Service Hotline: Vent Residue, Plugged Cap Tubes and Pipes Arrangement

Vent Residue

From Russ Donnici Indoor Air Diagnostics Inc. San Jose, CA

I read the April 3 Hotline question relating to vent residue and the answer that linked the problem primarily to the efficiency rating of the air filters. I also read the comments in the June 5 Hotline, which agreed with that and also added that all air has dirt and the filtered supply air mixing with the unfiltered ambient room air is also a significant problem.

I agree that filter efficiency and cleanliness of the ambient room air can have some effect. However, no one has mentioned excessive air diffuser velocity. The focus of our business is the diagnosing of sick buildings as well as assessments of buildings where the perception is that there is an indoor air quality issue. Since the occupant easily sees air diffuser streaks, the leap is then made that they are breathing air that is filthy. Particulate air measurements for respirable particulates do not support that claim. However, the perception is still there and the problem needs to be resolved.

We have diagnosed hundreds of buildings in the more than 10 years we have been in business. When the issue of register streaking is brought up or noted during our review, we have found that in only a few rare cases that the streaking has been solely or primarily linked to poor-quality filters or dirty ambient room air. These two things can contribute to the problem.

However, 90% of the time we have found the problem to be excessive air register velocity. The excessive air velocity impacts the particulates in the airstream (all airstreams contain particulates) directly against the register and/or the ceiling. The combination of the airstream particulates and the ambient room air particulates can and will exacerbate the problem.

However, we have found that when the air velocity issue is corrected, the problem is resolved. The mixing of the room air with the airstream and its resulting distribution is referred to as the coanda effect.

We have found that when air registers are selected using the ASHRAE ADPI (Air Diffusion Performance Index) criteria, the problem of streaking does happen. In addition, it is important to remember that vav systems without vav box velocity controls can cause excessive air register velocity during low overall system load conditions with one or two zones calling for their full cfm delivery. The Titus Company website (www.titus-hvac.com) is a good reference for the ADPI criteria.

Plugged Cap Tubes

From Marcus K. Maclean Via e-mail

Over the past year, I’ve been encountering quite a few self-contained commercial reach-ins and sandwich tables (and one ice machine as well) in which I’ve been finding the capillary tubing either 100% or partially restricted. In the 20 years I’ve been in the refrigeration trade, I have only encountered about two plugged cap tubes that were used with both R-12 and R-502.

I’ve been told by the oem’s that when the condenser coil is not kept clean (How clean is clean?), the rise in the condensing temps is directly related to the rise in the POE oil temperature, which is somehow generating matter that seems to get past the filter-drier.

They’ve told me that the plugging of the cap tube happens over a long period of time.

But more importantly, is this problem just confined to the R-134a and R-404A systems I’ve been encountering? At present, I have two units in my shop needing their cap tubing replaced. Also, I’m going to be replacing another cap tube assembly in the field shortly. In the last year, I’ve found four instances of having either plugged cap tubes or a plugged TXV on an ice machine.

Since I get and read the News every week, I’d like to see an article or two about this problem. I believe it is more widespread than just here in Northern California.

I’d like to know just how many other contractors are seeing this equipment problem and what do the oem’s have to say about this ongoing problem.

Pipe Arrangement

From Vincent Cormier Via e-mail

We need a comment on a piping arrangement. We have installed a freezer (-20°F). The 40-hp semi-hermetic compressor is inside, just three feet above the receiver, which is at the ground level. The air-cooled condenser is 6 ft above ground level.

The piping leaves the compressor horizontally, travels through a vibra, oil separator, and check valve, then goes up 20 ft straight under the ceiling. After a 25-ft horizontal run, crossing a loading deck, the piping comes down to the condenser inlet. At the outlet of the condenser we installed a P-trap.

The piping returns up 20 ft straight under the ceiling, again with a 25-ft horizontal run, and comes down to the receiver inlet. The piping includes a head pressure-regulating valve, receiver pressure-regulating valve and ball valves.

The questions are:

Answer:

From Daniel Kramer Specialist grade RSES member and professional engineer

Piping the condenser outlet vertically upward for 20 ft suggests that a static pressure head could be imposed at the condenser outlet in addition to the pressure drops through the piping and valves.

At about 100° liquid temperature, the liquid density of R-404A (your refrigerant) is about 60.5 lb/cu ft. Therefore, the riser at the condenser outlet will cause a pressure gain of about 0.4 psi per foot rise. That’s a total of 8 psi for a 20-ft riser.

In the worst case, assuming the downward flowing leg is mostly vapor that cannot siphonically offset the pressure gain from the condenser outlet riser, the 8-psi pressure gain at the condenser outlet should only cause a 1.2° rise in the condensing temperature.

However, I noticed in the photos you provided that there is no service valve in the winter control bypass from the discharge line to the receiver. This bypass contains the spring-loaded check valve. I have found it always wise to provide such a service hand valve in the bypass allowing the system to operate even if the bypass check sticks open or otherwise fails.

Further, should you have occasion to open the system, I would install a purge valve at the top of the receiver opposite the liquid return from the condenser. This could be installed in a tee underneath the relief valve. In a system of this sort, noncondensibles, if any, would collect at that point.

You would notice noncondensibles because they cause liquid refrigerant to back up into the condenser, thereby raising the head pressure but simultaneously causing a high degree of subcooling in the receiver.

Publication Date: 10/02/2000