Service Hotline: Residential A/C Ideas, Connecting Circuits, Motor Sizing, Does Dry Ice Help?

September 11, 2000
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Residential A/C Ideas

Comment: By T. Bush Via E-mail

I have some comments on charging residential a/c. This relates to previous discussions of this topic. It is correct that charging by just adding 30°F to ambient will not yield a correct charge on most systems. The correct use of this rule of thumb is to add 20° for high-efficiency, 25° for medium-, and 30° for older, low-efficiency units.

But this only gives you an idea of the range the head pressure should be in. It is not the way to charge a system.

But, why only put 10 psi in the system and then turn on the unit? This would be a waste of time. Why not look at the unit data plate, and if it calls for 6 lb of refrigerant, put 4 or 5 lb in to start with and then fine-tune the charge?

I also question the idea of charging until there was a suction superheat of about 20° or until the suction line got cold, but not flooding back. First of all, if there is a fixed bore-metering device, then a superheat slide rule/charging chart needs to be used. One needs to take into account the indoor return air wetbulb temperature. The correct superheat on a fixed bore can vary from 5° to 35°, depending on the indoor and outdoor temperature.

I disagree with adjusting the TXV on a new unit to adjust the charge. This is not a refrigeration system. The TXV on a residential a/c system has been properly set at the factory. If it needs adjusting, then it needs to be replaced.

These valves don’t arbitrarily vibrate themselves out of adjustment, and even if one did, it will just happen again. Most manufacturers recommend charging a TXV system by subcooling. Carrier/Bryant prints the correct subcooling on the condensing unit data plate. Others provide charging charts in the equipment literature.

When dealing with an older system, where the air handler and condensing unit may not be matched with the system, I realize there are no hard and fast rules on the proper charging. But if it is a fixed-bore system, superheat charts still work quite well (excluding other problems such as low airflow, inefficient compressor, or dirty coils). If an older unit has a TXV, an average sub-cooling is around 10° to 15°.

Reply

By Daniel Kramer Specialist Grade RSES Member and Professional Engineer

There is a problem if the technician does not know the factory charge. If the system is an older one, it may have been made before there was a requirement for the charge to be listed in the label. Most older systems had TXV expansion devices.

Regarding your idea to start with a 4- or 5-lb charge, your technique may save a little time, but for a beginner I still feel, that more would be gained in charging accuracy and control by starting with a slim charge and proceeding from there.

Regarding your idea to use a superheat slide rule chart, I spent years charging systems by pressure and feel. I still do it that way, even though I have a modern Concept Technology “4-in-1” superheat gauge with plug-ins for various refrigerants. I believe this technique can and should be learned. Even with modern instruments, it prevents gross mistakes.

Further, I strongly disagree that factory TXV settings should be sacrosanct and that the valve should be replaced if it is merely out of adjustment. After all, the last technician on the job may have been a less-informed technician who misadjusted the valve. Before replacing it, why not try, at least, to correctly adjust it? I would.



Connecting Circuits

Question: (Name Withheld by Request) Raleigh, NC

Can the neutral legs of two or more 24-V circuits served by different power sources be tied together? Here’s the scenario: An office building for which we provide hvacr service has a heat box that is powered off the 277-V lighting system. A transformer within the heat box steps down the voltage from 277 V to 24 V for control purposes. We’ll call this control circuit #1. Some additional 24-V controls (a relay and contractor) were installed inside this heat box, but were powered from another transformer in another part of the building. This other transformer was 120 V on the primary stepped down to 24 V on the secondary (control circuit #2). My co-worker says that the neutral legs of control circuit #1 and control circuit #2 can be tied together because “neutral is neutral.” I say this would be illegal and against electrical code because both circuits utilize a different power source. Is he right, or am I right?

Answer: By Dave Borowski Wiegold & Sons A/C Naples, FL

Yes, the neutral legs of two or more 24-V circuits served by different sources can be tied together — if the windings are isolated and only after the two (or more) transformers are brought into phase.

Simply connecting a terminal that happens to be marked R or C to another R or C can cause a very undesirable burnout of one or both transformers. The designations on a transformer are for convenience purposes, none other. In other words, you can unknowingly hook up an R from one transformer to a C from another and cause a short.

I’m sure this may sound confusing at first. Those technicians with oil experience know exactly what I’m speaking of in terms of phasing in two transformers (i.e., one with a VA capable of handling the A/C along with the existing low voltage output on a stack control).

The directions are as follows — energize both transformers with the low voltage disconnected and test with a meter trans. #1R to trans. #2R. If any voltage is noted, do not connect them.

Pay no attention to the designations from the factory. One extra wrap of a winding or a left-handed factory assembler versus a righty will vary your results.

This method of phasing a trans-former is also a common practice when you need a 75-VA transformer on a jobsite and all available are 40 VA.



Does Dry Ice Help?

Question: From Phillip Woodall Redlands, CA

A supermarket lost power, and it was going to be off for at least 10 hrs. The store manager had dry ice spread evenly on top of the frozen food cases and covered that with flat cardboard.

One ice cream case got covered with cardboard before any dry ice was laid on the ice cream. When the power was restored and the cardboard was removed, the ice cream in the case that did not get any dry ice was no worse off than the cases that did get dry ice.

Is the latent heat of dry ice sufficient enough to do any real good? What is the latent heat of dry ice?

Answer:

By Dave Burgener Airliquide Via e-mail submission

I have been involved in the CO2 business for the last 25 years and have had quite a bit of experience with railcars and storage containers that use dry ice as the expendable chilling media. The reason that I think that the ice cream case with the dry ice was no colder than the other one was the way that the ice was used to chill the freezer.

Dry ice will cool products by both conduction and convection. If you set dry ice directly on the product it will hard freeze the ice cream within only a few inches of the dry ice. This causes the dry ice to sublime on the bottom and will release about 8.5 cu ft of cold C02 gas at -11°F that can do convection cooling.

The method described in the question was to cover the ice cream with layers of cardboard, and then place the dry ice on top. This effectively insulated the ice cream from the dry ice. This meant that there was probably very little direct conduction from the -11° dry ice to the freezer contents and maybe more to the surrounding air. In addition, the cold gas created by the sublimed dry ice could not circulate over the warming product due to the obstructing cover created by the cardboard.

I would guess that the majority of the latent and sensible heat from the dry ice went towards cooling the outer surface of the freezer and the surrounding air, and not into the thawing contents of the freezer.

Should there be a future power outage, I would suggest that others faced with similar circumstances consider placing the dry ice directly over the top of the freezer contents. In that way the top rows of product will be hard frozen by the dry ice and the sublimation gases will circulate over the rest of the product doing considerable convection cooling. Extensive tests have shown that direct contact of dry ice with frozen product will not damage the packaging or the product.

Fresh and unfrozen foods are a different matter. Dry ice can be used to provide temporary refrigeration for chilled products by placing the dry ice inside a cardboard box and setting that in the top section of the chilled space. This insulates the dry ice from the heat load in the refrigerator and slowly sublimes the dry ice to better match the heat input. This will prevent the dry ice from freezing products in the area around it. Our company has even used this method in Australia to ship heat sensitive chocolate across the desert in non- refrigerated trailers.

A note of caution: Don’t use this with fresh fruits and vegetables. The high C02 concentration will “kill” the produce. It will turn lettuce brown and end up carbonating oranges, for just two examples. It has no effect on frozen foods.



Motor Sizing

Question: From Brian Hall Lewiston, NY

In certain motor sizes (ratings), motors are usually Wye (star)-connected; while in other sizes, they are Delta-connected.

I would really appreciate knowing at what size (horsepower) are three-wire Wye-connected motors used and at what size are three-wire Delta-connected motors used.

Answer: By Richard Beard Consultant There is no answer to your question. The reason is that whether a motor is manufactured with a Wye- or Delta-connected winding depends on many factors. The motor’s size (hp) is not necessarily one of the factors.

The winding design involves the number of turns of wire in each coil and this is related to the connection used. There are limitations on the slot size in the stator in which the coils are inserted. The physical constraints make it impractical to use a Delta connection on certain motors with high voltage ratings.

On the other hand, high horsepower, low-voltage motors have few turns of wire in each coil. If Wye connected, they would require a very large size wire and be extremely difficult to form and insert into the stator slots.

The motor’s speed rating deter-mines the number of poles in the winding design and thus the number of connections which must be made. The connection, Wye or Delta, must be chosen so that there is sufficient space available for the external cables used to connect the motor to the power supply. Economical use of interconnecting cables or stator coil groups is influenced by the Wye or Delta selection.

There are many factors that the motor designer must consider when selecting either a Wye or Delta connection. For further information, I would recommend the article, “Wye versus Delta: Does the Motor Really Care?” by Richard L. Nailen, P.E. It appeared in the August 1986 issue of Electrical Apparatus, a Barks Publication.

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