The following article was taken from a larger work on psychrometrics by. Norm Christopherson COOLING WITH DEHUMIDIFICATION

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1 The following article was taken from a larger work on psychrometrics by Norm Christopherson COOLING WITH DEHUMIDIFICATION Cooling is a sensible heat process. Cooling with dehumidification is a sensible and latent heat combination. Strictly cooling air raises the relative humidity. This may require that some dehumidification be accomplished to reduce the humidity to comfortable levels. The chart in figure 8-1 shows a typical process line for cooling with dehumidification. Figure 8-1

2 The state point on the right is warmer and lower in relative humidity than the point on the left. Moving to the left on the chart indicates a reduction in temperature as well as an increase in relative humidity. Also notice that the state point on the right is higher on the chart than that on the left. As this air was cooled it moved to the left but, it also dropped to a lower specific humidity (grains) line. This indicates that the air was not only cooled but, some actual grains of moisture were removed. The number of grains removed per pound of air can be determined by finding the difference between the number of grains at each state point. The following example of a typical cooling with dehumidification process illustrates how the process works.

3 COOLING WITH DEHUMIDIFICATION Figure 8-2 The cooling system in figure 8-2 is cooling and dehumidifying the air simultaneously. This is the most common air conditioning process. This system is moving 2000 CFM of air. Dry bulb and wet bulb temperatures are taken of the supply and return air and found to be as follows:

4 RETURN AIR SUPPLY AIR DB 80 degrees DB 60 degrees WB 65 degrees WB 55 degrees The state points for each of these conditions are located on a psychrometric chart. (See figure 8-3) From the chart the following additional conditions are read and recorded. %RH about 46% %RH about 73% Grains about 68.5 gr Grains about 57 gr Notice as the air was cooled the relative humidity increased. This is due to the contraction of the air. The moisture is contained in a smaller volume of air thus the relative humidity is higher. Also notice that the actual humidity in grains decreased. The cooling coil removed grains ( ) 11.5 grains of moisture removed for each pound of air treated by the coil. The moisture removed attached itself to the coil and fins, ran down the fins and into the condensate pan where it is drained off.

5 THE SYSTEM COOLING CAPACITY Since this system is removing both sensible and latent heat the total heat formula must be used to determine the operating capacity. Using the wet bulb temperatures for the supply and return air conditions, look up the matching enthalpy for each on the wet bulb to enthalpy conversion chart found on page xxx. The enthalpy values from the chart are found to be as follows: RETURN AIR SUPPLY AIR WB 65 = an enthalpy of btu\lb WB 55 = an enthalpy of btu\lb Find the difference in enthalpy by subtracting = 6.84 btu\lb Now we know that 6.84 btu of heat was removed from each pound of air that passed over the evaporator coil. Some of this heat was sensible heat because the air was cooled, and some was latent heat because grains of moisture were removed. Now the total cooling capacity of the coil can be determined.

6 The Total Heat Formula

7 APPARATUS DEW POINT OR EFFECTIVE COIL TEMPERATURE The ADP (apparatus dew point) or ETC (effective coil temperature) is the temperature of the evaporator coil for this system operating under these conditions. The temperature is determined by drawing a straight line between the two state points and extending this line to the saturation (100%) line on the chart as in figure 8-4. The coil temp, apparatus dew point or effective coil temperature is read at the saturation curve. Figure 8-4 You should be able to read this temperature as somewhere between 47 and 48 degrees on a psychrometric chart. A very careful evaluation will show the reading to be 47.5 degrees. The coil temperature is useful in determining the coil bypass factor.

8 COIL BYPASS FACTOR The bypass factor is the percentage of air passing over the coil that is not affected by the coil. This is air that passes between the coil tubes and fins without making physical contact thus is untreated by the evaporator. The leaving air dry bulb (LA DB) is the dry bulb temperature of the air leaving the coil. This is the supply air dry bulb of 60 degrees. The entering air dry bulb temperature is the dry bulb temperature of the air entering the coil. This is the return air dry bulb temperature of 80 degrees. The ECT is the temperature of 47.5 degrees.

9 The bypass factor of.384 means that 38.4% of the air moving over the coil is unaffected by the coil. Typical bypass factors for normally operating coils vary from.1 to.35, or from 10% to 35%. A high bypass factor may indicate the air flow in cfm is too high or it may simply indicate that the system air flow is adjusted to prevent over dehumidifying the space. High sensible heat applications of air conditioning may be so adjusted. An example is a computer room. A knowledge of psychrometrics and airflow integrated with the fan laws is a great service tool for the technician. This article just begins to illustrate the fundamental cooling and dehumidification process. Many practical uses of psychrometrics abound.