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Cooling Tower Control Based on “Approach”


Cooling Tower Approach

By Mike Donlon
Director of Research and Development, Computrols

The most common Cooling Tower Control System uses a fixed temperature setting for the condenser water supplied back to the chillers. For convenience, let’s label this temperature:

TO = Tower Outlet temperature

To control this value, most controls technicians would program a control loop (PID or similar) to vary the overall Cooling Tower Fan usage in order to maintain this temperature. In most large buildings, multiple Cooling Tower Fans are available for this purpose. These fans can be:

  • Simple two-state fans (ON/OFF)
  • Multi-speed fans (OFF/HIGH/LOW…MED)
  • Variable speed fans (VFD)
  • Any combination above

Most controls techs are familiar with how to program multiple fans under a single loop to maintain Tower Outlet temperature (TO), so we won’t cover this in this article. What we will be discussing is how a single constant TO setpoint is not the best practice for cooling tower control.
Best practice dictates that the TO setpoint be programmed as a moving target that varies with outside air conditions. But, before we cover how to program this moving TO setpoint, let’s first cover a little theory behind this phenomenon.

A Tower Outlet (TO) temperature setpoint that is optimal under some outside air conditions, can NEVER be achieved in others.

Generally speaking, a Cooling Tower fan system can achieve a TO at or below the dry bulb temperature of the outside air. However, it CANNOT achieve a TO at or below the wet bulb temperature of the outside air. This limitation is because of the physics behind how Cooling Tower fans cool the condenser water. For convenience, let’s label the outside air wet bulb temperature as:

TWB = Wet Bulb Temperature

In the HVAC industry, the term “Approach” is used to describe the difference between the tower outlet temperature (TO) and the outside air wet bulb temperature (TWB). So let’s define Approach as:
Approach = TO – TWB

Some Key Cooling Tower Concepts:
»     When in doubt, use a 7 °F Minimum Approach
»     Using colder condenser water reduces the chiller KW by 1% to 1.5% per °F. So the colder the water, the better the KW efficiency. So set the low limit on the TO setpoint based on what the chillers can handle. Raise it only when outside air conditions make that setpoint unachievable.
»     For undersized cooling towers, diminishing returns will reduce much of the chiller savings.
»     Cooling Tower Efficiency:
μ = (ti – to) 100 / (ti – twb)
(Commonly 70 – 75%)
»     Where:
μ = Tower Efficiency
TI = Inlet water Temp
TO = Outlet water Temp
TWB = Wet Bulb Temp

Using our definitions, we will never be able to get the TO to reach the TWB. Or in other words, we will never get the Approach down to zero.
How close can we get TO to the wet bulb temperature (TWB)? That depends on several factors including the individual equipment. You can typically use 7° F as a general rule. Or put another way:

You will probably be wasting fan energy if you try to achieve an Approach below 7° F.

For instance, if the wet bulb temperature (TWB) is 78 °F, and you want to get an 82 °F condenser water temperature back to the chillers (TO), your Approach would have to be 4 °F. But according to our rule, you will never be able to achieve a 4 °F Approach. So if you leave the TO setpoint at 82 °F your fans will continue to work overtime trying to achieve an impossible goal. The bottom line: You’d be wasting fan energy!
A better method would be to use a setpoint that is a moving target. This would be analogous to programming a Supply Air Reset for an air handler, a concept that most of you are familiar with. The fan usage varies quickly to meet the TO setpoint, and the TO setpoint moves slowly to adjust to outside air changes.
The best practice in Cooling Tower programming is to first make a simple calculation point for the “Minimum Achievable TO” based on Approach and outside air temperature. This quantity could be embedded inside the setpoint logic, but making it a separate point may be valuable to those who understand the concept. Logic on this calculated point would look something like:
  Command “Minimum Achievable TO” to equation begin (
+ “Outside TWB”
) Equation End

The point “Minimum Achievable TO” will reflect the best you can do for TO. If you have access to the Approach value from the Cooling Tower’s initial design, you can use this value. Otherwise, a 7 °F Approach is a good rule-of-them.
Finally, program logic for the TO setpoint itself:
  Command “TO Setpoint” to equation begin (
Maximum of begin (
“Minimum Achievable TO”
) End MinMaxAverage
) Equation End

Chiller specs will limit the range for TO and you should consult these specs before programming. Usually, the number previously used for a constant setpoint will now be considered the minimum setpoint. For our example, we will use 82 °F as the minimum acceptable temperature for the chiller. This final statement will incorporate the concepts discussed in this article.

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