ACHRNEWS

Sizing a Refrigerant Pressure Regulator

January 5, 2001
When sizing a refrigerant pressure regulator, precise size selection is important but not critical.

To size a refrigerant pressure regulator, determine maximum normal refrigeration load, desired pressure drop (frequently 2 psi or 0.14 bar) and nominal evaporator temperature. Consult the manufacturer’s regulator selection tables and choose the closest, not the next larger, regulator port size.

NOT EXACTLY THE SAME

In some cases, the regulator’s available 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.

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 tight 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, choose a smaller regulator with slightly more pressure drop at full load.

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 or higher) opening pressure.

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



DANGERS OF OVERSIZING

In general, it requires 2-psi minimum pressure drop to reliably open a normal pilot-operated regulator. Sizing a regulator larger (oversizing) in an attempt to reduce pressure drop merely causes hunting with no actual decrease in pressure drop. If a regulator is ever internally damaged (broken seat or piston parts), this is typically not caused by the regulator hunting; rather, the damage is a result of liquid and gas slugs entering the regulator at very high velocity, encouraged by high pressure drop across the regulator seat port. This situation tends to occur with gross oversizing accompanied by liquid slugging from heat exchangers, accumulators, or poor piping practices. In 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 an insufficient liquid level;
  • An evaporative condenser feeds liquid backwards to a hot gas defrost or reheat line;
  • A hot gas defrost line contains substantial liquid;
  • 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; or
  • If a 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 design of these valves has been proven sound in hundreds of thousands of installations over the years. One final note: Always refer to the manufacturer’s recommendations for the installation, operation, and maintenance of any specific piece of equipment. All efforts have been made to provide the most accurate information possible.

    Chason, with Hansen Technologies Corp., is currently a member of the International Institute of Ammonia Refrigeration (IIAR) and a member of the Refrigeration Engineering Technicians Association (RETA). This paper was presented at the 2000 RETA conference in Ocean City, MD.

    Publication date: 01/08/2001