ASHRAE Journal - August 2012 - 36

Condenser Water Temperature (°F)

the chiller plant cooling output Water Temperature (°F) Percentage vs. energy input consistent with Cooling Capacity Steam Flow Steam COP CHWR CHWS CWS CWR (tons) (lb/h) Load ASHRAE Guideline 22-20085 Flow (Entering) (Leaving) (Entering) (Leaving) and implement a control strategy that modulates the input energy 15% 406 11% 1.54 2,744 45.5 44 65 66.4 from the cooling tower fans vs. 20% 540 14% 1.62 3,473 46 44 65 66.9 the chiller energy. This control 30% 810 19% 1.79 4,722 47 44 65 68.7 strategy is possible with a traditional proportional integral de40% 1,080 24% 1.89 5,946 48 44 65 68.7 rivative (PID) algorithm. 50% 1,350 29% 1.99 7,085 49 44 65 69.7 Since many older buildings 60% 1,620 38% 1.80 9,461 50 44 69 74.7 do not have meters on their plant equipment, nor in many cases is 70% 1,890 50% 1.62 12,305 51 44 73 79.7 it practical to install them, a rea80% 2,160 63% 1.48 15,553 52 44 77 84.8 sonable approximation of cooling tower performance is to use a 90% 2,430 79% 1.34 19,498 53 44 81 89.9 7°F (3.9°C) DT approach. This is 100% 2,700 100% 1.18 24,647 54 44 85 95 clearly not possible if the cooling load is constant, as in a process Table 5: Steam chiller performance data: declining load and condenser water temperature. application, but for comfort cooling, the cooling load typically falls off proportionally with de100 clining wet bulb. Following a 7°F (3.9°C) DT approach can, 90 in many cases, significantly reduce cooling tower fan energy. 80 Figure 5 is an extrapolation of the performance curves for a 70 3,000 ton (10 551 kW) cooling tower.6
60 50 40 30 20 10 0 40 45 50 55 60 65 70 75 80 85

Cooling Tower Improvements
Adding VSDs and variable flow strategies to cooling tower operation (as opposed to cycling fans on and off or shutting off flow to cells) improves performance significantly. This is because when multiple cells are active, the entire heat transfer surface is used. Additionally, fans operating at part load through VSDs use less energy than fewer fans at full load. Adding extended cups (or weirs) to cross-flow style cooling tower distribution pans allows for flow reductions of 50% or more and ensures a uniform flow of water at the “perimeter” of the cooling tower during lower condenser water flows (Photo 1). This maximizes contact between water and air and reduces non-contact dry areas within the fill.

Outside Air Temperature (°F Wet Bulb)

Figure 5: Typical cooling tower approach as load and outdoor wet-bulb decline.

Variable Chilled and Condenser Water Flow
Adding VSDs to pumps is an effective measure that reduces plant energy use, allowing the pump speed to vary in response to the building load. New premium efficiency motors also improve energy use and are compatible with the VSDs. Because of very conservative pump friction calculations, most of the balancing valves on these older chilled and condensed water systems are partially closed to compensate for excess pump capacity. Opening these valves and using VSDs can reduce energy consumption significantly. Also, to minimize chilled water pump energy use, older chiller controls can sometimes be retrofitted to permit variable primary pumping in cases when throttling chilled
36 ASHRAE Journal

Photo 1: Adding cup extensions to the center pan orifices maximizes contact during low flow at the tower perimeter. water valves close off. When the minimum flow is met, a bypass valve must allow circulation of flow through the chiller. On the other hand, reducing condenser water flow
ashrae.org August 2012

ASHRAE Journal - August 2012

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