Scroll Compressor


From BrooklynDave Via e-mail

We have a scroll compressor that has been changed recently and it is not pumping. The compressor gets really hot and the head pressure never goes up.

The compressor is rated for 11.5 running amps. Instead, it shoots up to 48 amps and very slowly drops to 24 amps before going off on internal overload. The unit is a split system made by Mitsubishi (the Mister Slim design). I tried recovering the refrigerant charge and putting a fresh charge but that didn’t help.


By Gary Nettinger National Service Manager Mitsubishi Electric & Electronics Lawrenceville, GA

It is difficult to identify the specific cause of the problem without knowing more about the unit involved and its history.

I’d recommend you start by checking the true high- and low-side pressures when the unit is running. These ports are located just above the ball valves.

Install another low-side pressure gauge on the service port attached to the liquid line ball valve. The system you are working on has the metering orifice located in the outdoor unit. This pressure will actually be on the low side of the system.

On an 85?F day, normal pressure to look for would be a high side of about 220, a low side of 75?, and a liquid line pressure of about 105.

According to what you told us, I would suspect you have one of three problems: a miswired compressor, a bad capacitor, or a faulty reversing valve. Mitsubishi’s factory service support staff would be glad to help direct you through the diagnosis of the product. That information can be obtained by calling 800-811-3112.



From Kevin Brennen Frankford, DE

Why does a “dry” nitrogen tank exhibit properties of having temperature loss when the gas escapes rapidly? Is it because there are some vapor particles of liquid nitrogen in the tank?


By Dan Kramer, P.E. Specialist Grade Member of RSES, Professional Engineer

When you pump up your bike tire with a hand pump, I’m sure you have noticed that the end of the pump cylinder gets hot. Letting nitrogen out of a cylinder full of high-pressure nitrogen simply produces the reverse reaction; that is, the gas in the cylinder, having had its pressure reduced, gets cool.

When we pump up the tire by taking low-pressure air (atmospheric pressure, 15 psia) and pumping to a higher pressure, we are putting energy into the air by compressing it. The energy does two things: It makes the gas hot and it raises the pressure. The pressure increase results both because we have compressed the gas and also because we have warmed it by the energy of compression. When the warm air in the tire cools, its initial pressure will drop a little. The amount of the drop is proportional to the drop in absolute temperature; that is, about 1/490 of the initial pressure for each 1?F the gas cools.

When we let gas out of the cylinder, we are removing some gas and, therefore, reducing both the amount of gas and removing some of its internal energy. Therefore, it gets cool even though there is no liquid present to evaporate. There are some types of airplane air conditioning systems that work on the same principle: A compressor compresses the hot ambient air, thereby heating it at the higher pressure. The hot compressed air is passed through a heat exchanger, where it is cooled to near the temperature of ambient air. Then the high-pressure cooled air is expanded through a nozzle or turbine. The air gets cooler when it is expanded through a turbine than through a nozzle. The turbine reduces the pressure more efficiently. The air, in expanding, gives up some of its energy to the turbine blades. The air leaving the turbine is colder than the air would have been if expanded through a nozzle.

If you want to know more about this process, look in a book of elementary physics under Ideal Gas Laws, Boyle’s Law, and Joule’s Law.

A/C Settings


From Michael Smith Via e-mail

Please explain the proper procedure for charging an air conditioning system when the TXV is “not in control.” By this I mean, you just finished an installation, the air conditioning has been off all day, and it is 90?F in the house. You do not have the time to wait for the inside of the house to cool down.


By Jeff Staub Application Engineer Danfoss Baltimore, MD

Charging procedures should be followed as recommended by the equipment manufacturer. There are many variables, such as line size and length of liquid and suction lines, that need to be taken into account.

The TXV will not be in control, and you cannot properly diagnose TXV control, until the system is fully charged. When charging a system, or when the system is undercharged, you will not get adequate subcooling in the liquid line. There will be flash gas in the liquid line and refrigerant vapor will enter the TXV. This will cause the system to have a large superheat and the TXV will be fully open.

If you have finished charging a system to the recommended specifications, but do not want to spend time waiting for the house air temperature to drop from 90? to 72? to set the TXV, do not readjust the TXV. TXV suppliers work closely with oem’s to ensure that TXVs from factories are set to the oem’s specifications. This setting is optimum and makes the system as efficient as possible.

If the TXV has been replaced, find out what the oem originally set the valve for and readjust the TXV so that it matches the original valve’s setting. Valve manufacturers can provide you with information on how many degrees of superheat change you get when you adjust the superheat spindle.

If you feel the need to adjust the valve, you can do so without waiting for the air temperature to drop. As long as the evaporator temperature is within the specified range of the valve, it is OK to set the valve.

Once the system is fully charged, it should not take long for the evaporator temperature to drop. Actually, it is more appropriate to set the valve when the indoor air temperature is 90?, because the system is under a relatively high load. You will run a greater risk of system hunting if you set the valve at a low load (when, say, the indoor air temperature is 65?) and later the system operates at a higher load. That can easily happen when people turn off their air conditioning for a weekend out of town and return home to find indoor air temperature around 85?, or if a large number of people enter the cooled area.

To set the expansion valve, continually reduce the superheat setting until the system becomes unstable, which is when the valve starts to hunt. Next, increase the superheat spindle one-half turn so the valve will become stable and stop hunting. This is the optimum superheat setting.

Publication date: 09/03/2001