Service Hotline: 06/03/2002

June 2, 2002
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What’s The Difference?

QUESTION:
By T. Parrucci
Owens Heating & Air Conditioning
Laurel, MD

Could you please give me some information on the difference between a solid-state hard-start kit and a start capacitor with a potential relay? I have been told the use of a solid-state hard-start kit is not a good idea because it does not have a resistor like the start cap does.

ANSWER:
From Dan Kramer
Patent Attorney and Specialist Grade Member of RSES

It is possible that “hard start” is a trademarked name. If so, I apologize to the owner of the mark for employing hard start as a generic title.

Such a kit can be used to replace a faulty start capacitor and potential relay where an exact replacement is not available. The start kit can also be used to replace an internal centrifugal switch that has failed in a specialized motor that is no longer available. (See the fourth and fifth items below for high starting torque applications.)

Hard-start kits can also be used to soup up the starting torque in a permanent split capacitor (PSC) motor, such as employed in some small hermetic compressors.

The following comments apply:

  • Not all start kits are solid-state. While some employ thermistors, as described below, at least one company makes a kit employing a potential relay. In this case, the potential leads are connected across its capacitor instead of the usual connection across the start winding itself. The kits are generally made in various ratings for the different motor sizes.

  • The solid-state kits employ a thermistor as a current control instead of a potential relay. The thermistor is a solid-state resistor-like element that has a low cold resistance. But, under the flow of a high starting current, it heats up and develops a high resistance when hot. This cuts off flow of the large starting current through the capacitor and the start winding. The resistor is also known as a PTC, or positive temperature coefficient resistor.

  • The solid-state hard-start kit does not need a bleed resistor across the capacitor because the hot thermistor still has a high, but not infinite, resistance. Therefore, it acts just like the bleed resistor in a potential relay situation. The kit having an internal potential relay does have a bleed capacitor.

  • The solid-state start kit thermistor acts more like a timer in cutting off current flow to the start winding after a brief predetermined time period. Therefore, if applied to a motor with a high inertial load that gets up to speed slowly, it is likely to cut off current flow to the start winding before the motor gets up to the pull-in speed.

  • By contrast, a motor or a start kit with a potential relay would not be a pull-in and open the starting circuit until the correct motor speed, accompanied by the correct pull-in voltage, was reached. Therefore, in such high inertia, slow starting applications — such as pulley-driven blowers — a solid-state start kit might not produce reliable starts.

    When To Change The Oil

    QUESTION:
    By Frank Schiller
    Chandler, AZ

    I read once that you do not have to change the oil from a mineral to an alkylbenzene if the evaporator is running at or above 0 degrees F. I also have read that this is true in packaged units, but that in a removal installation you do need to change to the alkylbenzene oil.

    My situation is that I installed two ice cream machines with remote belt-driven compressors 15 ft above the evaporators. These were units from another store, each containing a holding charge of R-12. The refrigerant piping was 1-1/8-in. suction line and 5/8-in. liquid line, with both suction lines being trapped.

    If I was to change the oil and use

    R-409A and then had a leak in the system, I would have to recover and weigh the refrigerant to see if there was not more than a 30% loss. If less than 30% was lost, I would be able to add to the charge. If more than 30% was lost, I would have to charge with virgin R-409A.

    With a system charge of 11 lb in each machine, the probability of a leak at the shaft seal was pretty high, given how old these units were. I therefore elected to stay with R-12. My figures indicated this was the least costly way to go.

    Did I do the right thing? If not, what should I have done differently?

    ANSWER:
    From Gus Rolotti
    Atofina Chemicals
    Philadelphia, PA

    Oil circulation with Forane® R-409A is almost as good as that of R-12 with mineral oil. Packaged systems, because of their compact size, tend to have better oil circulation than remote systems. In general, if you have a remote system where oil circulation may be a problem, then using R-409A will not improve the situation.

    If your oil circulation is adequate with R-12, then it will probably be adequate with R-409A as well. Below 0 degrees F, the miscibility of the oil and the R-409A refrigerant may be under the minimum necessary for your system. Therefore, we recommend replacing some of the oil with alkylbenzene to ensure continued adequate oil circulation.

    You also had a question regarding leaks of 30% or higher. We have conducted testing in our labs in which we allowed the system to leak slowly 30% of its charge in the vapor phase. Next, we topped off the system

    with fresh refrigerant. We repeated this cycle five times. We discovered that at the end, even though the refrigerant was out of spec (as expected), the composition was so close to that of the original R-409A that the resulting performance changes in a system would have been very small.

    We concluded that topping off a system after a leak was not so bad as originally thought. If the leaks are in the liquid phase, little or no decomposition would occur.

    I agree with you that with a shaft seal in an open drive compressor, you have more chances for leaks. However, if the system is operating properly, the seals should work fine.

    If your customer was happy after you finished your work, I would say you did the right thing. However, you need to consider future costs to repair the system now that R-12 is becoming more and more expensive. At some point, you will probably be better off substituting the R-12 with R-409A.

    Motor Service Factor

    QUESTION:
    By Jon D’Agostino
    San Mateo, CA

    How is motor service factor determined and how would you apply it in the field?

    ANSWER:
    From Terry Glass
    A.O. Smith
    Tipp City, OH

    The service factor is the number by which the horsepower rating is multiplied to determine the maximum safe load that a motor may be expected to carry continuously.

    That is taken from a publication in the A.O. Smith Motor Mastery University (titled “The ACs and DCs of Electric Motors”) that covers the issue in detail. It can be obtained by contacting A.O. Smith at 937-667-6800.

    Auto Air Conditioning

    QUESTION:
    By Ralph White
    Danville, CA

    In recent years, I have read numerous reports in automobile and trade magazines which state categorically that closing the windows and turning on the air conditioning in a modern automobile during hot weather will not increase fuel consumption. In fact, some declare this will increase gas mileage.

    However, in the June 2000 issue of Kiplinger’s Personal Finance Magazine, a statement says just the opposite. On page 104, the author of an article on keeping auto operating costs down states, “Roll down the windows. Shutting off the car’s air conditioning reduces fuel use by 10% to 20%.”

    My question is, “Who’s kidding whom?”

    ANSWER:
    From Daniel Kramer
    Patent Attorney and Specialist Grade Member of RSES

    First of all, be assured that you get nothing for nothing.

    Running the a/c takes horsepower and you must provide extra fuel for the engine to provide the extra horsepower.

    However, you have introduced another variable: Does running the car with the windows open require more fuel than running it with the windows closed? I have read reports of how the auto manufacturers test the fuel economy of their cars, and they invariably do it with the windows up and with the a/c off.

    Now, at high speeds, a large fraction of the engine horsepower is used to move the car against air and road resistance. Therefore, the relatively small amount of energy required to operate the compressor and condenser fan may not degrade the fuel economy noticeably. In fact, the energy saving from closing the windows may offset the energy used by the compressor and condenser fan motor. So, at high speeds, you might actually observe better fuel economy with the windows shut and the a/c on. Of course, with the windows closed and the a/c off, you would observe still better fuel economy.

    However, when running at low road speeds or in traffic, wind resistance is a minor factor. So closing the windows would not have a significant power-saving effect.

    Further, at low speeds the energy consumed by the compressor and condenser fan motor could comprise a large fraction of the total energy output of the engine. So running the a/c could sharply degrade your city mileage.

    Therefore, expect your city fuel economy to drop sharply when you run the a/c. On the highway, your best fuel economy is secured running with the windows shut without the a/c.

    Run the a/c if you have to. But always remember, you get nothing for nothing. So in hot weather, you’d better grit your teeth, run the a/c, and be willing to pull up to the gas pump more often and say, “Fill ‘er up.”



    Sidebar: Contacting The Service Hotline

    Do you have a technical question for the pros? Submit your Service Hotline questions online at The News home page. You may also contact refrigeration editor Peter Powell at 847-622-7260; 847-622-7266 (fax); or peterpowell@achrnews.com (e-mail).

    Publication date: 06/03/2002

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