Plant personnel often asked Jake if they should install variable speed drive motors on their cooling tower fans. He usually replied, “I have normally found that one horsepower saved on the fan requires three horsepower more in compressor power.” Jake’s experience on multiple field tests and computer optimizations led him to this rule of thumb.
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However, in the late 80s and 90s, as the phase-out of chlorofluorocarbons began, refrigeration manufacturers tightened up not only the containment of their equipment but also the performance. Prior to that, power consumption varied within the 0.7 to 1.2 kW/ton range. Steady improvements brought chiller power consumption down in the 0.4 to 0.6 kW/ton range. So, Jake’s old rule of thumb probably didn’t work anymore.
To check this, Jake first thought about how individual components acted in the system and their constraints. He broke it down to the cooling tower, the refrigeration compressor and condenser. He then identified how these would interact.
For the cooling tower, Jake looked at some readily available samples of manufacturers’ curves. For optimal cooling, the fan blades are fixed at an airflow that maximizes the available motor horsepower. The curves are set to correlate to water flow and ambient air wet bulb temperature (WBT). The constraints on the tower include the air’s ability to absorb water vapor for the incoming water and the sensible temperature rise of the moist air leaving the tower. As the ambient air WBT increases, the air’s ability to absorb more water vapor — as well as the sensible temperature — decreases. Towers are designed with a set approach temperature, typically the difference between the WBT and the exiting water temperature, of 7–10°F. However, as the tower’s heat load decreases with high WBT, the approach temperature will fall, but not in proportion to the tower’s airflow. So in effect, air power doesn’t show full benefit versus the exiting water temperature. At that point, reductions in airflow will result in less power, which won’t impact the refrigeration condenser.
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In the condenser, the design conditions dictate the design kW/ton. That is but one point on the refrigeration compressor map. The kW/ton can vary considerably, depending on the evaporator and condenser operating conditions. The compressor has to lift the refrigerant gas from the evaporator to the condenser where it is condensed at a temperature largely dependent on the cooling tower exiting water temperature. The lower that temperature is, the lower the lift on the compressor, and the lower the compressor hp/ton. Generally, each 1°F reduction in condensing temperature trims compressor power 1.5 to 2.5%.
The major constraint on the condenser is a minimum pressure drop across the thermal expansion device. This could be a fixed orifice, a float valve or a control valve. Falling below the pressure drop could restrict flow to the evaporator thus increasing refrigerant liquid level in the condenser, a condition known as stacking. This would reduce the surface area in the condenser, raising the pressure and eventually slightly increasing flow.
The second constraint is the compressor operating curve. This can be modified by using inlet guide vanes to change the capability of the compressor to lift the gas. A variable speed drive either with a variable speed motor or a turbine drive also can serve to optimize compressor efficiency.
Jake recognized this analysis was more complicated than his old guidance. He developed a computer model to accurately assess the lowered condenser temperature impact versus the reduced cooling tower airflow impact. He picked several good test candidates where the tower and the condenser were uniquely connected; parallel towers and chillers would make the job difficult.
What he found was it “usually” made more sense to run the cooling tower to minimize condenser temperature and pressure. In his case, the exceptions were high ambient WBT with light loads, off peak season operations where lowering the exit cooling tower water temperature came up against minimum required condenser pressures, and a few others. The results surprised Jake. He attributed it to the advances made in refrigeration equipment efficiencies.
So, start collecting your data on the systems under consideration. Develop a model that looks at the constraints on the chiller as well as the cooling tower. Look at the interactions of the cooling tower, the refrigeration compressor and condenser. Compare your various options and the capital required to the savings achieved. Use this to make an informed decision on how to proceed. Happy energy hunting.
