N Solid-Fuel-Fired Heaters
Furnace-type air heaters are manufactured from cast iron or steel, and cased in brickwork or steel. The cast-iron heaters are sectional, cemented and bolted together. The steel type are welded or riveted. It is essentia] that the joints are airtight, so that the cold air can pass over the heated surfaces of the furnace and flues without contamination by the flue gases.
To reduce the possibility of combustion products being drawn into the air if the heat exchanger burns through, the air is blown and not induced through the heater.
Adequate draft is necessary, not only for efficient combustion, but also to reduce the risk of soot being deposited on the heating surfaces. Provision must be made for a cleaning door in the flue and for the tubes.
With a fan-furnace heating system, once the temperature is reached, the fan is controlled to start automatically. Should fan failure occur, provision should be made to damp down the furnace automatically to avoid overheating’s causing tube damage.
When selecting air-heater batteries, the following factors common to all types must be considered.
A. Air volume to be handled (and conditions at which it is measured)
B. Entering air conditions (EAT) (dry-bulb temperature, air density), air quality (possibility of dirt, corrosive, or hazardous atmospheres); whether the coils are suitable to operate in temperatures below freezing
C. Required leaving-air dry-bulb temperature (LAT)
D. The heating medium being used, considering
•Water and steam entering temperatures •Operating pressure
•Permissible temperature drop of the heating medium
• Electricity supply, voltage, and phase and whether a standby supply is required
• Maximum element operating temperature
E. Allowable air-side velocity
F. Allowable air-side resistance (as it affects fan power)
G. Heating medium connections, size and type, location, circuiting
B. Provision for air release and drainage
I. Provision for cleaning tubes
J. Electricity: type and location, circuit arrangement; terminal box specification
K. Air-side connections (flanges for duct mounting, etc.)
1. Construction: materials for beaters, coils, and extended surface; number of rows and arrangements; type of extended surface and spacing
M. Division of coil to suit control, manufacturing, or installing arrangement, i. e., number of sections and rows n. Air pressure at point of installation (and air leakage through coil casting), particularly when located at fan discharge on a high-pressure system
J. Economy of coil selection as related to the system, such as flow rates, temperature rise, pressure drops, and fan and pump power
If a preheater coil is provided and positioned before the air filter, the heater should be either a plain tube or have wide spread fins for easy cleaning
188.8.131.52 Selection of Direct-Fired Air Heaters Flued Heaters
This method of heating is the recommended one, and the following factors should be considered.
A. Air volume to be handled (qv), m3 s-1
B. Entering-air conditions (9ao) (EAT)
C. Leaving-air condition (0jat) (LAT)
D. Will leaving-air temperature cause discomfort or other problems?
E. Allowable air-side resistance
F. Is the unit provided with all the necessary safety controls?
G. Is the flue outlet suitable for easy discharge of the products of combustion into the atmosphere?
H. Are the ductwork connection flanges provided?
1. Is an access door provided for inspection of the pilot and cleaning the main burner?
J. Are sight glasses or observation ports provided for inspection of pilot and main burner flames? k. Is a suitable temperature control provided?
1. Does the fan within the unit have sufficient pressure to overcome the system ductwork resistance? m. Will moisture from the combustion products adversely affect the space humidity requirements? n. The concentrations of combustion products liberated into the occupied space must be well below the accepted tolerance levels. This is the factor that limits the use of flueless heaters for makeup air supply.
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