tailieunhanh - HVAC Systems Design Handbook part 6

All air-handling units (AHUs) and many terminal units, if they are not self-contained, require a source of heating and/or cooling energy. This source is called a central plant, and the means by which thermal energy is transferred between the central plant and the AHU is usually a fluid conveyed through a piping system. The fluids used in HVAC practice are steam, hot or cold water, brine, refrigerant, or a combination of these. The equipment used to generate the thermal energy is described in Chap. 7. In this chapter we discuss the transport systems. . | Source HVAC Systems Design Handbook Chapter 6 Design Procedures Part 4 Fluid-Handling Systems Introduction All air-handling units AHUs and many terminal units if they are not self-contained require a source of heating and or cooling energy. This source is called a central plant and the means by which thermal energy is transferred between the central plant and the AHU is usually a fluid conveyed through a piping system. The fluids used in HVAC practice are steam hot or cold water brine refrigerant or a combination of these. The equipment used to generate the thermal energy is described in Chap. 7. In this chapter we discuss the transport systems. Steam Steam is water in vapor form. Because it expands to fill the piping system steam requires no pumping except for condensate return and boiler feed. The specific heat of water vapor is quite low but the latent heat of vaporization is high. As a result steam conveys heat very efficiently. Steam may be used directly at the AHU in steam-to-air finned-tube coils or a steam-to-water heat exchanger may be used to provide the hot water used in AHU coils or in radiation. Steam radiation is also employed. When used directly steam pressures are usually 15 lb in2 gauge or less. When used with a heat exchanger steam pressures up to 100 lb in2 gauge are common. Higher pressures allow smaller piping but create piping expansion and support problems. In- 145 Downloaded from Digital Engineering Library @ McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Design Procedures Part 4 146 Chapter Six dustrial plants often use high-pressure steam for heating as well as for process purposes. Steam properties Table shows thermodynamic properties of water at saturation temperatures and corresponding pressures from 0 to 250oF. Complete tables in the American Society of Mechanical Engineers ASME steam tables1 .