ACHRNEWS

Sizing up refrigeration pressure regulators

April 10, 2000
In practically 99% of the cases, precise size selection is important but not critical when it comes to refrigeration pressure regulators. Just determine maximum normal refrigeration load, desired pressure drop (frequently 2 psi or 0.14 bar), and nominal evaporator temperature. Enter the regulator manufacturer’s selection tables and choose the closest — not the next larger — regulator port size.

In some cases, the available regulator connections may not be the same size. However, if the system pipe size is materially different, say more than two sizes, the regulator and system size calculations should be rechecked.

Changing loads

When chosen in this manner, the regulator should perform well even at reduced loads of 15% or less of full load. For example, if a regulator is chosen for a capacity (C) at 2-psi drop, it should control well at a capacity of 0.15 C at the same 2-psi drop.

However, at light loads, the pressure drop presented by the system to the regulator may be greater than 2 psi. For example, a single compressor system may experience a much lower suction pressure under light load than during normal operating loads.

In these cases, a correction should be made in the original selection in order to accommodate the light load, perhaps at the expense of slightly more pressure drop at full load as the result of choosing a smaller regulator.

In extreme cases, two regulators could be used in parallel, with a smaller one sized for the light load and set for a slightly higher (1- or 2-psi) opening pressure.

Actually, in many cases a greatly oversized regulator, although causing fluctuating upstream pressure due to hunting regulator action, would not materially affect overall system operation. In most cases, the regulator’s construction will tolerate hunting without damage, although the hunting noise may be bothersome.

In general, it requires 2-psi minimum pressure drop to reliably open a normal pilot-operated regulator. Sizing a regulator larger (oversize) in an attempt to reduce pressure drop merely causes hunting, with no actual decrease in pressure drop.

What damages a regulator?

If a regulator is ever internally damaged (broken seat or piston parts), this is typically not caused by the regulator hunting, but rather by liquid and gas slugs entering the regulator at very high velocities and encouraged by high pressure drop across the regulator seat port.

Such would only tend to occur with gross oversizing, accompanied by liquid slugging from heat exchangers, accumulators, or poor piping practices.

As a matter of fact, whenever internal valve damage or parts breakage occurs, it is usually caused by very-high-velocity liquid refrigerant slugs mixed with very-high-velocity gas flow through a valve or regulator. This usually occurs when:

  • A high-pressure receiver has insufficient liquid level.
  • An evaporative condenser feeds liquid backwards to a hot gas defrost or reheat line.
  • An evaporator or suction accumulator is overflowing onto the valve.
  • A defrosting evaporator containing liquid is suddenly exhausted through a valve to the suction pressure.
  • System flow direction is abnormally reversed through the regulator.

When pressure regulators or solenoid valves exhibit erratic operation or short-term abnormal wear or damage, it is usually caused by excessive dirt particles or by application problems rather than an inherent product flaw of valve design or construction.

The existing designs of these valves have been shown to be sound during the past 10 years, as proven in the field by thousands of installations.

This information comes from Hansen Technologies Corp., 6827 High Grove Blvd., Burr Ridge, IL 60521.