Suction Gauges

By Dave Anderlik

The Service Hotline in recent years has had questions on the term “retard” as it relates to suction gauges. One definition of retard is a holding back or slowing down.

That prevents damage to the suction gauge if exposed to high pressures, usually at 120 psig on standard gauges. One problem is determining this if you have gauges connected to the wrong sides of the system and the gauges were incorrectly reading high side pressure.

Fan Cycling

From George Knight
Beebe, Ariz.

I have many systems used in classrooms with thermostatic expansion valves and low ambient controls. How will a low charge affect the valve? The condenser fan cycling will cause the head to rise/fall and it is hard to tell if the charge is low or the valve is starving the system. I could weigh the charge in, but is this the only way to make sure the valve isn’t bad?

By Emerson Climate Technologies

A system that uses fan cycling for head pressure control will be affected by low system charges much like any other system. The thermostatic expansion valve (TXV) will receive poor-quality refrigerant (flash gas) resulting in a valve with reduced capacity.

In order to determine if this is the case, you need to first prevent the fans from cycling by bypassing the pressure control. Once bypassed and with the system at stable conditions, check the subcooling of the liquid line. To do this, measure the temperature and pressure entering the TXV. If the measured liquid temperature is lower than the saturation temperature, then the liquid is subcooled and the TXV should be seeing a solid column of liquid. If the measured temperature is higher than the saturation temperature, then there is flash gas present, which is a good indication that the unit is low on charge.

Flash gas will also be created by a restriction in the liquid line, so be sure the filter drier is not clogged or the tubing is kinked.

Once it is verified that the system charge is proper, you can check the TXV by measuring the superheat at the outlet of the evaporator coil. The system manufacturer should be contacted to determine the proper superheat for the system in question. However, as a rule of thumb, 10-12 is quite common for air conditioning equipment.

Motor Troubles

From John West
Chesterfield, Va.

My question involves an air handler unit motor and premature failure. The motor is an A.O. Smith 7-850008-OJ. We are having to replace this motor each year and this is very time-consuming and costly. We have a minimum staff and these instances start to drag us down from our preventive maintenance program.

By A.O. Smith

The motor is stock catalog number E451. It is a NEMA premium-efficient 15-hp one, which is designed, tested, and warranted to be corona-free for compatible inverter duty. One failure could be the motor’s fault, but multiple failures would make us look harder at the total system. In any failure, it is important to try to determine the cause, not just the result. We recommend that the analysis start with the drive, and would be glad to examine the motor if it is still available.

Traps Two Lines

From John West
Chesterfield, Va.

My question involves traps that must be installed for proper oil return when the condenser is much higher than the AHU [air-handling unit]. I have been taught in school to trap every 20 feet of height. Manufacturer reps have also instructed me that if the install is correct, then you do not need the traps.

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

With respect to the different responses about installation of traps in the hot gas riser to elevated air-cooled condensers from your tech school and the manufacturer, I suggest you think about it as follows:

A discharge line trap has important functions both during the compressor “on” period and during its “off” period. During the “on” period, the trap acts as a turbulator to entrain oil droplets and carry them efficiently up the elevated discharge line. Therefore, traps would be most useful in unloading of multicompressor systems to better ensure proper oil circulation during low system loads. Dual risers are sometimes applied in those cases.

During the “off” period, the traps act to catch and retain oil residing on the pipe walls that would otherwise possibly drain back to the compressor head and cause damage on startup.

The manufacturer was addressing the needs of its unit only, which probably has a single non-unloading compressor. The tech school was teaching the most conservative de- sign procedure.

Both were correct.

At the least, I would always install a single trap at the foot of the riser along with a check valve between the compressor head and the trap.

Discus Compressor

From Mohamad Bassiouny
Cairo, Egypt

I have a 35-kW Copeland Discus compressor with an evaporator temperature of 0°C [32°F] and a condensing temperature of 55°C [131°F]. I need to regulate its capacity according to the load. Could I use a frequency drive with 4-20 mA control signal to reduce the compressor speed?

By Emerson Climate Technologies

The 35-kW compressor is presumably a six-cylinder compressor. One way to regulate its capacity is using variable speed. Copeland discus compressors are approved to operate down to a 30-hertz rating.

Control Techniques is one company that makes a frequency drive like you mentioned. Either a low voltage or a milliamp signal can be used. The 4-20 mA signal that you suggested is proper.

But a word of caution: If the mA signal is used, it can be affected by electrical interference. If the voltage signal is used, it will not be suspect to this type of interference.

Publication date:08/11/2008