Document MMqQkzKer0bqVz8b0najOQo7z

EXHAUST SYSTEMS, GENERAL b COMFORT VENTILATION timn the 5,440 cfm required before, even though the hood is 50 per cent larger in area. The distance of die hood from the range (X) u 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. Hie amount a canopy hood should ex tend beyond the cooking equipment is regu lated in many cibes by ordinance, and of course in 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 oper ating 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 ftyeis and gnlls 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 oper ation. Grease filters are desirable in all kitchen hoods, including the lateral exhaust type, for two reasons they prevent grease hom entering the duct, where it condenses, caus ing a fire hazard, and when placed in the hood itself, they serve as excellent diffusers to assure a uniform distribution of air over the hood opening, which is generally large m area. Grease filters, however, add resist ance to the system, sometimes as much as 0 5m water gauge The exhaust fan should be able to handle the necessary volume of air over the range of filter resistance pres sures Grease filters should be cleaned regu larly. An approved fire damper with a fusible link should be installed in the mam exhaust duct or branch adjacent to the range hood Hus 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 suggestion may be made to recirculate the cleaned air One of two reasons may he put forward. It is often argued that the dollars- and-cents savings in heat will be so worth while that recirculation is attractive, or the existing heating system may have so little reserve capacity that recirculation will be the only way to mamtarn a comfortable working temperature There are some bas ic principles to follow in deciding whether to recirculate or to provide tempered make up air First, savings in fuel or in heating plant capacity are never a valid reason for re circulating air if the original air contami nant is a potential health hazard For ex ample, if tiie exhaust system is installed to control free silica dust, lead dust, other toxic dusts, harmful gases, or solvent va pors, the effluent should be exhausted out doors. Even if the dust filter or other ar rester is operating at high efficiency, so the concentration of contaminant downstream is normally within safe limits, a breakdown can always occur which would cause dan gerous amounts of material to be blown bade 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, recirculation would be safe. Third, there should be no recirculation during toarm weather, even if the contami nant is of the most harmless sort The local exhaust system, discharging outdoors, will add to the general room ventilation, and so make the area more comfortable This is particularly true if any of the proc esses discharge heat or steam Fourth, in every recirculation system, therefore, a Y-connecbon should be includ ed in the discharge of the arrester One leg should go to the outdoors, and the other should lead to the recirculation ductwork. Adjustable dampers should be arranged so that when the control is in one position, 100 per cent of the air will Ire discharged outdoors, and when in the opposite posi tion, 100 per cent will be sent back to the room Various in between adjustments can be made depending on the outdoor air tem perature. A further advantage of this Y is that should the arrester break down, the uncol lected contaminant can immediately be blown outdoors. Instead of the workroom. 28-47