Handbook of air conditioning and refrigeration / Shan K

Comparison between Various Air-to-Air Heat Exchangers
According to ASHRAE Handbook 1996, HVAC Systems and Equipment, among the air-to-air heat
recovery heat exchangers:
● Fixed-plate, runaround coil loop, and rotary sensible heat exchangers are all limited to sensible
heat transfer and recovery; only the rotary enthalpy exchangers can recover latent heat from the
exhaust airstream.
● There are no moving parts in fixed-plate and heat pipe heat exchangers.
● Both heat pipe and runaround coil loop heat exchangers show no cross-contamination or crossleakage.
Fixed-plate heat exchangers have a cross-contamination and cross-leakage from 0 to 5
percent, and rotary heat exchangers from 0 to 10 percent. If cross-contamination is strictly prohibited,
a purge section can be installed to reduce it.
● The effectivenesses of fixed-plate and rotary heat exchangers are higher than those of heat pipe
and runaround coil loop heat exchangers.
● Rotary heat exchangers show the lowest air pressure drop among the various types of heat exchanger.
12.6 GAS COOLING AND COGENERATION
Gas Cooling
Cogeneration
HEAT PUMPS, HEAT RECOVERY, GAS COOLING, AND COGENERATION SYSTEMS 12.25
High electricity demand charges, high peak electricity rates, and the development of high-efficiency
direct-fired equipment, highly reliable gas engines, and more sophisticated and cost-effective
desiccant-based air conditioning systems enable gas cooling systems to compete with electric
compressors after the decline of gas cooling in the 1970s. Current gas cooling systems include the
following:
● Double-effect, direct-fired, lithium bromide (LiBr) absorption chillers and chiller/heaters
● Desiccant-based air conditioning using evaporative cooling, refrigeration, and direct-fired gas
heaters for regeneration
● Gas engine chiller systems and cogeneration
Usually, a gas cooling system has a higher initial cost and a lower operating cost than an electricitydriven
refrigeration system. Accurate calculation of the operating costs of a gas cooling system is
important in this comparison.
The LiBr absorption chillers are discussed in Chap. 14, and desiccant cooling systems are
discussed in Chap. 29. Gas-engine chiller systems and cogeneration are discussed in this section.
Cogeneration is the sequential use of energy from a primary source, including natural gas, oil, and
coal, to produce power and heat. Power can be electric or mechanical power, or both. In a cogeneration
system, the sequential use of the heat released from the flue gas and engine jacket significantly
increases system efficiency and makes the cogeneration system economically attractive.
In 1978 in the United States, the Public Utility Regulatory Policies Act (PURPA) permitted the
interconnection of electric power lines of cogeneration systems with electric utility systems. This
provides flexibility for cogeneration plants. They can either use or sell their electric power to the