Ask Tim: What should my set point be for my cooling tower water temperature?

“What is the best temperature for my water condenser?” I have encountered this question a number of times during my first weeks at Stromquist & Company.

It is common knowledge that lowering the cooling tower sump temperature set point increases a chillers efficiency. As we lower the cooling tower sump temperature the condenser becomes sub cooled.

Sub cooling can increase the chiller’s efficiency about 1% per degree of sub cooling. Increased chiller efficiency means energy savings.

It is important to remember that there is a limit to this type of energy savings.

For example, on Trane chillers efficiency improvement bottoms out at around a 75 degree set point.
Lowering the set point past 75 degrees results in an increase in power consumption from the tower fan and pumps without improving the chiller efficiency.

It is important to maintain accurate temperature settings with all chillers. Since my background is Trane, I will speak specifically about set points when using a Trane Chiller.

If the temperature is too low, within the 60 degree range, you risk the oil carrying over into the evaporator. If the temperature is too high, above 75 degrees, the chiller is consuming excess energy and increasing your monthly energy costs.

The correct temperature range for Trane Chillers is between 72 and 75 degrees. Trane chillers that operate within these dimensions are maximizing energy savings and minimizing the risk of oil carrying over into the evaporator. In conclusion, save on energy costs and the environment and check your chiller set points.

Using a good remote bulb temperature controller like the Johnson System 450 or the Honeywell T775 is another important thing to consider. If your temperature controller is not accurate or does not control properly then any possible energy savings will be negated. So make sure you have a quality remote bulb temperature control.

I will be blogging at controltrends.com so send your questions about chillers, motors, and variable frequency drives to me. My email address is tim@stromquist.com.Loom for your answers at Ask Tim.

Also, check out my chiller classes.

Comments

  1. Harris Bynum says

    HVAC system designers got excited when affordable Variable Speed Drives (VSD) became available. They remembered that horsepower varied with the cube root of flow (fan/pump laws #3). So at half flow, 1/8 horsepower was required. But at closer examination, this was only true for a static distribution (one with no valves or dampers that restricted flow in order to maintain control) systems. To approach the savings predicted by law #3, applying this to a VAV AHU fan required a control system that tried to keep the VAV box dampers as open as possible; i.e. the fan pressure should be reset down in order to keep the most demanding box damper full open. If all boxes throttled their dampers to less-than-open positions, energy would be wasted.

    Throughout HVAC systems, controls typically throttle valves and dampers to control flow, and the throttling effect chews up horsepower. With DDC systems and network communications of today’s control systems, the better fan controllers can monitor the box damper positions and adjust the fan pressure setpoint to minimize this wasted horsepower.

    Looking around large HVAC systems, there are few loads that can approach efficiencies suggested by law #3, and one is the cooling tower fan. In my opinion, this is a perfect place for a VSD. As condenser water temperature varies, the fan speed can be modulated with no losses through modulating valves or dampers. And with PI control, temperature can be maintained at relatively constant setpoints. Compare this to using an on-off (or 2-stage) cooling tower thermostat that cycles the fan(s). To minimize fan short cycling, significant water temperature differentials (3 to 4 degrees?) between on and off setpoints are required, so the chiller efficiency and output continuously varies (minimally). Using VSDs on tower fans make lots of sense. If towers must operate in freezing outside air periods, at minimum fan speeds (dictated by the VSD specifications), a 2-position (on-off) algorithm may be invoked to cycle the fan on and off (but at the minimum speed) until the load rises. Typical Honeywell and Johnson remote bulb controllers will probably not have network communications and may not have PI control capability. A central DDC system may be necessary.

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