Document kD5b7M5Q0dddZ254Lg5K96eby

854 CHAPTER 54 1965 Guide And.Data Book Industrial centrifugal fan units are applied where heating capacities and space volumes are large, or where filtration of the heated air, operation against static resistance, or the introduction of outdoor air are required. Down-blow or horizontal-blow units may be used, depending on the re quirement. Cabinet unit heaters are used for applications in which a more attractive appearance is desired. They are suitable for free delivery or low static pressure duct applications, they may be equipped with filters, and can be arranged to dis charge either horizontally, or vertically up or down. Location for Proper Heat Distribution Location of the units for proper heat distribution is of utmost importance in the application of unit heaters. In general, the units must be selected, located, and arranged so as to provide complete heat coverage and at the same timft maintain acceptable air motion and temperature in the working or occupied zone. Proper application depends on a number of factors: the size, number,' type and direction of blow of the units, the mounting height, outlet velocity, outlet temperature, air volume, and the type of directional outlet employed. Many of these factors are interrelated, and manufacturers therefore publish data for specific units, show ing heat coverage and mounting heights for various outlet velocities and outlet arrangements. Such data should normally be consulted in selecting unit heaters for a particular appli^ cation. The mounting height may be governed by space limitations or by the presence of equipment such as display cases or ma chinery. Both noise and air velocity in the occupied zone generally increase with increased outlet velocities and must therefore be considered. The blow is dependent to a marked degree on the* tan* pcrature.of air lowing the heater as well as-upon its velocity and volume. Increased final temperature will reduce the area of heat coverage and the mounting.height. Care must-be exercised in the application of down-blow unit heaters. The higher the mounting height, the lower must be the outlet temperature of the air leaving the heater, in order that the heated air will be forced down into the occupied zone. . Since the outlet temperature of air from a unit heater, in creases with the temperature of the heating medium, unit heaters for high pressure steam or high temperature hot water should be designed to produce approximately the same leaving air temperature as would be obtained from a lower temperature heating medium. In order to obtain the desired air distribution and heat diffusion, unit heaters are commonly .equipped with direc tional outlets, adjustable louvers, or fixed types of diffusers. For a given unit with a given discharge temperature mid outlet velocity, the mounting height and heat coverage can vary widely with the type of directional outlet, adjustable louver, or diffuser employed. Other factors which may influence the heat coverage must also be considered. Obstructions, such as columns, be^ms, partitions, or machinery, either in the discharge air stream or in the approach area to the unit, can reduce the heat coverage substantially. The presence of strong drafts or other air currents will also reduce the coverage. Exposures such as large glass areas or outside doors, especially on the windward side ol the building, require special attention, and units should be arranged so as to blanket such exposures with a curtain of heated air which win intercept the cold drafts. For area heating, horizontal-blow unit heaters in exterior zones should be located so as to'Mow along the exposure or toward it at a slight angle. Where possible, multiple'units should be arranged so that the discharge air streams sup port each other and create a general circulatory .motion in the space. Interior zones under exposed roofs' or skylights should be completely blanketed. Down-blow units should be arranged so that the heated areas from adjacent unite over lap slightly, so as to provide complete coverage. For spot heating of individual spaces in larger unheated areas, single unit heaters may be employed, but allowance must be made for the inflow of unheated air from adjacent spaces and the consequent reduction in heat coverage: Iso^ lation of such spaces by partitioning or enclosure is always desirable, from a heating standpoint. Horizontal unit heaters should have discharge outlets lo cated well above head level, and both horizontal and vertical units should be so applied that the heated air stream will be delivered to the occupied zone at temperatures and velocities which are not objectionable. Where possible, units should be located so that the discharge air flow is into open spaces, such as *!<**, and not directly on the occupants! , Manufacturers' catalogs usually give suggestions for.the best arrangements of various unit heaters, recommended mounting heights, heat coverages for various outlet velocities, final temperatures, and directional outlets. , Sound Level Consideration must be given to the problem of the noise generated by unit heaters, if they are applied in spaces hav ing a requirement for low sound levels. All unit heaters genererate some noise, and whether or not that noise will be ac ceptable depends upon the type of space in which it is applied: For example, units with noise levels-acceptable in a: manu facturing plant might be too noisy for use in a show-room. A unit heater for a particular application must therefore be selected for satisfactory sound level as well as for adequate heating capacity and proper heat distribution. Ratings of Unit Heaters- It is standard practice to rate all types of unit heaters on the basis of the amount of heat delivered by the-air in Btu per hour above an entering air temperature of 60 F. Steam. Rating of steam unit heaters has been standard ized by a code* in which the following items are the basis of rating: dry saturated steam at 2 prig pressure at the heater coil, air at 60 F (29.92 in. Hg barometric pressure) entering the heater, and heater operating free of external resistance to air flow. The capacity of a heater increases as the steam pressure increases, and decreases as the entering air temperature in creases. The heating capacity for any condition of steam pressure and the entering air temperature other than standard may also be determined by means of a standard procedure prescribed by this code. Hot Water. A standard for the rating of hot water type unit heaters has also been established by a code4 in which the following items are the basis of rating: entering water at 200 F, water temperature drop 20 F entering air at 60 F (29.92 in. Hg barometric pressure), and heater operating free of external resistance to air flow. Variations in entering water temperature, entering air temperature, and water flow rate frill affect capacity. This code prescribes a method of translating the heating capacity, as obtained under test conditions, to other conditions of air and water temperature. Gas-fired. Gas-fired unit..heaters are rated in terms of both input and output, in accordance with the approval re quirements of the American Gas Association. a ; Unit-Ventilators and Unit Heaters 855 RCTUAM CATC *IV<^ --6 supput Fig. 15'.... Connection of Vertical Unit Heater to Hot Woter System Oil-Fired. Ratings of oil-fired unit heaters are based on heat delivered at heater outlet in Btu per hour. Electric. Electric unit heaters are rated on the energy input to the heating element, expressed in terms of kilowatts, or Btu per hour. Effect of Resistance Upon Capacity. Unit heaters are customarily rated as free delivery type units. If outside air intakes, air filters, or ducts on the inlet or discharge are used, a reduction in air and heating capacity will result, be cause of this added resistance to air flow, inlw^ compensat ing changes are made in fan Bpeed. This reduction in capac ity will depend upon the characteristics of the heater, and on the type, design, and speed of the fans, so that no spe cific percentage reduction ean be assigned for all heaters at a given' added resistance. The heat output to be expected under other than free delivery conditions should be. secured from the manufacturer. Effect of Inlet Temperature. Changes in entering air tem perature will influence the total heating capacity in' most unit heaters and the final temperature in- all units. Since many unit heaters are located some distance from the occu pied zone,' consideration should be given to the possible differences in temperature of the air actually entering the unit fro that being maintained in the heated area. This-is especially true with down-blow unit heaters. In general, the temperature gradient (temperature difference per foot of elevation) when using unit heaters is less than that when using gravity heating unite.* Higher velocity units and units with lower discharge air temperature will maintain lower temperature gradients than unite with higher discharge temperatures. Valve-con trolled or bypass-controlled units employing continuous fan operation will maintain lower temperature gradients than will units employing intermittent fan operation. Directional control of the discharged air from a unit heater can also be an important factor in effecting satisfactory distribution of heat and in reducing Aoor-to-ceiling temperature gradients. . When some outdoor air is introduced, the temperature of the mixture of outdoor and recirculated air must <be calcu lated and used as the entering air temperature at the heater. For recirculating heaters located at the floor, or with intakes Fig. 17.... Unit Heater Connection for Low-Pressure Steam Open Gravity or Vacuum Return System at the floor, the temperature of air entering the heater should be assumed to be the same as that to be mrinteitral in the room itself. Automatic Control of Unit Heaters The controls for a steam or hot water unit- heater ran either provide (1) on-off operation of the unit fan or (2) continuous fan operation with modulation of heat output. For on-off operation, a room thermostat is used to start and stop the fan motor. A limit thermostat, often strapped to the supply or return pipe, - prevents fan operation in the event that heat is not being supplied to the unit. Continuous fan operation eliminates the intermittent blasts of hot air resulting from on-off operation, as well as the stratification of temperature from floor to wiling, which often occurs during the off periods. In this arrangement, a pro portional room thermostat controls a valve modulating the heat supply to the coil or a bypass around the heating ele ment. A limit thermostat or auxiliary switch stops the fan when heat is no longer available. . Control of direct-fired and electric units is usually accom plished by intermittent' operation of the heat source under control of the room thermostat, with a separate fan switch to run the fan when heat is being supplied. For a more general discussion of automatic control, refer to Chapter 13! Unit heaters may be used in summer as a means of provid ing air motion. In such cases, the source of heat is shut off and the thermostat is provided with a bypass switch that will permit fan operation independently of toe controls. Piping Connections For Unit Heaters Piping connections for steam unit heaters are similar to those for other types of fan Mast heaters. The piping of unit heaters must conform strictly to toe system require ments, while at the game time permitting the heaters to