Coil Design

My question concerns the use of a two-ton high temperature coil (straight design) for residential cooling.

I would like to know the difference in efficiency and Btuh output when the coil is fed bottom-to-top flow vs. top-to-bottom flow. In this instance, assume that the coil has three circuits and is controlled by a TXV.

With the bottom feed design, will the coil circuits flood out due to a greater pressure drop? What about coil capacity? Will superheat settings be consistent? Will efficiency be on the downside if a bottom feed coil is installed on the negative side of the blower?

My interest is in building an economical test model for use in very high ambient using a special triple-flow heat exchanger that I hope will greatly increase the net refrigeration effect.

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

Your three-circuit, three-row coil with 3/8-in. tubing, 2 sq ft of area, and 400-fpm air velocity would have about the required capacity. I calculate about 0.25 in water airside pressure drop and about 3.25 refrigerant pressure drop with R-22.

While you don’t define the circuiting you prefer, the fact that you are questioning the merits of top or bottom feed suggests that you plan to feed the tops or the bottoms of each row with crossovers about halfway up (or down) the coil. This means that the feed that started in the first row ends up in the third row and the feed that started in the third row ends in the first.

With that arrangement, and with the relatively high refrigerant pressure drops, I suggest it would make no difference whether you feed at the top or bottom of the coil. Further, I don’t believe that the pressure drop would vary much from the top feed to the bottom feed arrangement.

By contrast, if you were to employ three circuits of 1/2-in. tube, your refrigerant pressure drop would only be about 0.75 psi. In that case, I might prefer to bottom feed.

I don’t think coil efficiency would vary much whether you put the coil on the air inlet or the air discharge side of the blower and whether you bottom or top fed the 3/8-in. tube coil. I might prefer to put the coil on the air inlet side of the blower because I might get more uniform air distribution over the face of the coil.

I would be cautious about trying to get too much improvement in refrigerating effect with a suction liquid heat exchanger. With R-22, for every 1 degree F drop in liquid temperature, the suction vapor temperature will rise about 1.8 degrees F.

Since you plan for this system to work in very high ambients, it would be easy to overheat the suction vapor, possibly causing compressor problems.

I would certainly test your pro-posed system and get the compressor manufacturer’s approval before I made very many of them.

Unloader

I have a 10-ton Carrier compressor with an unloader, split evaporator, and coil; one of which is operated by a thermostat. The condenser is on the roof with the evaporator in the basement. In the first stage, the coil is freezing when the compressor is running unloaded with 50 lb back-pressure. (One fan is off on the condenser due to mild weather.) When the compressor is running fully loaded, there is 68 lb of back-pressure with both coils operating.

ANSWER:
From Doug Bates
Carrier Corp.

I assume this is a four-cylinder compressor with a 50/50 split evaporator coil.

At first, it sounds as though you might have a restriction in your first stage. You may have a blockage in your TXV, cap tube, nozzle, filter-drier, liquid line service valve, or elsewhere.

Next, I might ask you about your airflow and load. If I understand your question correctly, you have a split evaporator with a solenoid valve that shuts off. I’ll assume this is in the top half of the coil when the load is light.

In this scenario, especially when you have fairly low airflow and high humidity, you will find that the bottom half of the coil will be wet and the top dry. The wet half of the coil has a greater airflow resistance. Because of this, more air will bypass the lower half and travel through the top half. The less airflow, the more moisture; the more moisture, the less airflow. As the airflow through the activated half is steadily reduced, the suction pressure lowers, resulting in lower saturated suction temperature and eventually freezing the lower coil. With the full coil activated, the airflow through both halves is enough to keep you going.

Is the unloader operated by suction pressure or is it an electric unloader? Are you sure it is actually unloading? If the unloader were inoperable, you would have 100% compressor capacity working with an evaporator at 50%. As you might imagine, the suction pressure would drop significantly. If it is a suction unloader, you may consider replacing it with an electric unloader that is switched with the evaporator solenoid coil.

At the beginning, I assumed a four-cylinder compressor and a 50/50 split coil. If, for instance, you had a 40/60 split, and the unloader was working, you could have 50% compressor and only 40% evaporator.

Hot Gas Bypass

We have a hot gas bypass and heaters to defrost. What would be the purpose of both in a low-temp walk-in freezer? Also, only the defrost timer — and not the Klixon — was hooked up on the evaporator for temperature termination. Were those that did the installation cutting corners or were they trying to achieve something else?

ANSWER:
From Gene Silberstein
Consulting Engineer
Whitestone, NY

The reason for having both hot gas and electric defrost is mainly to save energy and efficiently defrost the evaporator coil.

In most cases, hot gas defrost is adequate to effectively defrost the coil. However, when large amounts of product are added to the box, or when a high load condition exists, the humidity in the box can drastically increase. This increases the amount of ice that forms on the coil’s surface. It is under these circumstances that, after a predetermined period of hot gas defrost, the electric heaters will energize to complete the defrost cycle.

Since electric defrost is generally more expensive to operate than hot gas, it is used primarily as a supplementary defrost only when needed. It is common, however, to have both the hot gas and electric defrost operating at the same time when used on ultra- low-temperature systems.

The defrost termination switch should be strapped securely to the suction line at the outlet of the evaporator. This device will de-energize the electric heaters when all of the ice has melted from the coil and put the system back in the refrigeration mode. The defrost termination switch also prevents the evaporator fan from operating until the evaporator has cooled down to the desired temperature after defrost. If the Klixon is presently hanging, chances are that the evaporator fans are operating while the evaporator is still warm, circulating this warm air throughout the box.

Publication date: 05/06/2002