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General and Comfort Ventilation 2 The system must be able to throttle smoothly over the total range of turndown (which can be as much as 25 1) 3 Burner must be able to produce complete combustion of the fuel throughout the turn down range 4 Flame retention and stability must be maintained in high velocity flow of air So-called "line burners" are almost ex clusively used because they give uniform temperature distribution across the air stream Line burners are sectional burners available in a variety of shapes which are flanged so they can be bolted together into assemblies to fit the size and shape of the unit Air-gas proportional' and premixing equipment is required, as is a positive flame retention fea ture A single flame failure or combustion safeguard device is usually satisfactory, as is a single gas pilot for ignition The widerange type line burner was developed espe cially to meet the needs of air heating as it utilizes oxygen from the stream of air being heated to supplement that provided m the primary air-gas mixture in order to extend the range of turndown to the 25 1 figure re quired Be sure that heater manufacturer's recommendations arc followed in installa tion, especially with regard to velocity of air flow across burner Steam Where excess steam capacity is available, use of steam coils or finned heaters should be considered carefully as steam heaters are relatively simple, inexpensive, easily maintained, and require no combus tion safety equipment Precautions which must be taken when using steam include 1 Avoid long heating coils These may give uneven temperature distribution from side to side in the duct as the steam supply is throttled over the wide range of turndown required (20 1 to 25 1) 2 Protect against freezeup Outside air at subfireezing temperatures can be drawn over the heating coils 3 Consider use of oil- or gas-fired tempenng air heaters ahead of steam coils to reduce load during extreme low-temperature weather (which can already put a maxi mum heating load on boilers) It is important that a qualified ventilation engineer be consulted References American Conference of Governmental Industrial Hygienists, Committee on Industrial Ventila tion, PO Box 453, Lansing, Mich 48902 Industrial Ventilation--A Manual of Recom mended Practice, 9th ed 1966 American Cas Association, 420 Lexington Ave , New York, NY A Complete Guide to Direct Gas-Fired Heating of Process Makeup Air 1961 American Society of Heating, Refrigerating and Air-Conditioning Engineers, 345 East 47th St, New York, N Y Guide and Data Book, `Fundamentals and Equipment' (Issued annually ) Bedford, T Environmental Warmth and Its Measurement Medical Research Council War Memorandum No 17 1946 Belding. H S , and T F Hatch 'hides for Evaluating Heat Stress in Terms of Resulting Phys iological Strains " Heating, Piping and Air Conditioning, 27 129 (Aug 1955) Belding, Harwood S , Bruce A Hertig, and Marsm L Riedesel "Laboratory Simulation of Hot Industrial Job To Find Effective Heat Stress and Resulting Physiologic Strain Amer Indust Hygiene Assoc J ,21 25 (Feb 1960) Burkhardt, Charles H Residential and Commercial Air Conditioning New York, McGraw-Hill Book Company, 1959 Fuller, Frank H , and L Brouha, "New Engineering Methods for Evaluating the Job Environ ment" ASHRA Journal (Feb 1966) 916