Tank Heaters.
—When the demand for hot water is sufficient to warrant a separate hot-water heater the apparatus similar to Fig. 119 is used. With such a heater, the conditions of overheated water—to be described later—may be almost entirely avoided. In this case the connections are arranged similarly to those of the range boiler but a separate furnace takes the place of the water-back. The heater is simply a small furnace made expressly for heating water. Connected with the discharge pipe p is a draft-regulating valve which controls the drafts of the heater. The draft-regulator is set to so control the furnace that water at the desired temperature will always be in the tank. The mechanism of this regulator is the same as the draft-regulator described under hot-water heating plants.
Overheated Water.
—Under ordinary conditions the water contained in the range boiler is below the atmospheric boiling point (212°F.) but at times when a hot fire is kept up in the range for a considerable period, the temperature will rise to a degree much above that amount. The temperature to which the water will rise will depend on the pressure of the water supply. As an example—suppose the gage pressure of the water supply is 25 pounds. The temperature corresponding to that pressure is 258°F. The temperature of the water in the tank will rise to that amount but not further because any additional temperature will produce a higher pressure, but a higher pressure would be greater than the pressure of the water supply and hence will back the water into the supply pipe. This condition of things, then, acts as a safety valve to the tank to prevent excessive pressures.
Fig. 119.—Independent hot-water heater with temperature regulator.
When the water at a high temperature is drawn from the tap a considerable part of it will instantly vaporize, because of the reduced pressure. If water at a pressure of 25 pounds is drawn from the faucet, the temperature, 258°F., is sufficient to send all of the water instantly into steam. This high temperature will scald at the slightest touch. The water drawn from the faucet will continue to vaporize as it comes into the air until the water in the tank is cooled by the incoming cold water. The only means of relieving the overheated condition is to open the faucet a slight amount and allow a portion of the heated water to be drawn off.
It is evident from what has been said of the range boiler that it operates under a variety of conditions. It is first a storage tank in which is accumulated the water, heated from a greater or less period of use of the range. Should the range fire be maintained through the day or night the supply of hot water will be excessive and superheating is the result. If the heater is to be used during short periods of time, the piping should be arranged to produce the best circulation; on the contrary, should the heater be used continuously—as in the case of a furnace coil—a slow circulation through the tank is most to be desired and the piping should be arranged for that purpose.
In the use of furnace heaters, superheating is likely to occur during cold weather when a hot fire must be used over a long period of time. In order to conserve the heat accumulated under such conditions a hot-water radiator is frequently connected with the range boiler through which to dispose of the excess heat. This radiator may be placed in any desired position and so connected by a valve as to discontinue its use at any time.
