Almost any canteen, except perhaps the day school canteen, will be required to undertake the making of tea and probably also of coffee, and therefore facilities for the supply of boiling water must be available. Naturally requirements will vary depending on whether the demands for tea and coffee must be met within a very short period, as in the service of breakfast or tea in a boarding school, or whether hot drinks must be available over a much longer period, as in a railway buffet. But whatever the service the facilities provided must be adequate to meet maximum demands without undue delay.
It is obvious that the heating of water for tea making in movable containers such as large kettles is not practicable except for small numbers. The average quantity of boiling water to allow at teatime where two or more cups will be taken is 1 pint per head, which means that for 100 persons 121 gallons will be required. This quantity can be provided by a battery of 1-2 gallon kettles but such a method is clearly not very convenient for general everyday use. A large container is required which can be filled, heated and emptied in a fixed position; in this way the difficult ties and risks involved in the lifting of large quantities of boiling water are avoided. The problem can be solved very simply by using the ordinary domestic gas, electric or solid fuel washboiler with draw-off tap and fixing it at a convenient height for operation, but it is usually more convenient to employ a piece of apparatus specially designed for the purpose. Such equipment is known as the water boiler (bulk type) tea water boiler, or water boiling urn. The sectional diagram in Fig. 19 shows the general construction of a gas heated water boiler of this type. It consists of an inner pan usually made of tinned copper, with a chromium plated outer casing and lid; the water in the inner pan is heated by a gas burner under thermostatic control. A water inlet pipe for connection to the main water supply, a calibrated gauge glass and two draw-off taps are also provided. Water is admitted to the inner pan by manual control of a cock on the inlet supply until the gauge glass shows that sufficient has been run in. The burner is then lighted and the contents heated to boiling point, indicated by the thermometer near the draw-off tap. This thermometer must of course be fixed near the bottom of the container since owing to convection currents, there will be a difference in temperature between the top and the bottom until the total contents reach 212°F., and unless the reading is taken near the draw-off tap there is a possibility that water before 212°F may be drawn off in error.
Bulk water boilers can be obtained in sizes varying from 5 gallons to 50 gallons capacity. The time taken to raise the one tents to boiling point will vary according to the thermal capacity of the heating unit in relation to total capacity of the container, The initial temperature of the water from the main supply will also have an influence but the following times are given as a useful guide.
These bulk water boilers are also made as single containers without an outer casing, a type of construction which is less expensive but which has two disadvantages. The outside surface will, as the contents are heated, become much hotter and unless care is taken burns may result. This higher temperature will also Cause greater heat losses through radiation and a lowering of efficiency.
These boilers are simple and easy to use, but there is one point which requires emphasis. The supply of cold water to the container is not automatic as this would result in water ceasing to boil as soon as any boiling water was drawn off. It becomes necessary therefore for the stop-cock to be opened and the boiler refilled before all boiling water is drawn off, otherwise it will burn dry with a risk of overheating. In addition to this bulk type boiler which may be heated by gas, electricity or steam coils, there is also the automatic electric storage type of boiler in which an immersion heater boils the water which is drawn off at the top of the container. The principle is the same as in the water heater the difference being that the temperature is carried up to boiling point by the provision of a heater of sufficient loading and by the appropriate setting of the thermostat.
The bulk water heater is excellent for certain purposes such as the production of large quantities of boiling water at a given time in an industrial canteen where all the tea required must be ready for use within the tea interval of about 15 minutes. But for canteens where a continuous supply of boiling water is required over a period of, say, 1-2 hours it is less suitable. As indicated above a considerable length of time is required to raise the temperature of the whole of the contents to the maximum. Moreover, when all the water has been used a similar period of time must elapse before the same quantity is again available. One other affect arises which relates particularly to the water to be used for tea making. It is a well known and widely accepted fact that only freshly boiled water should be used for tea making. With the bulk water boiler there is the danger that the water will be boiling some time before it is required so that when it is used it will be flat and the tea less acceptable.
Equipment is, however, available in which these difficulties do not arise and which, after a short initial period, will deliver an almost continuous supply of boiling water for an indefinite period. Such a piece of equipment is known as the instantaneous water boiler and may be obtained singly or as part of a café set as shown in the illustration in Plate XII, where the center container is the boiler and the two containers on either side are designed with water jackets for milk warming and coffee making.
Instantaneous water boilers which are heated by gas, electricity or steam coils, may be one of two types, operating on quite different principles. One of these is known as the non-pressure expansion type and the other as the pressure type. Both types are equally satisfactory, a point about the latter being that because it operates under a few lb. pressure it is subject to periodic inspection and insurance.
(a) Boilers of the expansion type differ to some extent from one manufacturer to another in the detail of their design, but all depend for their operation on the fact that water expands when heated to boiling point. Rapidity of boiling is achieved by providing a large heating surface to a small amount of water. The diagram in Fig. 20 shows the general design. The boiler, which is usually constructed of heavy gauge hard rolled copper, is fed with cold water from a feed tank. This tank automatically controls the level of the water in the boiler by means of a ball valve. It will be noticed that the water in the boiler first enters the outer jacket and then flows into the lower end of the interior chamber, which consists of several inter-connected compartments designed to present a large heating surface When heat is applied, the water in this inner compartment boils and expands up into the boiling water chamber at the top whence it can be drawn off through the delivery tap. The height of the heating chamber is such that water cannot be drawn off until it is actually boiling.
(b) The operation of the pressure type boiler depends on the fact that when heat is applied to water in an enclosed vessel the steam generated on the surface ses up a pressure sufficient to force the water downwards to the lower end of a draw-off column. The diagram in Fig. Il shows the general design. The water presses up this column to the outlet pipe and although it is not boiling inside the vessel, its temperature is above 212°F., so that it boils as it emerges and the pressure drops to atmospheric.
Output of Instantaneous Boilers
The main concern of the caterer is the rate of flow of boiling water, expressed as the quantity required within a given time. Thus if 100 pints of water are required for tea making at a fixed time, this amount must be available during a short period of, say 15 minutes before that time. This means that a machine must be available which will deliver at the rate of approximately 7 pints per minute or, as it is usually stated, 400 pints per hour. If on the other hand 100 pints are required over a longer period of, say, 1 hour, the demands on the equipment would be less and would be met by a machine delivering boiling water at 11 to 2 pints per minute or 100 pints per hour.
It must be remembered that water boilers of both the pressure and expansion type are intended to give what is called a continuous intermittent flow. It is usual in practice for a tea making container to be filled and then for the boiler to be turned off while this container is removed and another put in its place. The performance of the boiler fits in with this procedure for the short breaks are essential to permit recovery. Consideration of the working operation of these boilers shows that this short intermittent demand is important in order to permit recovery. There is one other point which the user will quickly appreciate. When the equipment has been resting for a short time, the initial output for the first few minutes is higher than from then onwards; an expansion type boiler rated at 240 pints per hour is capable at the outset of giving 5 pints per minute for several successive minutes. This is because of the reserve of hot water available, which is characteristic of this type of boiler. Cold water is not admitted direct to the heating chamber but flows first through the outer jacket, where it is partially heated. When, however, the boiler is continuously used over a long period, the water flows through this jacket so rapidly that it picks up very little heat and therefore reaches the boiling chamber at a temperature very little above that of the mains supply. Clearly when this point is reached the time taken to boil the water in the heating chamber itself will be longer, and the delivery will be slower.
It is evident from the description of the operation of these boilers already given that the rate of flow of boiling water through the apparatus is directly related to the capacity of the heating unit, and that the rate of flow in pints per minute can be raised by increasing the supply of heat and lowered by decreasing it. It is not usual, however, for this to be under the control of the Operator. Equipment is supplied to deliver automatically a maximum rated output, and if a rate of delivery greater than this is required, another model with a more powerful heating element must be used. It cannot be emphasized too strongly how important it is to have a boiler which is equal to the maximum demands which may arise. All too frequently through mistaken economy, a boiler is installed which is not adequate for the output required, the result being that difficulties arise and the performance of the boiler is blamed unreasonably.
Instantaneous boilers are made in a wide range of sizes, outputs from 150 pints to 800 pints per hour being available in the pressure type, and 100 pints to 1,400 pints per hour in the expansion type. This latter output, however, is only obtainable when high pressure steam of at least 60 lbs. per sq. in. is available for the heating coils. High pressure steam is of course used also for the normal range of models but in these a pressure of 15 lbs. per sq. in. is usually employed. Where electricity is used, loadings of 7 kW. for 100 pints per hour and 15 kW for 240 pints per hour are common. There is some variation of practice between manufacturers of gas boilers, but one well known manufacturer of the expansion type boiler gives the gas consumption as 5 cu. ft. per gallon of boiling water, with a by-pass rating of 8-15 cu. ft. per hour according to the size of the boiler. Comparison of the thermal efficiencies of comparable expansion and pressure type boilers shows that there is no appreciable difference between the two.
Maintenance of water boilers is very important especially in hard water districts. It will be appreciated that a piece of apparatus designed to supply boiling water will, if hard water is used, collect a deposit of lime which will adversely affect the efficiency of the equipment. In the expansion type particularly, where efficiency depends upon the heating of water in narrow channels the furring up of these tubes will obviously impair efficiency. The speed at which the deposit forms will naturally depend on the degree and type of hardness of the water used and the frequency of use of the boiler, but the manufacturer is usually very willing to advise on this matter and to supply a maintenance service of which the Caterer would be well advised to avail herself. In the London area it is usually found necessary to have the expansion type equipment de-scaled about every three months but this period naturally varies in different parts of the country according to the degree of hardness of the water. Two methods are used. Either the equipment is taken to pieces and the scale chipped off manually, or dilute hydrochloric acid is used to dissolve the lime, any acid remaining, is then neutralized with caustic soda. The equipment is finally boiled out with water.
Two appliances are available which help in this problem by reducing the formation of scale and the need for frequent servicing. One type consists of a small cartridge filled with a slowly soluble form of a metaphosphate known commercially as Micromet. This is fitted into the supply pipe to the boiler so that the water when it reaches the boiler contains minute quantities of metaphosphate. The effect of this low concentration, which amounts to only a few parts per million, is to reduce the formation of scale, perhaps by preventing crystal growth. It appears that although this threshold treatment is of some value with the expansion type of boiler it is unsuited for the pressure type. In the second appliance an electrical method is used. A current, so small that an ordinary flash light cell will supply it for a period of six months is passed across the inlet stream, with results that are similar to those of the chemical scale reducer already mentioned. This device is equally satisfactory with both types of instantaneous boiler.