cooling_with_dehumidification.pdf

8/10/2019 Cooling_with_Dehumidification.pdf

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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 toreduce the humidity to comfortable levels. The chart in figure 8-1 shows a

typical process line for cooling with dehumidification.

F ig u r e 8 - 1


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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 notonly cooled but, some actual grains of moisture were removed. The number

of grains removed per pound of air can be determined by finding thedifference between the number of grains at each state point.

The following example of a typical cooling with dehumidification process

illustrates how the process works.


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COOLING WITH DEHUMIDIFICATION

Fi g u r e 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 aretaken of the supply and return air and found to be as follows:


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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 psychrometricchart. (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 ofair thus the relative humidity is higher.

Also notice that the actual humidity in grains decreased. The cooling coil

removed grains (68.5 - 57) 11.5 grains of moisture removed for each poundof 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 drainedoff.


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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 arefound to be as follows:

RETURN AIR SUPPLY AIR

WB 65 = an enthalpy of 30.06 btu\lb WB 55 = an enthalpy of 23.22 btu\lb

Find the difference in enthalpy by subtracting. 30.06 - 23.22 = 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 heatbecause the air was cooled, and some was latent heat because grains of

moisture were removed. Now the total cooling capacity of the coil can bedetermined.


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The Total Heat Formula


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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 theseconditions. 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 atthe saturation curve.

F ig u r e 8 - 4

You should be able to read this temperature as somewhere between 47 and48 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.


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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 finswithout 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 enteringair 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 isthe temperature of 47.5 degrees.


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cooling_with_dehumidification.pdf

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