This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
This is used under the same conditions as indirect steam, and the heaters used are similar to those already described. Special attention is given to the form of the sections in order that there may be an even distribution of water through all parts of them. Fig.. 14 and .15 show typical hot-water radiators for indirect work. As the stacks are placed in the basement of a building, and only a short distance above the boiler, extra large pipes must be used to secure a proper circulation, for the "head" producing flow is small. The stack casings, cold and warm-air pipes and registers are the same as in steam heating. Exhaust Steam. Exhaust steam is used for heating in connection with power plants, as in factories and shops or in office buildings which have their own lighting plants. There are two methods of using exhaust steam for heating purposes. One is to carry a back pressure on the engines of from 5 to 10 pounds, depending on the length and size of the pipe mains, and the other is to use some form of "vacuum system" which consists of a pump or ejector attached to the returns from the radiators; this draws the steam through the radiators and tends to reduce the back pressure on the engines rather than to increase it.
Where the first method is used, and a back pressure carried, either the boiler pressure or the cut-off of the engines must be increased to keep the "mean effective pressure" the same and not reduce the horse-power delivered. In general it is more economical to utilize the exhaust steam for heating. There are instances, however, where the relation between the quantities of steam required for heating and for power are such, especially if the engines are run condensing, that it is better to throw the exhaust away and heat with live steam. Where the vacuum method is used these difficulties are avoided, and for this reason it is coming into more common use. If the condensation from the exhaust steam is returned to the boilers the oil must first be removed; this is usually accomplished by passing the steam through some form of grease extractor as it leaves the engine. The water of condensation is then passed through a separating tank before it is delivered to the return pumps. It is better to remove a portion of the oil before the steam enters the pipes and radiators, else a coating will be formed on their inner surfaces which will reduce their heating efficiency.