Document xjD44O94vOjj9L7XayaQ86dLJ

EXHAUST SYSTEMS, GENERAL b COMFORT VENTILATION TABLE 28-P HOOD FACE VELOCITY (FPM) TO MAINTAIN A CONTROL VELOCITY OF 30 FPM AT RANGE Side Ratio (XT'L) Distance X from Hood Opening to Worenc Area 045JV 05W 075W W 1.25W 15W 1:1 Free hanging....................... 40 1:2 Shed type............................. 1.2 Free hanging....................... 24 1:4 Shed type............................. 1 6 Shed type............................. 120 40 60 24 16 800 77 120 40 27 120 215 60 40 170 355 90 60 240 120 80 Bsprlnted with permission from 3. M. DoBoVeUc. Nchausl Boodo. 2nd ed New York Clip, The Industrial Press. 1952. being of the canopy type, suffer from all their troubles. As has been mentioned pre viously, exhaust hoods cannot "reach out" more than a few Inches and trap the air contaminant, and yet the kitchen hood is often four feet or more from the range surface. Cross drafts can easily disperse the smoke to the room before it comes un der die influence of the hood On the plus side, fortunately, range hoods, in large kitchens at least, are often ex tremely long for their width, covering a row or Tanges, ovens, gnlls, deep fat fryers, etc. Frequently they can be set so the long side is against a wall (although the sound ness of doing so is often overlooked), and the hood becomes a shed-roof type hood The bad effect of unbaffled ends is then diminished Also, the temperature gradient set up by heat of the stoves aids in creating air movement into the hood Finally, as DallaValle0 has written, the control veloci ties needed at the surface of the range are low compared with those needed, for exam ple, in dust control where particles must he transported into the duct DallaValle recommends 30 fpm (Table 28-P) as the control velocity to be estab lished at the range surface. For different shaped hoods, he has calculated the face velocity, that is, the velocity of air at die hood opening necessary to set up this 30 fpm control velocity. His dimensions are in terms of hood width, W. Hood shape is expressed as "side ratio." A side ratio of 1 4, for exam ple, means a rectangular hood that is four times as long as it is wide. Two examples will show the extrava gance of a freely suspended hood Assume a freely suspended hood, 4 ft wide by 8 ft long, is located 5 ft above a range, and that a velocity of 30 fpm at the range will control the cooking vapors Ratio of sides (4 ft by 8 ft) = 1.2 X = 5 = 1.25W According to Table 28-P, the hood face velocity should be 355 fpm Since the face is 82 sq ft m area, the air volume needed is 32X355, or 11,360 cfm This figure is actually too low, according to good design standards, for the hood edge should over hang the range edge. (See following para graphs.) Now assume the hood and range can be hacked up against a wall, so the hood be comes a shed type The face velocity needed will be 170 fpm Since the face area is still 32 sq ft, the air volume will he 32x170, or 5,440 cfm--less than half that needed for a freely suspended hood. Again, suppose the hood is made larger, but kept against the wall at the same height above the range Now it is 5x10 ft, and a shed type The side ratio is still 1.2 X=5=W. From Table 28-P, the face veloc ity should be 120 fpm The face area is 5x10 or 50 sq ft, and the air volume is 50x120 or 6,000 cfm--not much higher DallaValle, J. M. "Design of Kitchen Range Hoods" Heating and Ventilating, 95 100 (Aug 1953). 28-46