air leaving temperature-psychrometric

[moideen]

The method of find out the actual leaving air temperature of cooling apparatus, here an example takes an FCU. The chilled water entering temperature 47F, bypass factor .20 and entering air temperature is 75F. Please see the calculation as per the psychometric formula
Tldb=Tadp+bf(Tedb-Tadp), then 47+.02(75-47)=52.6F( 11.4C[sup][sup][/sup][/sup]) is it practical in field?

 

[chicopee]

Compare your results with those equations used in the examples shown in the Perry Chemical Eng. Handbook.

 

[moideen]

chicopee: can you specific the page number

 

[lilliput1]

No because Tadp, apparatus dew point temperature is not equal to the entering chilled water temperature. Tadp is higher than the leaving chilled water temperature. You should instead use the manufacturer's rated sensible and latent heat capacity at their rated conditions of entering chilled water temperature, chilled water flow, airflow and entering air temperature and humidity.

Chicopee X = 49 by trial & error in excel.

 

[moideen]

lilliput: Tldb=Tadp+bf(Tedb-Tadp), then 47+.02(75-47)=52.6F( 11.4C). Did you mean like that the
Apparatus dew point temperature is not equal to the entering chilled water temperature to the fan coil unit.

 

[moideen]

How to find the actual apparatus dew point of chilled water fan coil unit

 

[lilliput1]

See Carrier Chapter 8 Applied Psychrometrics pages 1-120 to 1-123.

Apparatus dewpoint temperature and bypass factors are now not used in rating fan coils. Google Trane chilled water fan coils and see ratings base on unit size, CFM, EWT, WTR, Entering Air EDM/EWB. The ratings are given in TC (total capacity MBH), SC (sensible capacity MH), GPM, WTR (Water temperature rise, PD water pressure drop ft of water.

SC x 1000 = 1.1 x CFM x (Tedb - Tldb)

Tldb = Tedb - (SC x 1000)/ (CFM x 1.1)

 

[moideen]

lilliput1: then i have calculated as below and imaged

 

[lilliput1]

Correct

 

[moideen]

then what is the role of chilled water temperature. in this formula chiller water temp is not subjected.

 

[lilliput1]

A set of table is specific to an EWT. Notell leftmost column designating EWT

 

[moideen]

Can be used the same formula for dx[sup][/sup] system?

 

[willard3]

DX systems do not have an Entering Water Temperature

 

[moideen]

willard3: here we discussed entering air temperature.

 

[lilliput1]

If you use the rated sensible heat capacity of the DX system at the conditions at which the rating applies yes the formula applies because it is the definition of sensible heat capacity.

 

[chicopee]

In my Perry's Chemical Engineering Hdbk,4th edition, chapter 15 under the subject of psychrometry, you should check out the examples to determine suitability to your OP.

 

[moideen]

I have imaged the air entering mixed temperature-psychrometric formula[sup][/sup]

 

[lilliput1]

So use the table at the entering air temperature of 79.4F instead of 75F to get the correct LAT. You should study and understand psychrometrics. What condition of air (CFM, dry bulb temperature, wet bulb temperature, moisture in grains/lb, points on psychrometric chart) would be needed to off set the space and outdoor air sensible heat and latent heat load. Understand how to plot the process on a psychrometric chart. See Carrier Handbook of Air Conditioning System Design Chapter 8 Applied Psychrometrics.

 

[moideen]

lilliput1: I prepared above example based on that same book.

 

[chicopee]

The one component missing in your drawing and which was inferred by Lilliput1 is the moisture content. If you can defined some relative humidity, you can plot the entire process on a psychrometric diagram and get the answers needed. Right now the formula that you presented is for dry air which is unrealistic in the real world unless you live in some of the driest places on earth. If you check out some of the used book dealers such as Abebooks, you may want to get a copy of "Mechanical Engineering License Review" authored by R.K. Pefley and D.G. Newman. This soft cover book has sample problems that would benefit your learning.