The need for head pressure control

June 1, 2000
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There are various methods of head pressure control, some of which contractors may not be aware of. They can have a dramatic effect on the performance of refrigeration systems and components.

Refrigeration systems are designed around certain criteria:

  • Evaporator-compressor capacity;

  • Type of refrigerant used;

  • Evaporator temperature;

  • Condensing temperature;

  • Total system pressure drop;

  • Pressure drop across the TX valve; and

  • Liquid temperature.

Let’s look at 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.

Convert the condensing and evaporator temperatures to pressures and we get 226 over 43, which is 183-psi pressure difference from high side to low side. Then subtract for system pressure drop of 50 psi, which is 35 psi for the distributor and 15 psi for system components and line pressure drop.

This leaves us with a net pressure drop across the TX valve of 133 psi and makes the TX valve selection a 1-ton externally equalized valve. This delivers approximately 1.06 tons with no correction for liquid temperature, as the liquid temperature will be approximately 100°.

This makes a perfect match to evaporator capacity at the design conditions of 90° ambient with a 20° condensing temperature difference.

Summer to winter conditions

Now, not being a perfect world out there, the ambient is going to change dramatically from summer conditions of 90° to winter conditions averaging say, 40°.

If we assume that condensing temperature could follow the ambient at 20° temperature difference above the ambient, then the condensing temperature is now 60°. Liquid temperatures will also follow the ambient from the design temperature of 100° at 90° ambient, to approximately 50° at a 40° ambient.

Following the same formula as before we now have 102 over 43, which is 59-psi pressure difference. Flow rate decreases and density of the refrigerant increases slightly, so the distributor pressure drop falls to approximately 11 psi and line losses fall to approximately 7 for a net pressure drop across the TX valve of 40 psi.

Now, the same 1-ton valve selected for the application will only deliver 0.79 tons. That is 0.61 tons capacity from the TX valve sizing tables, times the liquid temperature correction factor of 1.29 for 50° liquid temperature, or 0.61 x 1.29 = 0.79.

The liquid correction factors are a function of the density of the refrigerant at a given temperature, being more dense at lower temperatures. The valve is now undersized for the 12,000-Btuh load by 2,520 Btuh, or about 21%.

Adding head pressure control

What I have just illustrated is a common problem with systems that have no method of head pressure control. Compounding this basic problem is that the condenser becomes more efficient and refrigerant tends to stay there, which causes the system to appear to be short of charge.

Some of the various ways of falsely loading the condenser to raise high-side pressure are:

  • Fan cycling controls;

  • Fan modulating controls;

  • Dampers and heaters;

  • Split condensers; and

  • Head pressure control valves.

Fan cycling controls are pressure switches that sense high-side pressure, or temperature controls that sense liquid line temperature that turn the fan off when the condensing pressure-temperature falls, and on when the pressure-temperature reaches the upper setting on the fan cycling switch. This causes fluctuation of the head pressure and can result in expansion valve hunting and poor superheat control.

Another problem with fan cycling controls is that when the fans are turned on and off, the liquid line pressure after the receiver fluctuates, which may cause the refrigerant’s saturation point to change and result in flash gas, or bubbles in the liquid line. This decreases the capacity of the TX valve due to vapor bubbles passing through the expansion valve orifice and decreased pressure drop.

Fan modulating controls are electronic devices that sense either the temperature of the liquid line or the ambient and slow down or speed up the condenser fans. This gives better superheat control, but it’s only effective to the point that shutting off the fans completely will not help the head pressure rise. In some cases this method can also cause a flashing problem in the liquid line.

Both fan cycling and modulating controls may be affected by prevailing winds that can blow through the condensers, rendering them useless. In addition, PSC fan motors may, in fact, start in the wrong direction if the wind is turning the fan blades backwards.

Dampers are used after fan cycling or modulation has become ineffectual and stops the wind from helping the condenser. The heaters add heat to falsely load the condenser and raise head pressures.

Dampers have two serious downfalls to be considered:

1. During periods when the dampers would be in effect, the opportunity for ice formation can render them inoperative.

2. The actuators for the dampers, which are normally pressure operated, have a tendency to leak, causing loss of refrigerant.

Next week: Split condensers, control valves, and compression ratios.

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