Fixed Orifice

From David Gardner P.E.
Process Engineering, Inc.
Renton, WA

I would like to respond to a Hotline question about setting superheat on fixed orifice air conditioning systems when the weather was cool. The concern was that the contractor could not get the suction pressure high enough to correspond to the manufacturer’s calculator.

The manufacturer’s data (several sources) presume that when the air temperature is at certain listed conditions, then the ambient outdoor temperature is warmer and will provide the necessary head pressure to operate the system. There was no reference to head pressure to operate the system, but I’ll bet it was low, mostly due to cool ambient.

The solution, along with the information given by Dan Kramer, is to artificially raise the head pressure to an anticipated operating condition. With lower head pressures, there is not enough driving force across the orifice to allow the suction pressure to rise. The orifice is pretty much a fixed pressure drop device and when the head pressure goes down, the suction pressure follows along.

Since these units don’t have any form of head pressure control, I find that a blanket, or sheets of newspaper, laid over the outside of the condenser surface is a very effective method of raising the head pressure to get to the desired setting. You may have to cover 70% to 80% of the condenser surface to get the job done, but keep an eye on the high pressure gauge so it doesn’t get too high.

A good nominal reference is 105 degrees F condensing temperature, which is 210 psig on R-22. The 105 degrees condensing will equate to about 85 degrees ambient for most current equipment, which is close enough to get a good setting. With this head pressure, the suction pressure should fall into the range on the charts and allow the proper charge to be installed in the system.

Dan’s point about watching the condenser subcooling is a good one. The evaporator conditions may check out OK if the charge is just on the line (one or two degrees subcooled) but the subcooling is essential to the overall system performance and economic operation. With a temperature sensor on the condenser liquid outlet, add enough charge to provide 12-15 degrees subcooling.

With these numbers, the system should operate through the full range of summer cooling conditions, from 105 degrees ambient at peak daytime temps down to 75 degrees night times, when the temperature in the house is still warm due to the heat soaking effect of the structure.

From Dan Kramer
Patent Attorney and Specialist Grade Member of RSES

The original question seemed to be grounded in the fact that while the airside parameters met the requirements of the charging calculator, the observed evaporator pressures weren’t as high as the chart predicted. The original writer of the question then asked a series of questions directly related to that specific problem.

While blocking the condenser airflow to raise the head and condenser pressure is an obvious expedient, the author of the question obviously didn’t want to do this. I was trying to work within the framework of his existing conditions and the limitations of his charging calculator.


By Robert Cormier
L’Assomption, PQ, Canada

A compressor contactor is stuck on, but the liquid solenoid is closed. What would happen after about six hours of operation? What is going to be the first defect — the compressor motor or the cylinder rings?

From Sheri Wilkerson
Manager, Service Training
Copeland Corp.
Sidney, OH

The refrigerant is a coolant for the motor, oil, and cylinders. If there is no refrigerant flow to the compressor, then the oil could thin as it heats. This would reduce its ability to support the load. The piston rings, cylinder walls, bearings, wrist pins, etc. may wear as a result. This could cause increased friction and amp draw as the wear increases.

The motor protector may cycle the compressor off if the internal heat increases. However, the type of protector should be noted. If the protector is line break, the compressor will cycle off even though the contactor is stuck. If the protector is pilot duty, the compressor motor could burn due to the contactor being stuck.

Specifically, the lack of coolant is no doubt causing internal friction due to the oil’s low viscosity and inability to support the load. The type of protector is the key. If it is the pilot-duty style, then the motor may burn due to the stuck contactor not breaking the line. Note that the line break protector may cycle the compressor after enough heat is generated. But if left to operate under these conditions for an extended time without correction, the heat may cause enough friction and debris to ultimately take the motor out as well.

Unfortunately, the time it will take for all this to occur is not cut and dry. The compressor is not identified as being air-cooled or refrigerant-cooled. Either compressor is susceptible to damage.

Publication date: 03/04/2002