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Cooling capacity of chiller and heat load calculation

Jigneshpadia2002 (Bioengineer) (OP)
20 Jun 08 9:49
Can I please ask one question!

I have a situation. I need to make sure that a particular cooling unit / Equipment is suitable for a process or not. I am bit new with this type of assignments! I am sure it won't be very challenging for the experts but I really wish to get some help with this one. So here is the problem.

I have a vessel. Working volume for the process wil be 20 L. Starting temp. 37C End temp for the vessel content needs to be at the 8 C. There is a time restriction. Vessel need to go from 37C to 8 C in 30 minutes.

Now I used Q - mCpdT equation to find out the amount of power required.

here is the calculation

M = mass of vessel content. I assumed to be water for the calculation purpose. Working volume is going to be 20 L =

M = 20 Kg

Cp = I used Cp of water at 40 C found from
http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html

Cp = 4.179    KJ/Kg/K

dT = 29    K

So Q = 2423.82    KJ
Time    30    minutes

So Rate of cooling heat removal = 1.346566667    KJ/sec

1 KJ    238.8459    Cal
        
Rate    321.6219274    Cal/ sec
        
1 Watt    0.2390585    cal /sec
        
Power    1345.369135    W
    1.345369135    KW

Now I know that the equipment specification is 0.5 KW at -20 C

Does this mean that I can conclude that Equipment won't be able to meet the cooling requirement in a given timeframe.

Please suggest and comment on this calculation.

Mant Thanks in Advance
JP
zerosum (Mechanical)
20 Jun 08 15:02
This might be better posted to the Heat Transfer and Thermodynamics forum.

Your numbers check down to the "Equipment Specification" (You went thru a couple of needless conversions since a watt is a Joule/sec by definition).

You need your "Equipment Specification" up in the 8 deg C temperature range to know whether it is adequate, assuming the wattage refers to the heat removal capability of the machine.  
djack77494 (Chemical)
20 Jun 08 19:21
It is hard for me to picture what it is you're trying to do. I'm thinking you have a vessel that you're running batches through trying to heat and/or cool over the specified temperature range within 30 minutes. If that's the case, then you're dealing with an unsteady state heat transfer problem. I don't know what you mean when you say, "the equipment specification is 0.5 KW at -20 C"; it makes no sense to me. If I'm right about this being a batch operation, then understand that the rate of heat transfer varies over your 30minute time period. It is initially fast, but then slows as the batch temperature responds to your efforts to heat/cool. Please provide additional information for additional help.
Jigneshpadia2002 (Bioengineer) (OP)
21 Jun 08 12:32
hi,

sorry for the confusions.

"Now I know that the equipment specification is 0.5 KW at -20 C"

This referes to the chiller - equipment which has a specification of 0.5 KW at -20 C

I agree with your comment on the heat transfer being faster in the initial phase and slower at the end. The cooling capacity at the lowest / slowest point must exceed the overall requirement. I hope this clarifies things a bit.

One more thing - I checked the manual and found that at 0C chiller is having slightly higher efficiency of 0.8KW but still this is not enough.


Now someone at work suggested to use water-glycol mix in the chiller to achieve better capacity. However this complicated the calculations. Is it possible that someone can quickly take me through this calcualtions.


Regards,
Jignesh
reena1957 (Chemical)
23 Jun 08 4:09
Dear Jigneshpadia,
Your problem is under" Batch Heat Transfer", handled using Kern's famous Heat transfer book with the same chapter title. You have not specified whether your chiller is constsnt boiling refrigerant ( at -20Deg C OR at 0 Deg C) OR a chilled water system with constant inlet temperature to your vessel and a variable outlet temperature. Each of the above case is handled differently by different formulas. Pl give more details.
Reena
Jigneshpadia2002 (Bioengineer) (OP)
25 Jun 08 7:12
Hi Reena,

Thank you for your time and suggestion.

The Equipment used in our facility is heating/cooling unit - which able to do both. We have several units, and they all utilize different coolant / cooling fluid.

Currently I am looking at the one which can potentially use the water-glycol 50 % mix for cooling.  I am new with the equipment and its terminology so I am not sure if I still managed to answer your question of constant boiling referigerant.  I will try to get the book from library. Is it the process heat transfer by D Q Kern?

Thanks
JP
reena1957 (Chemical)
25 Jun 08 8:13
Dear JP,
The title of book is right. If the cooling medium is water -glycol mixture, then it is constant inlet temperature to your vessel and a variable outlet temperature case. Get the formuls from Kern and use it. You will also need the overall heat transfer coefft,area of the coil, specific heat ( you already have it) to plug into the formula.
Reena
Jigneshpadia2002 (Bioengineer) (OP)
26 Jun 08 12:29
I got some good explanation from vendors. The equation used in my initial question is correct. Only thing is that vendor's don't have data on Cp of 50-50 glycol-water mix. Does anyone know this?

Thanks again for your help.
JP
Unotec (Chemical)
26 Jun 08 12:42
Check the web. Here's a link: http://www.engineeringtoolbox.com/ethylene-glycol-d_146.html

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>

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