Document O3jw25Db48QxdqMwdO0VDBxZj

390 Chapter 21 1945 Guide Velocities and decibel rating^ for the various types of unit heaters are given in Table 1. The centrifugal fan type, as illustrated by Figs. 1 and 2, delivers air at high velocity (1500 to 2500 fpm) and is equipped with outlets adjustable to deliver air in several directions. It is able to project the heating effect over distances of from 30 ft to as much as 200 ft from the unit. However, it is more suited to long than short blow, except when mounted high above the floor with discharge at a sharp angle downward. As a rule it can be located at considerable distances from other units thus reducing the piping and conserving floor space usually allotted to heating equipment. The propellerfan type is divided into two classes of horizontal and verti cal blow. The horizontal blow type, illustrated by Fig. 3, discharges air at from 400 to 1000 fpm. It takes care of medium distances of blow, up to approximately 100 ft where mounting height is not too great and where other conditions, such as final temperature and mass of air handled by the unit, are favorable. The vertical blow unit, illustrated by Fig. 4, discharges air at from 1200 to 2200 fpm. It is mounted on the ceiling of a room or up in the roof trusses and blows vertically downward., Various diffusers are available Table 1. Outlet Velocities, Distance of Blow and Decibel Ratings for Various Types of Unit Heaters Type of Unit Heater Outlet Velocities FPM 1500-2500 400-1000 1200-2200 Distances of Blow--Ft 20-200 30-100 10-50 Decibel Rating 34-90 44-84 50-82 to take care of the height and space conditions. These units are appliedto elevations of from approximately 10 to around 50 ft above the floor. Where mounted in roof trusses, as in industrial buildings, they do not interfere with traveling cranes. A code2 governing the number of sizes of propeller fan units as well as standardization of fan motors and the method of specifying outlet velocities has been adopted. Air Outlets In order to direct the air to points desired and to diffuse the air to avoid drafts, it is common practice to equip unit heaters with directional out lets, adjustable louvers or fixed types of diffusers. RATINGS OF UNIT HEATERS It is standard practice to rate unit heaters on the basis of the amount of heat delivered by the air in Btu per hour above an entering air tem perature of 60 F. This applies to all types of unit heaters, the steam or hot water type, the electric type and the direct fired type. There are, however, other factors which must be taken into account, especially when an attempt is made to compare one type of heater with another. These are the temperature of the heating element and the velocity of air through it. Consideration is given to these factors in the discussion of ratings for each type of unit heater in the following paragraphs. Standards for Propeller Type Unit Heaters prepared and adopted by the Industrial Unit Healer Asso. ciation, June, 1938. IV'* - ,' \ -t t Unit Heaters, Unit Ventilators, Unit Humidifiers_________________________ m. Fig. 1. Floor Mounted Unit Heater, Housed Type Fan 7T Fig. 4. Vertical Blow Unit Heater