In the HVAC industry, there are two basic approaches to evaluating central-chiller-plant performance: single-number evaluation methods, such as integrated part-load value, and hour-by-hour analysis programs, such as eQUEST, TRACE, and EnergyPlus. With single-number evaluation methods, chiller and cooling-tower characteristics with potentially significant impacts on plant kilowatt demand can be missed,1 while hour-by-hour analysis programs tend to be complex and time-consuming,2 generally failing to provide first-law-of-thermodynamics simulation of plant performance, including flow, temperature, kilowatt demand, and load.

This article will present an alternative central-chiller-plant model, one that is consistent with the laws of thermodynamics and can be executed with a laptop computer. The model will be demonstrated based on an assumed chiller plant operating at design weather conditions in Salt Lake City.

The model addresses:

• Hydraulic pressure drop through evaporators and condensers.
• Part-load performance, including pumping, of chillers and plants.
• The effect of cooling-tower approach temperature and chiller-motor size.

Thirty-two combinations of chiller motor/compressor, evaporator, and condenser will be evaluated, as will the effect of 5°F and 7°F cooling-tower approach temperatures.

Chiller-Performance Data
Figure 1 provides performance data for 32 combinations of chiller motor/compressor, evaporator, and condenser. Generally, the better the performance of a chiller (the lower the kilowatts per evaporator ton), the higher the cost. Thus, from the group of chillers in Figure 1, one can select 0.455-kw-per-ton Chiller 1 at minimum cost or 0.395-kw-per-ton Chiller 32 at maximum cost.

For the chillers in Figure 1, the entering-condenser-water temperature is 71°F, flow in the evaporator is 2,400 gpm, condenser flow is 3,000 gpm, and evaporator load is 1,000 tons. These conditions, especially the 71°F entering-condenser-water temperature and 1,000-ton evaporator load, are unlikely to exist in a real system, as explained below.

Plant With Chiller 32 Installed
Figure 2 illustrates the peak-load performance of a plant incorporating Chiller 32. Table 1 provides the nomenclature. The peak site load of 980.4 tons will be used for all of the plants discussed in this article.

The plant incorporating Chiller 32 is at energy equilibrium,3,4 or steady-state condition. Chiller performance is a little better than in Figure 1, primarily because the temperature of the water entering the condenser is less than 71°F. This is because the condenser load is 1,140 tons, below the load of 1,200 tons for which the cooling tower was selected. At a load of 1,200 tons, the capacity of the cooling tower is 104.6 percent. The site load requires Chiller 32 to operate at 100-percent capacity (395 kw) to provide approximately 44°F supply water, with an evaporator load of 1,014 tons. The cooling tower provides less-than-71°F water to the condenser, which results in better chiller performance than given in Figure 1. This is offset a bit by the flow in the bypass, which requires the chiller to provide a little less than 44°F supply water, and an evaporator load of 1,014 tons vs. the 1,000 tons in Figure 1.

The load on the condenser consists of the evaporator load, the compressor heat load, and the cooling-tower-pumping load. The load on the evaporator consists of the site load (980.4 tons) and the primary-and-secondary-pump load. The pump kilowatt heat load is a function of pump efficiency. The secondary-pump heat load is 80.3 percent of 121.8 kw. The chiller-pump efficiency is 81 percent, while the cooling-tower-pump efficiency is 83 percent. The primary, secondary, and cooling-tower pumps are operating at the values shown by:

(P)sec-kw = (2,435 gpm) × (213 ft) × 0.746 kw/hp/(3,960 × 0.803) = 121.8 kw

(P)c-kw = (2,400 gpm) × (48.2 ft) × 0.746 kw/hp/(3,960 × 0.81) = 26.9 kw

(P)t-kw = (3,000 gpm) × (83.8 ft) × 0.746 kw/hp/(3,960 × 0.83) = 57 kw

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