ACHRNEWS

Service Hotline: 10/01/2001

September 25, 2001

Line Temperature

Question:

From Rick Dempster Columbia, SC

When checking subcooling on an a/c system, should you check the pressure reading on the discharge line or the liquid line? Where is the best place to take the temperature of the line?

Answer:

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

It is always best to take the pressure at or very near the point where you will be measuring the temperature. This is because there can be either a significant pressure drop or even a pressure rise between the point where you measure the pressure and the point where you measure the temperature.

For example, suppose in an R-134a system there is a 5-psi pressure drop through the condenser. You measured 114 psig at the condenser inlet and 92.5?F at the condenser liquid outlet. You might calculate 2? to 3? sub-cooling when there was none.

On the other hand, let’s consider a condensing unit on the roof, 15 ft above grade. If you measured the liquid line pressure at the receiver outlet and the liquid temperature at the evaporator inlet on grade, you would have a pressure at the point of measurement about 7 psi higher than at the point of pressure measurement at the receiver outlet. Your measured temperature might then incorrectly indicate zero subcooling when there was really several degrees of subcooling. Of course, a liquid sight glass showing clear liquid at that point might help to raise a question about the correctness of that zero subcooling measurement.

If you were hearing periodic hissing at the thermostatic expansion valve (TXV) and possibly observing a starved evaporator, you might suspect little or no subcooling at the TXV inlet. It would be important to measure the pressure and temperature right there at the TXV inlet. This is because of the possibility (or even probability) of a restricted liquid line filter-drier causing a pressure drop and liquid flashing (zero subcooling).



Refrigerant Odors

Question:

From J. Schilling Via e-mail

I seem to be able to smell a chemical odor when I open the freezer door. I think it could be the refrigerant. The food that is in the refrigerator compartment is picking up this odor. Does refrigerant have an odor and do you know what affect this could have on my family?

Answer:

By Gary Zyhowski Genetron Refrigerants Technical Service Honeywell Buffalo, NY

Fluorocarbons are most often described as having a faint ether-like odor. Since foods also give off odors and the ability to smell odors varies with the individual, you don’t want to rely on your nose.

I would suggest having a service person check the system for leaks. Domestic refrigerator/freezers have a small refrigerant charge. If you did detect the presence of refrigerant, there may not be much left in the system now.

I would not expect the level of exposure to the refrigerants R-12 or R-134a commonly used in domestic systems to be a concern. Prior to the ban on CFCs, R-12 was used in direct-contact food freezing. In the case of 134a, it is currently used in metered-dose inhalers as a way to deliver, for example, asthma medication directly to the lungs.

Depending on the design of the refrigerator/freezer, there is also a possibility that a leak may have introduced compressor lubricant into the compartment. You may want to have a service person check for this possibility as well.



Voltage Imbalance

Question:

Rick Sones Vatterott College Des Moines, IA

I am an instructor at a technical college in Des Moines, IA, and I have a question. I cannot for the life of me remember how to figure voltage imbalance on a three-phase system.

This question is so basic that I must be overlooking a very simple step. I am trying to find the percentage of unbalance between the three legs of power.

Answer:

Gary Weeks SSAC Inc. Baldwinsville, NY

The effect that unbalanced voltages and currents have on three-phase motors is widely misunderstood. Not enough technicians and controls engineers understand the number of motor failures that are actually caused by unbalanced voltages.

Most understand the effect of phase loss (the most extreme level of unbalance possible), but don’t recognize the cumulative effect of moderate unbalance events.

Unbalanced voltage causes over-heating of a three-phase induction motor’s stator windings and rotor bars. When the voltages are unbalanced, the currents are unbalanced by six to 10 times the percent of voltage un-balance. A 5% voltage unbalance can mean a 30% to 50% current unbalance.

With currents unbalanced at these levels, the motor will begin to overheat and slip as if it was overloaded. The effect of overheating is said to be equal to twice the square of the unbalanced percentage. Often, when the voltages are unbalanced, the average voltage is also low, so a normal 100% load will appear like an overload to the motor. This is why is it always important to derate the motor’s horsepower when unbalances are present.

The best idea is to eliminate the unbalance so that the motor can operate without overheating. This is usually accomplished by redistributing the single-phase loads on the three-phrase supply transformer.

It is unfortunate that more technicians don’t understand how often unbalance and low voltage occur together. Too often, failures caused by repeated exposure to these events are blamed on a mysterious single-phase event or an overload that never actually occurs. SSAC manufactures voltage and current monitors that can detect unbalance, single phasing, low voltage, high voltage, and phase reversal. Some models also include overload protection. These provide three-phase motor protection and can eliminate failures caused by three events.

Unbalanced voltages are covered in NEMA motor specifications. It is relatively easy to calculate the percentage of unbalance. The most commonly used formula is also included in the NEMA guideline. That states that the percentage of voltage unbalance equals 100 times the maximum voltage deviation from average voltage, divided by the average voltage. For example, with voltages of 460, 467, and 450, the average is 459. The maximum deviation from average is 9 and the percentage unbalance = 100 x 9 over 459 = 1.96%.

This same formula can be used to calculate the percentage of current unbalance by substituting current readings for the voltage readings.

SSAC also has a training manual that shows the relationship between unbalance and overheating. That includes a graph that shows the relationship between overheating and motor insulation life span.

Voltage monitors are controls that provide much-needed protection for hvacr compressor systems. Their values have been proven for years in the field. Most systems include a voltage monitor so every hvacr technician needs to understand what they do and how they protect a three-phrase motor against premature failure.

Publication date: 10/01/2001