10.1 Introduction Heating is the first word in the acronym HVAC. It is the most impor-tant part because without heating mankind would have difficulty in surviving. Proper design of the heating system is even more critical than that of ventilation or cooling. Human history began to develop with the discovery and control of fire which increased people’s ability to survive in a harsh environment. In modern heating system design,two primary concerns are proper system sizing, to achieve comfort,and system reliability. Capital cost, operating cost, and pollution con-trol are secondary in consideration. Pollution control is addressed by code authorities. Energy conservation and operating costs go together and have a considerable effect on life-cycle costs.These concerns and many others are addressed in this chapter.10.2 General 23986
In a modern heating system, heating can be provided by 1. Fuel-fired or electric boilers that produce steam, hot water, or ther-mal liquids for direct or indirect use 2. Furnaces, unit heaters, duct heaters, and outside-air heaters which provide hot air for direct circulation to the conditioned space 3. Waste heat furnaces and boilers which utilize the waste energy from some other source, such as a process, an incinerator, or re-frigeration equipment 4. Solar energy collectors, both passive and active, which heat either
water or air and, in some cases, solid materials
5. Heat pumps, either liquid or air
6. Direct-fired radiant heaters, either electric or natural gas
7. Geothermal sources
End users are provided heat by
1. Direct air—furnaces, duct heaters, outside-air heaters, reheat
units, ducted heat pumps
2. Indirect air—coils and air-handling units, fan-coil units, unit ven-
tilators
3. Liquid—radiators, convectors, liquid-filled radiant heaters
4. Radiation—direct radiation from panels, floors, or other radiators
10.3 Boiler Applications
Boilers can produce low-, medium-, or high-temperature water; low-,
medium-, or high-pressure steam (including process steam); and ther-
mal liquid. Break points between categories are usually defined by
codes.
10.3.1 Hot water boilers
Low-temperature water boilers (to 250F) are the most widely used
type for residential, apartment, and commercial construction. Me-
dium-temperature water boilers (250 to 310F) are generally used in
industrial and campus-type facilities. High-temperature water (310 to
450F) is used for extended campus-type facilities and industrial pro-
cess facilities. Thermal liquid heaters are primarily found in industrial
applications where both space heating and process heating are signif-
icant loads.
10.3.2 Steam boilers
Low-pressure boilers (up to 15 lb/ in2
gauge) are generally found in
commercial, apartment house, and single-unit industrial facilities.
They are used for space heating and domestic hot water, through end-
use heat exchangers. Medium-pressure steam applications (15 to 150
lb/in2
gauge) are generally found in campus-type facilities, hospitals,
and industrial plants where there are significant process require-
ments. Power generation high-pressure steam boilers operate in the
range of 150 to 900 lb/ in2
gauge or more with some degree of super-heat in order to obtain good turbine efficiency. Waste heat from tur-
bines is often used for space heating, domestic hot water, and process
requirements. Steam at high pressure can be used to provide lower
pressure steam, either by direct pressure reduction or by indirect gen-
eration through a heat exchanger. The latter is useful if high quality
steam is required.
10.4 Boiler Types
Boilers can be categorized in many different ways. For this book, the
following categories are used.
10.4.1 Cast-iron sectional boilers
Cast-iron sectional boilers can produce hot water or steam at pres- 暖通空调英文文献和中文翻译:http://www.751com.cn/fanyi/lunwen_17290.html