Determine Glycol Outlet Temperature

[RJB32482]

I am trying to find out what the outlet temperature of a 30% propylene glycol solution after it exchanges heat with air through a finned coil. Here goes the knowns I have:

Glycol Solution Mass/Volumetric Flow Rate (using minimal hydraulic flow of the pump)
Glycol entering temperature (set point on chiller)
Heat capacity of 30% propylene glycol
U Value for the Heat Exchange Coil
Air Mass/Volumetric Flow Rate (design)
Air Entering Temperature (design)

What I'm trying to calculate:
Air Exiting Temperature
Glycol Exiting Temperature

Haven't done a heat transfer calculation like this in a while. Can I these 2 equations equal:

(m)(Cp)(deltaT) = UA(deltaT log mean)

And solve using excel to get the 2 outlet temperatures (air and glycol)? Or is there another method for this type of heat transfer calculation?

Thanks in advance.

 

[TD2K]

If I'm reading your post right you know the glycol flow rate and the air flow rate as well as the air inlet temperature and glycol inlet temperature.

Your glycol exiting temperature is linked to the air exit temperature due to the conservation of energy. I've used Excel as you mentioned, vary the glycol outlet temperature, calculate the air outlet temperature (based on the amount of energy given up by the glycol) and then see if my UAdTlm equation gives me the same amount of energy transferred.

Are you calculating the individual heat transfer coefficients to come up with the overall U or do you have some other way of estimating the U overall?

 

[srfish]

On the left side of the equation there is a delt T so you must know the inlet and outlet temperatures of the glycol. Then it is not necessary to use a trial and error procedure to calculate the outlet temperature of the air. You can use one of the thermal effectiveness charts to calculate P, the ratio of the glycol dela T to (glycol inlet temperature -air inlet temperature)

 

[RJB32482]

TD2K - I got the U value from the coil vendor.

Srfish - Can you show me on the internet where one of these charts are? I have the Process HEat Transfer Book from Donald Kern but can't find a chart in there.

Thanks.

 

[srfish]

To use the internet, try using the search term; e-NTU. One site is

http://www,mak.etu

but you need to enlarge the charts. TEMA has the charts for counterflow and two shell-and-tube configerations. Process Heat Transfer by Hewitt, etal and Heat Exchanger Design Handbook have the charts.

 

[ngb23]

If you know the inlet and outlet temperatures of glycol, you can just calculate the outlet temperature of air from the equation (or vice versa).