Handbook of air conditioning and refrigeration / Shan K

8.32 CHAPTER EIGHT
Gas Infrared Heaters
Electric Infrared Heaters
suitable environment for a manufacturing process. Heat radiates from an infrared heater in the form of
electromagnetic waves in all directions. Most infrared heaters have reflectors to focus the radiation on
a localized target; hence, they are often known as beam radiant heaters. Infrared heating is widely
used in factories, warehouses, garages, gymnasiums, skating rinks, outdoor loading docks, and racetrack
stands.
In environments with a low indoor ambient air temperature during the winter, the warmth that an
occupant feels depends on the radiant energy absorbed by the occupant from all sources that have
temperatures higher than that of the indoor ambient air. This warmth is closely related to the effective
radiant flux I rad, or ERF Btu / h�ft 2 /(W/m 2 ). And �ERF indicates the effective radiant flux that
must be added to an unheated space, Btu/h�ft 2 /(W/m 2 ). ERF and �ERF can be calculated as
ERF � hr(Trad � Ta) � h (To � Ta) (8.17a)
�ERF � h(To � Tuo) (8.17b)
where h r � radiative heat-transfer coefficient, Btu/h�ft 2 �°F (W/m 2 �°C)
h � convective and radiative combined heat-transfer coefficient, Btu/h�ft 2 �°F (W/m 2 �°C)
T rad � mean radiant temperature, °F (°C)
T a � indoor ambient air temperature, °F (°C)
T o � operative temperature as defined in Eq. (4.6), °F (°C)
T uo � operative temperature of unheated space, °F (°C)
In Eq. (8.17), T rad � T a indicates the amount T rad is above the indoor ambient air T a; it is important
because it influences the infrared radiant heat transfer to the occupant. There are two types of infrared
heaters: gas infrared heaters and electric infrared heaters.
Gas infrared heaters can be divided into indirect infrared heaters and porous matrix infrared heaters.
An indirect infrared heater consists of a burner, a radiating tube, and a reflector. Combustion takes
place within the radiating tube at a temperature up to 1200°F (650°C). In a porous matrix infrared
radiation heater, a gas-air mixture is supplied to an enclosure and distributed evenly through a
porous ceramic, stainless-steel, or metallic screen that is exposed at the other end. Combustion
takes place at the exposed surface and has a maximum temperature of about 1600°F (870°C).
Gas infrared heaters are usually vented and have a small conversion efficiency. Only 10 to 20
percent of the energy output of an open combustion gas infrared heater is in the form of infrared
radiant energy. Gas infrared heaters should not be operated under conditions in which the ambient
air contains ignitable gas or materials that may decompose to hazardous or toxic gases or
vapors.
Adequate combustion air must be provided. Venting is preferred in order to prevent a buildup of
combustion products. Usually, 4 cfm (1.9 L/s) of makeup air is required for 1000 Btu/h (293 W)
of gas input. If unvented infrared heaters are used, humidity and condensation control should be
provided to account for the accumulation of water vapor that forms during combustion.
A thermostat usually controls the supply of gas by means of a gas valve in on/off mode. For
standing-pilot ignition, a sensing element and controller are also used to cut off the gas supply
when the pilot flame extinguishes. If the combustion air is blocked, the gas supply is also cut off for
safety.
Electric infrared heaters are usually made of nickel-chromium wire or tungsten filament inside an
electrically insulated metal tube or quartz tube with or without an inert gas. The heaters also contain