Range and Approach for Evaporative Condenser

Part 3 of 11

http://jmpcoblog.com/hvac-blog/cooling-tower-and-condenser-water-design-part-3-understanding-tonnage-range-and-approach

Hi maverick

I am a little confused as there seems to be some difference from the way I would usually use these terms (and a common problem is that terms are often used interchangeably in different industries and countries)

My background is oil & gas and the utility systems that go with them

Range in particular is something that I would normally only use on a cooling tower.

In that case Range = Inlet Hot Cooling Water - Outlet Cooling Water

Clearly this could be used in any exchanger.

For a cooling tower Approach = Ambient Wet Bulb - Cold Cooling Water Out

The problem with asking for calculations is that we do not know what information you have; what you need and what your objectives are.

In most of the projects on which I have worked the range = 10C (18F). For systems where river / sea water is used this is based on limits for the returning stream. So normally this is defined for the process.

Approach typically varies depending on the type of heat exchanger. Normally as the approach narrows the heat transfer area increases. In a cooling tower a close approach is desirable to give the lowest possible cooling water temperature.

You mention an evaporative condenser with ammonia.

So I think this means you have cold ammonia vaporizing on one side and a process stream condensing on the other.

On the ammonia side the range would be a notional 0C - with pure ammonia at an approximately constant pressure vaporization takes place at a constant temperature.

On the process side the range depends on the composition of the stream. The purer the stream the narrower the range.

For the approach it depends on various issues. but typically for different types of exchanger the practical limits can b anything from 2-3C (plate exchangers) to 15-20C (Air Coolers).

Some more information would enable us to help you better

eg on range you can either fix range to determine flow or fix flow and determine range where

Duty = flow of coolant * range (not quite that simple if you have vaporisation but basically thats how it goes)

Approach is less of an issue with an ammonia cold side as you can vary the ammonia pressure to adjust the temperature and so widen the approach with your process side and not end up with huge areas.

For a cooling tower the wet bulb temp cannot be changed (although it will vary through the year)

In this case the closer the approach the larger the tower