Trane SYS-APM001-EN manual Chilled-Water System Variations, Heat Recovery

Chilled-Water System Variations

Figure 40. Sidestream plate-and-frame heat exchanger

Chiller 2
	Production
Chiller 1
Bypass Line
Plate-and-Frame	Distribution
Heat Exchanger
70

A number of chilled-water system variations can and should be used when appropriate. Each configuration offers specific advantages to solve problems and add value to the system.

Heat Recovery

ASHRAE/IESNA Standard 90.1–200720requires heat recovery in specific applications. Indoor air quality concerns have spurred the use of systems that subcool supply air to dehumidify, then temper, the air to satisfy space conditions. ASHRAE/IESNA Standard 90.1–2007 limits the amount of new reheat energy used in these applications. With these drivers, and energy costs, there has been a resurgence of heat recovery chillers. The example on page 75 describes a cost-effective recovered-heat strategy. This scheme is commonly used for service-water heating in resort hotels and for certain process loads.

A separate application manual21 discusses heat recovery, as does an Engineers Newsletter 22, so it is not discussed in depth in this manual. However, considerations for heat recovery on chilled-water system design are discussed.

Condenser “Free Cooling” or Water Economizer

There are several ways to accomplish free cooling through the use of a water economizer circuit. Three common techniques for chilled water systems are using a plate-and-frame heat exchanger, refrigerant migration, or well, river, or lake water. Each technique is discussed in more detail below and in an Engineers Newsletter 23.

Plate-and-frame heat exchanger

A plate-and-frame heat exchanger may be used in conjunction with a cooling tower to provide cooling during very low wet-bulb temperature conditions. When it is to be used for these purposes, designers often specify a cooling tower larger than necessary for design conditions so that it can be used for many hours with the plate-and-frame heat exchanger.

In this type of water economizer, the water from the cooling tower is kept separate from the chilled-water loop by a plate-and-frame heat exchanger. This is a popular configuration because it can achieve high heat-transfer efficiency without cross-contamination. With the addition of a second condenser-water pump and proper piping modifications, this heat exchanger can operate simultaneously with the chiller, provided the chiller is placed downstream of the heat exchanger (as shown in Figure 40). As much heat as possible is rejected through the heat exchanger while the chiller handles the rest of the cooling

Chiller System Design and Control SYS-APM001-EN