Service Hotline - April 2, 2001: Sight Glass

The used condensing unit that I sold him had a receiver. The original did not. I chose to leave the receiver in place. That allows me to take advantage of the built-in liquid line valve. The original had none.

I charged this unit with “extra” refrigerant to make up for the additional receiver charge. I knew charging this unit would be tricky with the additional receiver space. The cooler worked well after my installation. But, can the receiver cause problems in the capillary tube system — especially knowing that cap tubes typically have bubbles in the liquid line and a receiver can create a liquid seal?

Despite the fact the original compressor had acid due to the burnout, I chose not to install a suction line filter.

My thinking is that the condensing unit I sold has a clean oil charge, and I installed an oversized solid core liquid line filter. The old compressor had 10 of its 11 oz of oil charge still contained in the old compressor. It appears to me that the makers of suction line filter-driers do not market driers for the small (1¼4-hp and under) compressors. I checked the cooler after one week of run time. The unit holds 38°F and cycles on and off normally. My head pressure runs about 205 lbs and the suction runs around 7° at a pressure of 32 lbs.

Your opinions on this repair job will be appreciated.

ANSWER:

By Sherri Wilkerson Service Training Specialist Copeland Corporation Sidney, OH

To utilize a receiver on a cap tube or any other fixed orifice system invites migration and/or floodback problems. A receiver is to provide additional refrigerant storage and supply on a system that has a varying load. The system change will now require more refrigerant to clear the sight glass. The cap tube will not prevent floodback during operation or migration of vapor refrigerant during the off cycle. This is not a recommended practice.

Any time acid is in the system, the suction filter-drier should be used to clean the oil returning from the system that has not been replaced. Chances are, oil logged in the evaporator, etc., will return when you start the system back up and achieve a high mass flow.

It is true that the new condensing unit has an oil charge in it, but the oil that returns may contaminate it if the acid is not removed. Remember that the suction filter-drier should be replaced after 72 hours maximum or a 2°F temperature drop from when new, across the filter. If the system is still dirty/acidic, then change filter-drier oil and repeat until system is clean. You could then remove the filter-drier and use a filter only or nothing in the suction line.

By adding the oversized liquid line filter, the refrigerant charge will need to be more to clear the sight glass. This “extra refrigerant” is only adding to a recipe for disaster with this cap tube system.

In short, this system may be working well for the time being. However, it is highly recommended that these oversights be corrected before they cause your next compressor failure.

Sight Glass

QUESTION:

From ‘Look 4 Libby’ (screen name) Via the Internet

I am a subscriber to The News and I have been working in the hvacr field for a little over seven years. My question pertains to the use of field-installed sight glasses as a means to effectively charge an air conditioning system. Many are under the assumption that charging a system with a TXV metering device is the only time that you would charge to a full sight glass and that you could not do the same with a fixed-orifice-type metering device. Regardless of what type of system, I am aware that this type of charging method is only a tool, and you must incorporate other factors as well. Any input on this subject would be helpful and appreciated.

ANSWER:

By Sherri Wilkerson Service Training Specialist Copeland Corporation Sidney, OH

The thermostatic expansion valve (TXV) provides an immediate pressure/ temperature drop only if it is provided adequate refrigerant as a liquid. If the refrigerant is already a vapor entering the valve, then it does not achieve the saturated state desired to maximize heat transfer. The TXV is used on systems with a varying load, so we may have an additional charge for the receiver. Consider that this system must have enough refrigerant at high load to handle the load and maintain a liquid seal up to the TXV. Charge at high load by subcooling (clear the sight glass).

Here is another thought. It is important for liquid to be at the TXV, but the sight glass is at the receiver outlet. It could be possible for the refrigerant to flash in the liquid line prior to the TXV, reducing the valve capacity. It may be advantageous to add another sight glass right before the TXV to ensure that the valve receives liquid.

The fixed-orifice metering devices are critically charged systems; in other words, there is no load variation. The cap tube, for example, works off the principle of a constant pressure drop. There are no restrictive components to prevent liquid floodback during operation or vapor migration in the off cycle if this system is charged with “extra” refrigerant. At low load, there should be just enough refrigerant to handle the load without floodback. (A TXV would throttle down at low load.)

Keep it simple: Charge at low load by superheat in the evaporator.

OEMs

QUESTION:

From Don Vega Via E-Mail

I was wondering what and where would I go when I am asked to contact OEMs. How would I contact them?

Also, sometimes when I am working on a system malfunction, I can’t read any of the system’s vitals, such as refrigerant type, charge, voltage, amps, model, etc., because the sticker or plate is unreadable. Where do I go from there? How do I know what to do other than taking guesses?

ANSWER:

By Peter Powell Refrigeration Editor The News

I apologize if what follows seems like a blatant plug, but the best source I know of for contacting OEMs is the HVACR Directory & Source Guide, published by Business News Publishing. It is issued as the first issue each year by The News. It lists manufac-turers and includes names, addresses, phone numbers, websites, and key personnel.

Regarding “old soldiers” whose nameplates are missing or unreadable, I would suggest still going to the manufacturers for help. Some of the motor manufacturers have material on how to deal with service-related questions. I would certainly try to eliminate as much guesswork as possible — and that may mean giving the “old soldier” a decent burial.

Refrigerant 411B

QUESTION:

From V. Sridhar Bahwan Engineering Co. Sultanate of Oman

One of our customers has decided to charge R-411B in place of R-22. I need the properties of 411B. The local supplier claims the refrigerant will realize 30% power savings vs. R-22

ANSWER:

By Jim Lavelle National Refrigerants Philadelphia, PA

You should be able to get the properties of this refrigerant from the local supplier you mentioned or from the manufacturer, Greencool, directly.

R-411B is a blend of 96% R-22, with 3% propylene and some R-152a. Basically it is R-22 with ‘additives’ to help it run better. Thermodynamically speaking, you can not get 30% better efficiency from this blend compared to R-22 in a clean, well-maintained system.

The benefits of this blend come from the addition of the hydrocarbon, which will thin the oil, clean the heat transfer surfaces, and promote better oil return. Adding the R-152a will help lower the discharge pressure and temperature somewhat, reducing wear on the compressor. For all practical purposes, however, you are replacing R-22 with R-22. You might better spend your money on servicing the system (clean the heat exchangers, change the oil) and putting R-22 right back in.

R-411B has an ASHRAE Standard 34 safety rating of A1/A2, which means the blend is nonflammable as packaged, but it could separate into a flammable mixture during a leak. In the United States, this safety rating will limit the amount of this blend that can be used in residential or commercial air conditioning equipment. In some cases, it is not allowed at all. Other countries around the world may have different codes regarding the use of this type of refrigerant blend.

SEER

QUESTION: From Alan Gibson Bahlonega, GA

When a unit says “10 SEER,” what does SEER mean? What is the difference between 10 SEER and 8 SEER in regards to the efficiency?

ANSWER:

By Rick Roetken Manager Product Marketing Air Conditioning Systems Carrier Corp. Indianapolis, IN

SEER stands for Seasonal Energy Efficiency Ratio and is the measure by which the hvac industry rates central air conditioners. More specifically, a product’s SEER rating measures BTUs of cooling that are produced per watt of electricity consumed. When the SEER rating increases, you get more cooling for the same number of watts. For example, a 10 SEER unit is 20% more efficient than an 8 SEER system.

In 1992, a federal government raised the minimum efficiency standard for central air conditioners and heat pumps to 10 SEER, or 10 BTUs of cooling per watt of electricity. Before 1992, the minimum efficiency rating was 8 SEER. Today, Carrier offers energy-efficient central air conditioning systems that deliver greater than 17 SEER when properly matched with the correct indoor fan coil.

Got A Technical Question For The Pros?

You can submit your Service Hotline questions to The News several ways. Just log on to our website, www. achrnews.com and click on Service Hotline. You can also contact refrigeration editor Peter Powell at 847-622-7260; 847-622-7266 (fax); or PowellBNP@aol.com (e-mail).

Publication date: 04/02/2001