Approach in Chiller Evaporators and Condensers

A chillers approach value is a very useful tool. One of it's biggest functions is to indicate the rate of heat exchange.

We use difference in temperature to move heat from one fluid to another. A few things affect the temperature difference required to move a specific amount of heat. Two of the factors we can look at is efficiency of HX (how well it moves heat) and how much heat we need to move. 

BTW another term for heat is energy. In our industry, we are moving energy from one place to another by transferring that energy to different fluids. Ya I nerd on this stuff..... I like me so you don't have to. 

So if we have a less efficient evaporator, like a DX style, then we will need a higher TD than a flooded style would require to move the same volume of heat. 

Approach is the temperature difference between the refrigerant saturation temp and the leaving water temp. This is true whether evaporator or condenser. 

Don't forget to factor the load aspect when evaluating approach. With more load on the chiller the more TD is required to achieve the volume of transfer needed. 

When checking operating conditions, if your approach is elevated but within limits with a lite load then as the load increases the approach will likely increase. 

Things to consider when evaluating chiller performance. 

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