Selection Procedure
Trane Propeller Unit Heaters
Both Model S and Model P Propeller Unit Heaters are ideally suited to such a wide variety of installations that there can be no hard and fast rules regulating their selection.
In determining the type and size of the units for any building, the following points are generally considered:
BTU Requirements
While the choice of units depends upon several important factors, the total Btu requirement is usually determined first. In calculating heat loss, methods as recommended by the ASHRAE Guide may be used, or any other procedure which is known to be acceptable.
Type of Building
Once the total Btu requirement is known, the type of building together with its architecture and its purpose is considered.
In the many cases where a combination of space characteristics is found, a combination of Model S and Model P Unit Heaters may be used to create the most satisfactory heating system.
The Model P Unit Heater has the natural ability to tap the reservoir of heated air which collects at the ceiling level and return it to active service in the floor zone.
The Model P can be used on either high or low ceilings. From high mounting positions the Model P will allow ample clearance for moving objects and can project heat down into occupied areas regardless of obstacles which would restrict the flow of air from horizontal units.
From low mounting positions the Model P, with proper diffusion, can be used without disturbing nearby occupants with direct blasts of high velocity heated air, or requiring return lines so low as to leave insufficient head room.
The superior horizontal action of the Model S, on the other hand, is ideal for creating a wiping effect of warm air along exposed walls which neutralizes drafts at their source, beaming heat down narrow aisles and production lines, and blanketing large windows, doors and other points of high heat loss.
Spacing of Units
It is the usual practice to divide the building into areas with like exposures, or in relation to heat load distribution desired.
In general, Model S Unit Heaters may be selected for buildings where direct horizontal air currents are desired and where there are no obstacles to interrupt the flow of air from the heaters.
Model P Unit Heaters are successfully used where high or low mounting heights are required and should ordinarily be specified with diffusers, depending upon the mounting height and application.
Mounting Heights
The maximum effective mounting height is determined by the outlet temperature of the air, the outlet velocity, the cfm for which the heater is designed, and if a diffuser is used – the setting of its blades.
The higher the outlet temperature of the air, the more difficult it is to force it down into the living zone.
The cfm also affects the mounting height as a large volume of air will travel farther than a small volume under the same initial conditions of temperature and velocity.
In the preliminary planning stages, it is well to remember that the lowest possible mounting height is desirable in order to get the most heat down to the floor line and to allow the greatest possible diffusion adjustment to provide
In providing for the use of diffusers please remember, adjustment of a Louver Cone Diffuser to deflect the air toward the horizontal immediately lowers the mounting height limit. Adjustment for lateral deflection with the Louver Fin Diffuser shortens the distance of throw.
Diffusion
The use of Trane Louver Cone or Louver Fin Diffusers influences the selection of units in two principal ways: First: It is seldom necessary to install
oversized heaters to extend the maximum effective mounting height of the Model P or the distance of throw of the Model S. Conversely,Trane Diffusers enable extremely low mounting since the adjustable blades direct heat where it is wanted without creating uncomfortable hot spots near the units.
Second: After the original selection has been made, units may be located with confidence because adjustments in heat distribution are possible to accommodate future changes or unforeseen draft conditions.
Air Changes
Better diffusion and more even temperatures can be maintained in a heated space when the rate of air recirculation through the heaters is relatively high. For buildings where large numbers of people are engaged, it is desirable to provide for a greater number of air changes than for sparsely occupied areas.
A greater number of small units are used where wide diffusion and even temperatures are necessary. A few centrally located units of large capacity would be used where there are few occupants.
Comfort Conditions and Economy
Air circulation, diffusion and spacing of units are closely related to economy and comfort in the selection of unit heaters. The more units used to provide the required number of Btu’s, the more comfortable will be the conditions for personnel. On the other hand, a few large units can be selected to provide plenty of heat at low first cost, but may be slower in response and thorough distribution of heat.
Here again, adjustable diffusion equipment can go a long way toward saving the buyer first cost expense while still providing completely satisfactory comfort conditions.
11 |