Summary of Central Plants

Summary

This chapter has been concerned with central plant, specifically with boilers, producing steam or hot water, chillers producing chilled water and cooling towers that cool the chillers.

Introduction

Central plants generally require less maintenance than numerous smaller package systems and the equipment usually has a longer life. Other advantages include ease of operation and maintenance in a central location; efficiency; heat recovery options; less maintenance and a longer life. Cons include: cost of installation, space

requirements for the equipment and for the distribution pipes. Issues of seasonal efficiency were also raised.

Central Plant Versus Local Plant in a Building

Issues that can influence the choice include installation costs vs. operating costs. For minimum installation cost, the package approach usually wins. However, the central plant has several operational benefits.

Boilers

Boilers are pressure vessels used to produce steam or hot water. The critical design factor for boilers is pressure. A low-pressure steam boiler operates at a pressure of no more than 15 psig. Low-pressure hot water boilers are allowed up to 160 psig.

Boilers and system components are covered by local code requirements. The safety equipment and staff monitoring requirements are far less stringent for low-pressure boilers so there is a significant incentive to use low-pressure.

Boilers have two sections: The combustion section is the space where the fuel- air mixture burns; the second section of the boiler is the heat transfer section. In all boilers there is a need to modulate the heat input. On smaller units, the efficiency improves and cycling effect is reduced by having a “high-low-off” burner. On larger units, a modulating burner can adjust the output from 100% down to some minimum output. The burner modulation range is called the “turn-down ratio.” With a modulating burner, efficiency increases as the output drops and efficiency drops as the mean temperature of the heated fluid rises.

Boilers can run in parallel: With two water boilers, about half the water will flow through each boiler; with steam boilers, if one is running both will fill with steam to the same pressure.

In steam systems, there is a constant loss of water in the condensate return system. To prevent problems with solids build-up in the boiler and distribution pipe corrosion, continuous high quality water treatment is required.

Chillers

Chillers are refrigeration machines with water, or brine, heating the evaporator. The standard measure of chiller capacity is the ton, a heat absorption capacity of 12,000 Btu per hour. The main difference between chillers is the type of compres- sor. Smaller compressors are often reciprocating units, larger units may have screw or scroll positive-displacement compressors, and for 75 tons up to the largest machines, there is the centrifugal compressor.

Chillers should be sized to match the estimated load without a ‘safety’ factor. An oversized chiller will have a lower operating efficiency, so it will have a higher operating and maintenance cost, as well as more difficulty dealing with low loads. When designing a central plant, it is often worth having two 50% capacity chillers instead of a single chiller. If failure to meet the load is mission critical, use two units sized to 50% of the load each, with a third 50% unit as standby.

Cooling Towers

Cooling towers are a particular type of big evaporative cooler. In the cooling tower, warm water is exposed to a flow of air, causing evaporation and therefore, cooling of the water.

The psychrometric chart can be used to illustrate the workings of the cooling tower.

It is often considered worthwhile to over size the tower to ensure that full chiller capacity will always be available. The tower capacity can be reduced: by using a fan that can be cycled on and off; with a two-speed motor that can cycle between high, low and off; or a variable speed fan can be used.

A danger of cooling towers arises from the warm, nutrient rich environment that can propagate bacteria growth, therefore, the tower should be regularly cleaned of dirt buildup and treated, to prevent biological growth. In addition, some water must be bled off to prevent the build-up of dissolved solids.