ACHRNEWS

Service Hotline: More on Vent Residue, Ammonia Sensors, Condensation Problems, Training, Residential A/C and Pulling a Vacuum

September 11, 2000

More on Vent Residue

Comment:

By Michael Woolsey KE Fibertec North America In the April 3 Service Hotline section, a question was asked about vent residue. While keeping filters clean and increasing their efficiency seems like the logical solution, it overlooks a major, if not dominant, cause of smudging: particles in the room air.

Traditional mixing diffusers work by mixing supply air with room air. As supply air is blown out of a diffuser, room air is induced upwards, toward the center of the diffuser. The high-velocity stream of supply air pulls air from the room and mixes it near the diffuser.

This circulation pattern is what makes supply diffusers work. This same circulation pattern also causes particles in the room to be drawn to the face of the diffuser and the ceiling surfaces adjacent to the diffuser.

Over time, particles from the room air collect on the diffuser and the adjacent surfaces, contributing to smudging.

So, even the best maintenance plan may not prevent diffuser smudging if the room air contains particles anyway.

On the other hand, displacement diffusers work by creating a piston-like movement of air downward in the space. Without induction, displacement diffusers don’t pull room air particles toward the ceiling, greatly reducing smudging.

Some displacement diffusers are constructed of fabric, so that even if they do become smudged, they can be easily removed and washed.

Comment: By By Warren T. Hanna Worchester Air Conditioning Co. Framingham, MA

Regarding your April 3 Service Hotline section, a question was asked about the discoloration of ceilings immediately around the discharge area of the grille (diffusers).

Nearly 50 years ago, I had this problem, and tried various solutions, including an oil-soaked cloth in the air stream. This caused me to suspect that the contaminants were not from the primary air.

I then received an answer, I believe from Carrier, that indicated the dirt was not coming from the primary air, but from the secondary air (air in the room) that was being induced into the primary airstream as it came from the diffusers. The secondary air would rub up against the ceiling and deposit dirt at that point.

The likely culprit is the dirt that enters into the room from outside sources and is further fragmented by an inefficient vacuum cleaner, causing these particles to become airborne and part of the secondary air.

Comment: By Greg Tschida Air-Side Equipment Co. Metairie, LA

The April 3 answer on diffusers was partially correct. Filter selection and filter maintenance are factors. But maintenance of the space is too, as is the performance of the air distribution outlet.

The performance of the diffuser is the main factor in “smudging.” An improperly designed and constructed diffuser will create a lower pressure area and induce secondary air (room air) into the primary air stream. This could bring with it contaminants from the space and cause the smudging.

Thanks for a great publication. I enjoy it each week and use many of your articles to educate and inform. Keep up the good work.

Comment:

By Joe Turner Gaithersburg, MD In the April 3 issue, Dan Stanton asked what causes the “dirty streaking residue” on ceiling grilles and the ceiling around the discharge area of the grilles. While the answer that he was given is indeed true, the streaking that Dan is referring to is created even with ultra-clean supply air.

As the air from the diffuser (primary air) is blown out of the diffuser and into the space, the room air (secondary air), which is often full of dust particles, becomes entrained by the primary air and creates a swirling pattern that causes the secondary air to sweep the ceiling and deposit dust particles.

This condition is most noticeable in buildings that are under a negative pressure, such as a restaurant with exhaust fans and inadequate make-up air. This causes dirty, unfiltered air to enter the space through doors, etc. Correcting the air pressure in the building (thus keeping the dirty air from being sucked in through the front door) will greatly reduce the streaking.



Ammonia Sensors

Question:

From Phyl Leah Speser Via e-mail I am a contractor working on a new ammonia sensor with application in industrial refrigeration. As part of our work, we have to provide information on the downstream market for the technology. Thus, I am trying to get a handle on how many ammonia sensors are used in industrial refrigeration in America or even on how many industrial refrigeration units there are and whether the market for industrial refrigeration is growing or shrinking.

Answer:

By Joe Pillis York International Waynesboro, PA

Ammonia sensors are required to be installed in equipment machinery rooms per ASHRAE Standard 15-1994. I have no idea how many new ammonia engine rooms either go in or have to be brought up to code each year, but I suspect about 1,000 per year in the U.S. would not be out of line.

If you want further information, I would suggest you talk to current manufacturers of ammonia detectors, as there are many commercially available units in the market today. Industrial refrigeration in the U.S. is a well-developed market that is not really growing or shrinking but is certainly alive and well.



Condensation:

Question: From Loy Lobo Via e-mail

We are experiencing a case of severe condensation around the insulated suction line copper pipes. These pipes run between the fancoil unit and condensing unit of a direct expansion 1.5- or 2.0-ton split unit.

The condensing units are located out in the open. The units are installed for a residential application on a small island.

The pipes are installed in a corridor area which is not air conditioned or ventilated. The corridor area gets very humid several months a year. The floor of the corridor is built out of a natural limestone type of soil and appears to absorb the moisture from the sea water table below. In general, the area, being an island, gets very humid.

The copper pipes are insulated with 0.5-in.-thick close-celled insulation. The brand name is Armaflex. The pipes are taped together with the liquid line pipe, which runs alongside it.

The ambient reaches 43°C and humidity levels reach 90% to 95%. In normal applications (for example, at locations not on the island), the pipes do not condensate as in the above case when insulated with the above method of insulation.

We have now added two layers of fiberglass roll insulation 1-in. thick each, 24 kg/cubic meter density, wrapped with 4-oz. canvas cloth and sealant. Despite the increased thickness, the condensation continues to occur.

Answer: By Gene Silberstein Consultant

First let me clarify that con-densation is the moisture that forms on a surface that is below the dewpoint temperature of the surrounding air. The dewpoint is governed by humidity, as well as the temperature of the air. As the humidity increases, the dewpoint temperature also rises until, at 100% humidity, the dewpoint temperature is equal to the temperature of the air. At this point, the moisture quite literally falls out of the air. In an area of high humidity, therefore, condensation becomes a major problem.

There are, however, a few things that can be done to alleviate, or at least minimize, the situation.

By increasing the temperature of the suction line, you will reduce the amount of condensation on the line. The easiest way to accomplish this is to run the liquid line in direct contact with the suction line before insulating the line. The liquid line will now be on the inside of the Armaflex, along with the suction line. By doing this, the suction line will absorb heat from the liquid line, increasing its temperature while at the same time decreasing the temperature of the liquid line, thereby increasing the amount of subcooling in the liquid line. This provides a dual benefit.

It also appears that the seams between the Armaflex sections are not properly sealed. Creating a completely sealed insulation surface will greatly reduce the amount of condensation. A commonly used adhesive is Armstrong #520. It should be applied to the joining sections to create a continuous run of insulation. (This stuff smells real bad, so be sure to use in a well-ventilated area.)

By properly sealing the Armaflex, you can probably remove most, if not all, of the other fiberglass and canvas wrapping that is presently around the line.

One final solution, which may or may not be possible, is to condition the corridors. By reducing the temperature of the corridors, increasing the temperature of the suction line, and properly sealing the insulation, your condensation problem should be gone.



Training:

Comment: By Timothy Collier Lansing, IL I have some comments on previous discussions about where technicians can find the training that would be most useful.

I think the technician is responsible for his or her training. You need to acquire information from the field or from schools. You need to go to the programs that supply houses and manufacturers put on. Unions will provide training to some extent.

You can also check with the company you work for to see if the company will sponsor some training or schooling for you.



Residential A/C

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 need to take into account the indoor return air wet bulb 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°.

Answer: 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.



Pulling a Vacuum

Question: From Ted King Alpena, MI

Using a vacuum pump to remove air and moisture, it is necessary to use two tap valves for evacuation? I do, but a lot of service technicians don’t.

I know the cap tube is a small diameter device quite long in length. Can all air and moisture be removed from a low tap only? I’ve read articles that two valves should be applied to remove air and moisture from the entire system.

Answer:

By John Healy Past International RSES President Colorado Springs, CO It is most important to use evacuation from both the high and low side at the same time. This method not only saves time, but also is more through.

Yes, pull a complete vacuum by using two valves pulling from the high and low sides at the same time. It is the only way to completely prepare a system for recharging.