By Steve Murphy
Fort Walton Beach, FL

I have a question about contactors. I’ve been on a service call before where I found a bad three-pole, 30-amp contactor. The contacts were pitted.

I did not have a 30-amp, three-pole contactor on the van but I did have a 50-amp. So I installed it. I had the contactor wired right, so I turned it on. When I turned it on, the contactor started to chatter. I hurried to turn it back off. I then disconnected the coil leads to test for low voltage. When I did, I had 24 volts (perfect).

I didn’t know what to do next. So I called a guy I worked with to see if he had a 30-amp, three-pole, 24-volt contactor and he did. When he showed up, I installed it and turned it on. The contactor pulled in right away. I’ve seen this a couple of times in my five years of experience.

I’ve talked to some people who said a contactor should pull in no matter what the amperage is as long as you’re getting 24 volts to the coil.

From Gene Silberstein
Consulting Engineer
Whitestone, NY

The magnetic field required to pull in the armature of a contactor increases as the size of the contactor increases. The magnetic field generated depends directly on the voltage supplied by the control transformer, as well as the amount of current flowing in the control circuit. The amount of current will decrease as a result of the increased resistance of the new, larger holding coil of the 50-amp contactor. Replacing the 30-amp contactor with a 50-amp contactor did not work simply because there was not sufficient power to pull the armature of the replacement component.

In the event this should happen in the future, there are a number of possible solutions. The first being the most obvious: Always try to use the exact replacement for any component that becomes defective. Secondly, if you must use a replacement contactor that is larger than the original as far as contact ratings go, you may need to replace the control transformer as well with one that has a larger VA (volt-amp) rating. In essence, the VA rating is the power that is supplied by the device. A transformer with a larger VA rating will be able to pull in the larger contactor. Most transformers that are used on light commercial systems are rated at 40 VA, so a 75-VA transformer may very well be the answer. If a larger transformer or the exact replacement is not readily available, the next solution may get you out of a bind.

Connect the holding coil of the new contactor directly to the transformer. (See Figure 1, above). If the contactor pulls in, you are in luck. Connect the wires that would normally be connected to the holding coil of the contactor coil to the coil of a general-purpose relay, usually used for fan control. Connect the holding coil of the new contactor directly to the transformer, placing the normally open contacts of the general-purpose relay in series with the contactor’s holding coil. Set up this way, the thermostat will provide power to the holding coil of the general-purpose relay, which in turn will control the contactor coil circuit. The reason this often works is because the holding coil circuit has much less wiring in it now, thereby reducing the resistance of the circuit. This increases the current in the circuit, causing an increase in the power of the magnetic field generated in the holding coil. If this does not work, you must resort to the first or second solution.

Head Pressure

Name Withheld By Request

How do you determine head pressure for a water-cooled condenser? If your water temperature is 55 degrees F (incoming), what should the head pressure be on an R-22 system? It seems that if it were a 10 to 20 degree split, there would be a large waste of water.

Second, could I get an unbiased opinion of UV light’s ability to clean coils in an air handler? We have seen many claims, but are they realistic?

From Dan Kramer
Patent Attorney and Specialist Grade Member of RSES

While cold water can produce low condensing temperatures when circulated freely through a water-cooled condenser, there probably should be a water control valve that has a capillary sensing the head pressure and set to control the desired pressure.

Typical design conditions for water-cooled condensers assume a one-ton air conditioning situation with a heat rejection of 15,000 BTU per hour and three gallons per minute of 85 degrees F water. With a correctly designed condenser, this produces outlet water temperatures of 95 degrees and a condensing temperature of 105 degrees or 211 PSIG. The Log Mean Temperature Difference (LMTD) under these conditions is 14.5 degrees and UA, the latter the product of film coefficient U and surface area A.

For a one-ton condenser, UA would be about 1,035 BTUH-F. It is generally accepted that where the conditions affecting water-cooled condensers change drastically, the use of LMTDs is necessary. An ASHRAE Handbook or the RSES SAM Manual will explain these terms and show how to use them.

Now, assuming you had plenty of 55 degree water available, yet didn’t want to waste any, you would keep the same water control valve setting that produced the 211 head pressure (105 degrees condensing) with the warmer water. Under these conditions there would be a substantially reduced water flow. To estimate the water flow under your conditions, I estimated that the overall heat transfer coefficient in the condenser might drop about 30% with the reduced water flow (from the 3 GPM standard rate) and that a UA heat transfer coefficient for a one-ton condenser would be about 725.

Under these conditions, I estimated that your water outlet temperature would be about 80 degrees. Your LMTD would be about 21 degrees and your water flow rate would be about 1.2 GPM. Clearly this is a big reduction of water flow (over the 3 GPM needed with the 85 degree water) secured by keeping the condensing temperature at a reasonable level.

Since there are many ways to make these assumptions, others might calculate different conditions and arrive at different results. However, you can see that attempting to employ a straight or arithmetic TD to predict the operating conditions might not be too helpful.

Regarding your question of UV, ultraviolet light cannot “clean” dirty coils. UV light does kill bacteria, including slime. UV lights are frequency applied with some success in icemakers. If your coils and drain pans are slimy and smelly, UV might help.

But, if your coils have been run without filters and are coated with lint and other junk, UV won’t help at all. I would detail a person to get into the dirty area, do a thorough cleaning job with proper cleaning chemicals and tools, and then simply repeat the manual cleaning process on a regular schedule.

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Publication date: 09/02/2002