Heaters
Many shops, office buildings and factories use heaters, heat certain rooms or spaces. Fig. 21-28 shows compact infrared heater that can be operated on natural gas or propane. The device complies with the standard, two of the four panels of the ceiling grids. Polished reflector radiates infrared energy that is converted into heat, as it reaches room residents and work surfaces. The surrounding air is heated by heat conduction and convection. Depending on the design, these units can either be sold or unvented.
A special unit heaters vertically down the warm air flow. They are also used for high air velocity (about 2500 fpm) against the large door openings. Tin helps keep out cold air outside. As a rule, they are on three feet above and four feet from the opening. They are often used for passenger and cargo door or the shipping and receiving doors. Tyres unit is controlled door switch or a thermostat connected in parallel. Door less than 8' tall and 10' wide typically not protected by these devices. Some users are running a fan in the summer to help the ingress of dust and insects.
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Air-Conditioning 
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