This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The High-Pressure heating apparatus takes its name from the fact that it is either hermetically sealed or is closed with a heavily loaded valve, so that a considerable pressure occurs in the pipes and parts when the water is expanded by heat. What, to many, is a peculiar result of heating water under pressure is that this liquid then reaches a temperature far above 212° (the commonly recognised boiling-point) without boiling. In the ordinary way water will boil at 212° Fahr. (or closely thereabouts according to the height of the spot above sea-level), and once boiling-point is reached the water ceases to gain in temperature, however fast the boiling may be made to occur. If, however, anything can be done to prevent the boiling, then on suitable heat being applied the temperature of the water will continue rising, and reach quite high figures. It raises a feeling of doubt as to what temperature water could be raised to by heating it under suitable conditions and in a vessel of sufficient strength, but no hesitation need be felt in saying that something like 6oo° has been reached in a high-pressure hot-water apparatus (by bad work or firing), while 300° to 400° is a normal heat for the water in these pipes.
The action of boiling, or ebullition, which occurs when water is sufficiently heated, is controlled by the pressure exerted on the water. In open vessels at sea-level the pressure of the atmosphere is 14.7 lbs. per square inch, and this is exerted on the water, with the result that boiling occurs at 212° Fahr. On high lands, or below the sea-level, the boiling-point is lower or higher, owing to the pressure or weight of the atmosphere varying accordingly. If, however, a vessel containing water is soundly closed, then atmospheric pressure has no influence on the boiling-point, and it can be controlled quite independently.
In Fig. 56 are given the details and outline of a small apparatus, which will serve for explanation. In the first place, it will be seen to consist wholly of pipe, this being necessary on account of the pressure that is exerted by the water. No form of boiler would stand it, therefore a coil or arrangement of tube is always used as the heater. The tube, too, is of special strength, being of 1/4-inch substance, 1 5/16-inch external and 13/16-inch internal diameter ; it is generally spoken of as 7/8-inch tube. To give an idea of the strains the tube is expected to bear, it is customary to test a finished apparatus to a hydraulic pressure (cold) of 1800 to 2000 lbs. to the square inch.
In the illustration it will be seen that the top tube of the furnace coil goes away as a flow-pipe, and continues unbroken by branch circuits, coming back to the lower tube of the coil, thus making the apparatus consist of one endless line of pipe except for the expansion tube. This arrangement is of importance, it being necessary for the filling and subsequent proper working of the apparatus ; and all installations, however extensive, should consist of one endless pipe. In large works there may be several separate circulations, and the heater may be spoken of as a 5-pipe coil (with five flow and five return ends), yet, as will be shown directly, this may, and should, wholly consist of one single endless pipe.
It will be noticed in Fig. 56 that the flow-pipe passes but once around the first floor above the heater, and the second pipe is the tail end of the circuit after it has been twice round the second floor. This arrangement is not absolutely necessary, but is given to show how the heating may be equalised if the two floors are to be served equally well. Thus the first floor is given the hottest and coolest pipe, while the second floor has two pipes both within these two extremes, and it might be hoped by this plan that each floor would receive about the same number of heat units. As the runs of pipe often reach 500 to 600 feet it follows that, swift as the circulation may be (as it is with these high temperatures), there is a marked difference between the temperatures of the flow and return pipes where they are near the heater; consequently, in planning the runs of pipe it is proper to share out the flow and return as equally as possible in the various rooms or sections of the building. Exception to this can be made when some rooms are of greater importance than others ; or, when it appears best to increase the proportion of piping in those rooms which have an unfair share of return pipe.
The filling of the apparatus is done from a low point by the "pumping valve" shown there. It is not done at the point marked " filling cap." This latter term is a misleading one, but is used in the trade ; its correct name would be " replenishing cap," and its use is described a little farther on. In an apparatus such as this it would not be easy to fill properly from the top, and it is further found that, by pumping, the pipes are well scoured and freed of air, as they need to be. Again, this kind of apparatus has air gather in it, in course of time, and only by pumping water through can this be properly removed. The engineer usually books a contract to do this repumping once yearly, at the beginning of each winter season.
There are different forms of pumping valves, some engineers having a design made for their exclusive use. These usually take the form of a four-way gun-metal valve, which can have an ordinary portable lift-and-force pump attached, and have two outlets capped off when the pumping is done. What maybe termed the ordinary form of pumping valve, or pumping-through cock, which is obtainable from any factor or manufacturer of these goods, is a miniature lift-and-force pump itself. To the best of belief it is not claimed that one is better than the other, but the engineer's valve has the advantage of preventing strangers interfering with or attempting the repumping, as its use is not obvious. From the foregoing it will be seen how essential - certainly desirable - it is that the apparatus consist of an endless tube. There could not be the same certainty of the pumping removing all air from, and filling all parts of, the piping, if there were branch circulations; in fact, it takes a very skilled man to say when a branch circuit may be attempted without the risk of air being driven up to its remote end, if it has one, and being held there by the water.