ACHRNEWS

Btu Buddy 79: Lunchtime Lesson on Dew Point

October 19, 2009

Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers from the same confusion that all technicians occasionally do - the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminds him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.

Their last service call is continued as Bob and Btu Buddy meet for lunch at the restaurant where the call occurred. The duct through the dining room was dripping from either high humidity or discharge air that was too cold. The service call showed that the duct surface temperature was too cold because of reduced airflow, which lowered the leaving air temperature. The duct surface temperature was below the dew point temperature of the room and moisture formed on the duct surface.

Bob said, “Well, I see that the duct is not dripping today. That is a good thing. The manager said that it stopped dripping shortly after we left.”

Btu Buddy added, “That is a good sign that you knew what you were doing. Let’s get down to talking about dew point and why it is important.”

Bob said, “I seem to want to talk about humidity and I hear others talk about dew point when discussing air moisture content.”

Figure 1. This illustration shows the amount of moisture that a pound of air can hold when 100 percent saturated with moisture (A) and 50 percent saturated with moisture (B). A pound of air occupies about 13.5 cubic feet at standard conditions. (Figures are from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Cengage Learning.) (©Delmar Cengage Learning.) (Click on the image for an enlarged view.)

Btu Buddy responded, “They both pertain to the moisture content in the air and both have their place in the conversation. Humidity describes how much moisture there is in the air compared to how much moisture it can hold. In scientific terms, moisture content can be expressed in actual weight of moisture in the air. For example, a pound of air at 70°F can hold 110.5 grains of moisture when it is totally saturated with water vapor. This is called 100 percent humidity; it won’t hold any more moisture. The same pound of air at 50 percent humidity will contain 55.25 grains of moisture (Figure 1). When you look at a psychrometric chart, you will find that air at this condition has a dew point temperature of 50.5°.”

Bob then asked, “How much volume does a pound of air occupy?”

Btu Buddy said, “Great question; at standard atmospheric pressure, 1 pound of air will occupy a space of approximately 13.5 cubic feet. Now, let’s explore dew point.

“The dew point temperature contains the word ‘point.’ That implies that this is a point of departure. Let’s order a glass of water and a glass of ice to use as a demonstration. This restaurant has an icemaker that produces flake ice. That is what we want because we want the ice to quickly melt in the water.”

When the glass of water and glass of flake ice arrived, Btu Buddy took out his pocket thermometer and placed it in the water. The temperature read 71°. Btu Buddy then asked Bob to check the temperature and humidity of the room air.

Bob said, “The air temperature is 73° dry bulb and 61.6° wet bulb, which is a humidity of about 52 percent. The dew point temperature is 55°.”

Btu Buddy then said, “Add a few flakes of ice to the glass of room temperature water and start lowering the water temperature. Use the spoon to keep it stirred to keep the temperature constant. Keep adding and stirring until the water temperature reaches about 55° and watch what happens.”

Bob did as he was told.

Figure 2. This glass of water has ice in it and the surface of the glass is below the dew point temperature of the air surrounding it. (©Delmar Cengage Learning.) (Click on the image for an enlarged view.)

Btu Buddy said, “When the glass surface temperature reaches 55°, a slight fog of moisture should form on the surface of the glass (Figure 2). This is just a field experiment and is not really accurate to a fine degree. In a laboratory, much more elaborate equipment is used for great accuracy. A mirror surface is used so the slightest bit of moisture can be seen.”

Bob then said, “The water temperature is showing 53° on my pocket thermometer and I can see moisture in the form of sweat forming. That is pretty close. Now, what does it mean and how is it applied to our profession?”

Btu Buddy explained, “Our profession is to condition air. In this case, we are distributing the air to a conditioned space. The air has been cleaned in the filter and passed over a coil that cools the air and removes moisture from the air. When this air is distributed and mixed with the room air, it will lower the room temperature and the moisture content of the air in the room. We saw during the service call that moisture is added back to the room with the hot steaming food. Our breath also adds moisture to the room. Air from outside also infiltrates into the room and brings moisture. The broad lesson here is the air conditioning equipment must remove this moisture to a level that is acceptable.”

Bob commented, “That must be the condensate that we see going down the drain.”

Btu Buddy said, “Correct. The coil removes this moisture by operating below the dew point temperature of the room return air. The room return air in this room was 73° dry bulb and 55° dew point. The coil surface temperature must be below 55° for moisture to begin to form on the coil. Can you tell me what typical coil design temperature is?”

Figure 3. This coil is operating at design conditions of 40°F, which is well below the dew point temperature of the air passing over it. Moisture gathers on the coil surface and drips into the pan and is piped outdoors. (©Delmar Cengage Learning.) (Click on the image for an enlarged view.)

Bob said, “It is 40°, which is well below the dew point temperature in this room (Figure 3). How do we vary the coil temperature to remove more or less moisture for a particular application?”

Btu Buddy explained, “This is done in the engineering of the equipment. We have very little control over this in the field. As a rule of thumb, an air conditioning system has 400 cfm (cubic feet per minute) of air passing over a coil for each ton of air conditioning. A 3-ton unit typically moves 1,200 cfm of air (400 cfm/ton x 3 tons = 1,200 cfm). In the Southeastern coastal areas, you may find 350 cfm/ton. The lower cfm of airflow causes the coil temperature to drop to a lower temperature and remove more moisture. The temperature is lower than the dew point temperature of the air passing over it. In the dry desert areas you may find 450 cfm/ton used. More cfm will cause the coil temperature to be slightly higher and less moisture will be removed. It is already dry and there is no need to remove more moisture. Different parts of the country use different airflow per ton of air conditioning. These variables are used to control moisture in different parts of the country.”

Bob then said, “The more I learn, it seems the more there is to learn. I think I like that. It never gets dull because there are more horizons to expand my thinking.”

Btu Buddy noted, “When a technician stops learning, most likely his career will stop right there. The more that is learned, the more valuable you are to the career.”

Publication date: 10/19/2009