How to Maintain Good Ventilation in a Kitchen

Good ventilation consists in maintaining the atmosphere in a comfortable condition by the admission of fresh air and the removal of vitiated air. Among the factors that affect comfort are humidity, the presence of cooking smells, temperature and draughts, and the ventilating system in the kitchen must therefore be capable of controlling any, or at peak periods all these factors so as to contribute to the comfort of the occupants.

Let us first consider the question of humidity, which is, perhaps more than any other, a cause of discomfort. Where large quantities of water are kept at boiling point, water vapor is constantly being driven off into the atmosphere. Boiling pans and steaming ovens are the main sources of steam in a kitchen, but hot water taps discharging into sinks and the sinks themselves when full of hot water also provide a certain amount. Much of this water vapor is taken up by the air and the hotter the air the more water vapor it can hold. The increase in the humidity of the air reduces the rate of evaporation from the skin, and it is this that is the fundamental cause of discomfort. After saturation point is reached other disadvantages of a humid atmosphere become apparent. Condensation takes place on such cold surfaces as walls and ceilings, water and gas pipes, and the resulting moisture streaming down the walls will cause deterioration of decorations, rusting of metal work and damage to food and other things with which it comes in contact. All too often in large kitchens there is a cloud of steam which may be so serious as to interfere with visibility, especially on still days and even more on foggy days when the atmosphere is already so heavily laden with moisture that it cannot carry the heavy load imposed upon it by cooking processes. This is the extreme case which produces very unpleasant conditions and must obviously be guarded against, but a good ventilation system provides for more than the avoidance of the extreme. Comparatively dry atmospheric conditions are most conducive to comfort, and a good ventilating system should therefore be designed to keep the moisture content of the atmosphere as low as possible.

The elimination of all smells of cooking is, of course, impossible and generally speaking they cause no great inconvenience within the kitchen. It is only when these odors spread through other parts of the building that they become a source of annoyance. A good ventilation system must therefore be designed to avoid this defect. Frying, and particularly deep fat frying, is one cooking process which will load the atmosphere unpleasantly even for those working in the kitchen, and a very efficient system of ventilation is required to ensure that the atmosphere, and in fact the clothes of the staff, do not become permeated with this particular smell.

Although the prevention of uncomfortably high temperatures will be discussed later, it is convenient to consider here the joint effects of temperature and draughts. Heat is constantly discharged into the atmosphere by cooking processes, with obvious effects on the staff, who, when overheated, are more sensitive to draughts. This must be remembered when planning a ventilating system. It is difficult to provide for adequate circulation and exchange of air without creating draughts and a powerful ventilating system has not infrequently had to be suspended because of complaints from the staff that it causes them to have colds.

The measurement used in assessing the capacity of a ventilating system is based on the number of air changes in one hour. From what has already been said it is clear that this figure must be higher for a kitchen than for the general run of rooms. Ventilating systems designed to give change rates up to 30 per hour are now generally recommended with higher rates of up to 60 for those below ground. This is a high rate of change which should ensure good ventilation, but care must be taken to prevent it cooling the kitchen too much in cold weather. With these high rates provision is usually made for warming the air before its introduction into the kitchen. A system whereby the rate of air change can be varied is also useful. Thus during peak periods it can be increased to the maximum and during slack times slowed down to a lower rate.

Some Ventilating Systems used in Kitchens

Natural ventilation is generally suitable only for the smaller kitchens where the number of meals cooked is two hundred or less. In these kitchens if there is good cross ventilation and if the equipment is placed so that steam and cooking smells have easy exit, satisfactory conditions can be maintained for most days of the year, but trouble will still be experienced on damp misty days and on warm still days. Care must also be taken when planning kitchens with natural ventilation to ensure that windows are not so placed that whenever they are opened wind carrying dust, etc., blows on to the food during its various stages of preparation. Where single storey buildings are used, overhead lighting and ventilation are frequently provided by lantern lights in the ceiling. These have windows which when opened allow the escape of the rising steam and hot air. Fresh air is then drawn in at the lower levels through doors and windows thus producing a constant change of air.

It has been common practice for some years to erect over blocks of cooking equipment hoods which are connected to the fresh air outside by ducts. Plates I, II and III show kitchens where this method of ventilation has been adopted. Steam and hot air rise into the hood and escape to the outside either naturally or with the assistance of an extractor fan. There may be a fan for each hood or one serving a number of hoods through common ducting. Hoods are made of reinforced glass or of special metals resistant to corrosion by steam and the fumes of cooking. As steam generated during cooking is bound to condense on the inner surface of a cold hood, a gutter should be provided along the lower edge of the hood and connected through a small pipe to the floor drain. There is no doubt that such hoods are effective in eliminating steam but they have some rather serious disadvantages, the main one being that they tend to produce dark unsightly kitchens and to increase the cleaning work by creating extra surfaces which hold grease and dirt. In addition the metal of these hoods increases the radiating surfaces of the kitchen.

The position of the hood in relation to the block of equipment it covers and the height of the lower edge from the floor are points to be considered. The height above the floor must be sufficient to allow the staff to stand comfortably underneath the hood, and 6ft is normally adequate, except where high chefs’ caps are worn. There are two different points of view on the dimensions of the hood in relation to the equipment it covers. One is that it should extend -12″ beyond the cooking area and the other that it should only be sufficiently large to cover the actual area of the cooking apparatus and not to extend beyond; the staff will then be working outside the hood and so in a cooler atmosphere.

For those kitchens where a mechanical system of ventilation is required, the choice lies between the provision of hoods and a general controlled system for the whole of the kitchen. In many cases ventilation is controlled by fans at strategic points. The most efficient method is to install at appropriate points powerful extractor fans which create vigorous air currents and draw the steam and hot air out of the kitchen. In less efficient systems frequently employed through ignorance of the fundamental principles of ventilation, an odd fan is installed here and there to produce movement of the air. Fans vary in power and are rated according to the number of cubic feet of air discharged in one hour. It is obviously important to install fans which are adequate for the work for which they are intended. Unfortunately fans do not always give the rate of air change specified and it is important to insist on adequate tests by the maker to ensure that the required performance is secured.

In those cases already discussed where ventilation is secured by extraction of the steam and fumes, fresh air enters more or less by chance through open windows and cracks under doors. In some modern kitchens a system has been installed in which the entrance of fresh air is controlled, pressure fans forcing the air into the kitchen at selected points. An important feature of this controlled system is that all doors and windows are kept closed. Moreover it has the advantage that the air can be cleaned and if necessary adjusted to a suitable temperature before introduction into the kitchen, both of which operations are beneficial to kitchen work, particularly in industrial areas.

If a kitchen where such a system has been installed adjoins a dining room, care must be taken to ensure that steam and cooking smells from what is now virtually a pressurized kitchen do not enter the dining room. This can be achieved by installing extractor fans which are more powerful than those providing input.

This use of both input and extractor fans sounds ideal, and indeed there is no reason why it should not be so. Yet even with such safeguards failure will result unless the actual conditions in the kitchen are considered. Conditions vary from kitchen to kitchen according to the amount of equipment installed, the quantity of heat which it radiates, the amount of steam produced during cooking, and other factors. For instance, it is generally accepted that a kitchen where gas is used for cooking requires a more powerful ventilating system than one where electricity is the source of heat. This is because water vapor is one of the products of combustion of coal gas. It is therefore essential that the vent of any item of cooking equipment using a considerable quantity of gas should be near an extractor fan or underneath a hood with extract ducting or that the vent, suitably equipped with a baffle, should lead directly to the outside atmosphere. Actual conditions must be visualized and taken into account when planning the ventilating system. It was not a good advertisement either for the system or for the planning that preceded its installation to find recently in a new kitchen that the temperature of the pastry room was well over 90°F.

Ventilation of Ancillary Rooms

Good ventilation is necessary for storerooms, but as the atmosphere in these rooms is not affected by cooking processes, a mechanical system is not necessary and natural ventilation can usually be adopted. Care should be taken however to ensure that the storerooms do not become a pocket in which the hot steamy atmosphere of the kitchen can collect. Stores should obviously be kept cool; if steam from the kitchen can enter, it will condense in the cool atmosphere and streaming surfaces will result.

For offices and small staff dining rooms natural ventilation is adequate. The question of the ventilation of large dining rooms is of course another matter, and outside the range of the present discussion. It is of interest to note in passing, however, that when a number of people are collected in a comparatively small space, the temperature slowly rises-the heat generated by ten human beings is equivalent to that given off by a 1-kilowatt fire, while heat is also emitted by the dishes of food served. As the period of use in the middle of the day may extend over two hours it is clear that in large dining rooms a controlled system of ventilation will be necessary to maintain comfortable conditions throughout.