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) is it practical in field?

 

[moideen]

chicopee: thank you for your reference, I will try to collect that book. yet, how moisture content influences the dry bulb temperature? the moisture content influences the total capacity of cooling apparatus.

 

[chicopee]

The higher air temperature from a regular thermometer(dry bulb temperature), the higher the moisture content. Cold air by contrast can not hold much moisture. Only by studying psychrometric charts, you will understand the relationships between dry bulb temp, wet bulb temp, R.H , moisture content and the enthalpy of air.

 

[ChasBean1]

This is great but it’s academia. We can get a 55°F leaving dew point by limiting DAT to 55°F; or a 51°F leaving dew point via a DAT of 51°F. If you want to get technical, the 55°F LAT with a larger cooling coil bypass factor will have a lower dew point. So you can have a lower coil ADP temperature with less fins per inch and have a 55°F LAT with a 51°F dew point, or a higher coil ADP temperature with more fins per inch and have a 55°F LAT with a 54°F dew point.

To me, this is minutia. As a general rule, if you need a space (e.g., pharmacy or OR) that wants a 68°F indoor temperature and less than 55% RH (dew point of 51°F), supply 52°F air during the summer and reheat the air to the desired room condition. The supply air temperature is the key; the fins per inch, bypass factor and other parameters matter, but are generally academic.

Wait, I might've answered the wrong question again... Sorry, I like going off on tangents.

 

[lilliput1]

Thank goodness computerized coil selection makes the task so much easier without figuring out apparatus dewpoint and bypass ratio. You calculate the required cooling coil leaving air dry bulb temperature and moisture content to satisfy the following equations:

CFM dehumidified air = (Total Sensible Heat BTU/hr) / (1.08 x (TF ent clg coil - TF lvg clg coil)

CFM dehumidified air = (Total Latent Heat Btu/hr) / (0.68 x (W grains/lb ent clg coil - W grains/lb lvg clg coil)

Total Sensible Heat = Room Sens + OA Sens + Fan Heat Gain + Duct Heat Gain

Total Latent Heat = Room Latent + OA Latent

For space HVAC CFM dehumidified air is usually = Room Sensible Heat Btu/hr / (1.08 x 20) but this may be changed depending on the coil selection.

Select coil type, and fin arrangement and at maximum 500 fPM face velocity to avoid moisture carryover. Try out various available circuit arrangement to pick coil meeting desired leaving conditions with minimum air pressure drop and with less than 20 ft water pressure drop.