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7 comments
Active Contributor

Join Date: Feb 2008
Posts: 23

approach temperature

11/16/2008 10:09 AM

Can somebody explain about the " approach temperature" of a chiller. Is it possible to measure / calculate it by using inlet & outlet temperatures of condenser water?

Please explain.

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Guru
Engineering Fields - Control Engineering - New Member China - Member - New Member

Join Date: Sep 2006
Location: CHINA
Posts: 2972
Good Answers: 14
#1

Re: approach temperature

11/16/2008 7:56 PM

depend on what chiller you are using.

many measure methods.

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Active Contributor

Join Date: Feb 2008
Posts: 23
#2
In reply to #1

Re: approach temperature

11/16/2008 9:20 PM

Thank you,

it is Centrifugal compressor & water cooled chiller.

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Anonymous Poster
#3

Re: approach temperature

11/16/2008 11:21 PM

Approach temperature generally applies to evaporative cooling towers. It is the difference between the ambient wetbulb temp and the condenser water supply temp. The inlet and outlet of the condenser water is simply the condenser water deltaT,as is the difference between the supply and return temps for the chiller(s).

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Anonymous Poster
#4

Re: approach temperature

11/17/2008 3:27 AM

Or this little tidbit gleaned from a Google search...

"The compressor's approach temperature, the difference between the temperature of the fluid leaving the heat exchanger and the saturation temperature of the refrigerant being cooled or heated, is a good indicator of heat transfer efficiency. An increasing approach temperature is a prime indicator that heat transfer efficiency is decreasing. An accurate log sheet will reveal when temperatures start to change from efficient levels."

It appears that "approach temperature" means different things to different processes.

Cheers.

J.

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Associate

Join Date: Jan 2007
Location: Los Angeles, California
Posts: 35
Good Answers: 3
#5

Re: approach temperature

11/17/2008 10:57 AM

I agree with Guest. The aproach temperature of a chiller and the approach temperature of an evaporative condenser are two different things. For a chiller, imagine you have an entering water temperature of 54 F, a leaving water temperature of 44 F, and a saturated refrigerant temperature of 38 F. (To find saturated temperature, measure the refrigerant pressure and find the temperatue equivalent). In this example the Temperature Difference (TD) is 10 F. The approach is 6 F. Temperature difference is a function of water flow, approach is a function of efficiency of the heat exchanger.

Both TD and approach will change as the chiller capacity changes, so an accurate measurement also requires knowledge of capacity at that time. Also the approach will change at the heat exchanger becomes fouled on either the water side or refrigerant side. Rule-of-thumb for American equpment. In the 1980's a typical approach was 10 F. A chiller made today will be more efficient, and have an appreach in the 2-4 F range.

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Anonymous Poster
#6

Re: approach temperature

09/30/2010 4:29 PM

I understand Approach Temp for a cooling exchanger to be the difference between the inlet temp of the cooling fluid (or gas) and the discharge temp of the fluid being cooled. i.e. chiller water/glycol at 30 deg inlet with discharge of cooled condenser water from exchanger at 40 deg gives 40-30=10 deg approach. The smaller the approach temp, the bigger the exchanger... As exchanger gets fouled the approach will get larger.

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Anonymous Poster
#7

Re: approach temperature

11/25/2010 5:15 PM

It was said:

Gadget guy

I agree with Guest. The aproach temperature of a chiller and the approach temperature of an evaporative condenser are two different things. For a chiller, imagine you have an entering water temperature of 54 F, a leaving water temperature of 44 F, and a saturated refrigerant temperature of 38 F. (To find saturated temperature, measure the refrigerant pressure and find the temperatue equivalent). In this example the Temperature Difference (TD) is 10 F. The approach is 6 F. Temperature difference is a function of water flow, approach is a function of efficiency of the heat exchanger.

Both TD and approach will change as the chiller capacity changes, so an accurate measurement also requires knowledge of capacity at that time. Also the approach will change at the heat exchanger becomes fouled on either the water side or refrigerant side. Rule-of-thumb for American equpment. In the 1980's a typical approach was 10 F. A chiller made today will be more efficient, and have an appreach in the 2-4 F range.

Hello, it was said here that the approach temperature is 2-4 f,(would you like to buy a pet rock?) and saturated temperature was used. I am still in the 1980'S, and confused! are you saying that the saturated temperature can be 2 degrees above the condensing shell and tube l.w.t.? I think you are confused with sub cooled liquid TEMPERATURE, which is not saturated.

anyone…

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