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 - I have also been instructed by manufacturer reps 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 or 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 catch and retain oil residing on the pipe walls that would otherwise possibly drain back to the compressor head, causing damage on startup.

The manufacturer was addressing the needs of its unit only, which probably has a single, nonunloading compressor. The tech school was teaching the most conservative design procedure. Both were correct.

At the very 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.

Fan Cycling

From George Knight
Beebe, Ariz.

I have many systems used in classrooms with thermostatic expansion valves (TEVs) and low ambient controls. How will a low charge affect the valve? The condenser fan cycling will cause the head pressure to rise and fall; 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 Al Maier
Emerson Climate Controls

A system that uses fan cycling for head pressure control will be affected by a low system charge much like any other system. The TEV 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 first to prevent the fans from cycling by bypassing the pressure control. Once by-passed and with the system at stable conditions, check the subcooling of the liquid line.

To do this, measure the temperature and pressure entering the TEV. If the measured liquid temperature is lower than the saturation temperature, the liquid is subcooled and the TEV should be seeing a solid column of liquid. If the measured temperature is higher than the saturation temperature, flash gas is present, which is a good indication that the unit is low on charge.

Flash gas also will be created by a restriction in the liquid line. Make sure the filter-drier is not clogged and the tubing is not kinked.

Once you verify that the system's charge is proper, you can check the TEV by measuring superheat at the outlet of the evaporator coil. Contact the system manufacturer to determine the proper superheat for the system in question. However, as a rule of thumb, 10 degrees to 12 degrees F is common for air conditioning equipment.

Motor Troubles

From John West
Chesterfield, Va.

My question involves an air handler motor and premature failure. The motor is an A.O. Smith 7-850008-OJ used on a Carrier unit. We are having to replace this motor each year, which 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 Denny Bush
A.O. Smith

The motor is stock catalog number E451. It is a NEMA premium-efficient, 15-hp model 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. We would be glad to examine the motor if it is still available.

If you have a technical question, fax it to 847-622-7266 or submit it online by visiting The News' Extra Edition page and clicking on The Hotline link in the left-hand column.

Publication date: 03/07/2005