example in the first part of this article, the compression ratio at 90° ambient, expressed in psia, would be 241 over 58, or a 4.16-to-1 compression ratio The typical power requirement reduction can be significant in many cases. Another added benefit of lower compression ratios is extended compressor life expectancy.

Of course, design requirements would change, dictating that a larger expansion valve be sized for the system at the lower head pressure and net pressure drop. This is where a balanced port-type expansion valve is at its best. Sized for the lower pressure drop, a larger expansion valve at normal operating conditions must run at a smaller percentage of its capacity and still control superheat smoothly.

Balanced port valves, by design, can control their superheat setting down to approximately 20% of their rated capacity over an extremely wide range of head pressures.

Each of the different types of head pressure control has its place, but the head pressure control valve can supply the best performance under all ambient conditions.

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