In commercial kitchens, the steam and vapor carrying latent heat, and the sensible heat produced within the kitchen must be carried off by appropriate ventilating means to maintain reasonable temperature and humidity conditions within the kitchen so that these conditions are tolerable by the kitchen personnel and do not affect adversely the cooking process.
In other words, the ventilation must be so dimensioned that predetermined temperature and moisture levels in the kitchen are not exceeded.
This requires generally a certain air replacement rate with which fresh outside air is supplied to the kitchen and kitchen air with the sensible heat and steam and vapor with the latent heat are withdrawn.
During periods in which the facility must be heated, generally in the winter, the supplied air must be heated to prevent the kitchen personnel from being chilled and to prevent detrimental temperature drops which might interfere with the cooking process. Furthermore, because relatively large amounts of air must be circulated, the heating may be relatively costly with respect to the energy which must be supplied to the air.
In determining the amount of air which must be circulated, therefore, one must consider on the one hand the cooking units which are in operation in the kitchen and may contribute comparatively large amounts of water vapor and hence latent heat, namely, boiling equipment, frying and water baths and steam tables which are commonly present in such kitchens. It is also necessary to consider cooking devices which contribute primarily radiant heat and/or convection heat to the atmosphere in the kitchen, i.e. sensible heat. These can include grills, ovens, tilting pans and the like. The type of heat which makes the most significant contributions to the air circulation, therefore, will depend on the facilities in the kitchen.
To minimize the amount of heat energy which must be supplied for the heating of the cold supplied fresh air, two basically different approaches are employed. In one approach heat is removed from the discharged air and used to preheat the incoming air by heat exchangers. A drawback of this system is that the discharged air normally contains residual grease which can condense in the heat exchangers and requires maintenance thereof at high cost. The other approach utilizes a so-called short circuit or induction hood in which a part of the supplied air is blown via a nozzle directly into the vapor collection space and thus is immediately drawn out in a short circuiting of the fresh air supply. In winter in which less sensible heat must be extracted, this short circuiting allows less flow from the kitchen of already heated air while nevertheless supplying a sufficient velocity with the aid of the unheated external air to permit the grease filter in the hood to remain effective and maintain the high degree of grease removal which is necessary. It should be understood, in this regard that the grease filter generally operates as an impingement or baffle filter in which the relatively high velocity is necessary for a high degree of grease separation.
The drawback of this approach, however, is that the introduction of unheated outside air into the kitchen and hood requires expensive and careful control of the air intake because any of this air which is not immediately discharged will alter the climate within the kitchen.