Document gE4pp5o5LdGxyZRXnJLZJqX3

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 stall 1 2 X=5=W From Table 28-P, the face velocity should be 120 fpm The face area is 5X10 or 50 sq ft, and the air volume is 50x120 or 6000 cfm-- not much higher than the 5440 cfm required before, even though the hood is 50 per cent larger m area The distance of the hood from the range (X) is now the same as the hood width (W), that is, it is 1 0W, instead of being 1 25W as before, and results in more effective utilization of air The amount a canopy hood should extend beyond the cooking equipment is regulated in many cities by ordinance, and of course m such cases the ordinance should be complied with Where there is no code, it is customary to increase the dimension of a canopy hood 4 to 5 inches for every foot of distance between the hood face and operating surface (range, fryer, etc ) Some designers use a minimum overhang of 12 in A rectangular exhaust hood placed along the wall at the back of deep fat fryers and grills can be installed closer to the source of smoke and odor In effect it becomes a lateral exhaust for an "open-surface tank," and may be designed as already outlined m the section under that heading The smoke and oil vapor are trapped before they nse from the cooking unit, and they have less chance to spread through the room As al ways, end baffles further improve the op eration Crease filters are desirable in all kitchen hoods, including the lateral exhaust type, for two reasons they prevent grease from enter ing the duct, where it condenses, causing a fire hazard, and when placed m the hood it self, they serve as excellent diffusers to assure a uniform distribution of air over the hood opening, which is generally large in area Crease filters, however, add resistance to the system, sometimes as much as 05 in water gauge The exhaust fan should be able to handle the necessary volume of air over the range of filter resistance pressures Crease filters should be cleaned regularly An approved fire damper with a fusible link should be installed in the main exhaust duct or branch adjacent to the range hood This is required by code in many states Recirculation of "cleaned" air When the rate of air exhausted by a con taminant control system is high, either in absolute number of cfm or in comparison to the cubical contents of the room, the sugges tion may be made to recirculate the cleaned air One of two reasons may be put forward It is often argued that the dollars-and-cents savings in heat will be so worthwhile that recirculation is attractive, or the existing heat ing system may have so little reserve capacity that recirculation will be the only way to maintain a comfortable working temperature There are some basic principles to follow in deciding whether to recirculate or to provide tempered makeup air First, savings in fuel or in heating plant capacity are never a valid reason for recircu lating air if the original air contaminant is a potential health hazard For example, if the exhaust system is installed to control free silica dust, lead dust, other toxic dusts, harm ful gases, or solvent vapors, the effluent should be exhausted outdoors Even if the dust filter or other arrester is operating at high efficiency, so the concentration of contami nant downstream is normally within safe limits, a breakdown can always occur which would cause dangerous amounts of material to be blown back into the workroom Second, if the contaminant is harmless and, if after passing through a dust, mist, fume, or vapor separator, the effluent is no more contaminated than the outdoor air, re circulation would be safe Third, there should be no recirculation during warm weather, even ifthe contaminant is of the most harmless sort The local ex haust system, discharging outdoors, will add to the general room ventilation, and so make the area more comfortable This is particu larly true if any of the processes discharge heat or steam Fourth, in every recirculation system, therefore, a Y-connectaon should be included in the discharge of the arrester One leg should go to the outdoors, and the other should lead to the recirculation ductwork Adjust able dampers should be arranged so that 893