« PreviousContinue »
quite often not realized in many plants fitted for so doing, for the reason that a pump designed and used for handling very hot water is one of the most difficult appliances to keep in repair that the mechanic has to deal with, consequently the tendency generally is to neglect this important feature, and as the heating can be done without it, that is, by running the water to waste, the pump is allowed to stand idle for a great part of the time. This should not be permitted, as the expense of keeping it in first class order is a small item compared with the loss of the heated condensation.
In buildings equipped with either the direct or hot water systems where no wall space is required for storage or other purposes the pipes can be placed around the sides below the windows. As warm air naturally rises, this location insures good heat distribution and comfort for the operatives.
The prevailing custom at the present time is, however, to arrange them on the ceilings where they will not be injured by passing trucks or obscured by material stored in front of them.
With the overhead location, the coils are not as liable to become repositories for brooms, clothing, waste or other refuse which might ignite and cause serious damage by fire.
THE INDIRECT OR BLOWER SYSTEM.
The efficiency of any steam heating system depends on the amount of steam condensed or relieved of its heat per square foot of surface exposed.
It is therefore clear that pipes of a given area arranged as in the fan system where cool air passes through them very rapidly, are many times more efficient than a radiating surface of the same area exposed under ordinary conditions.
The fresh air for use in this system is usually supplied from out of doors, and in heating weather is always cool enough to absorb heat from almost any source of even moderate temperature. Warm water from condensers circulated through pipes, furnace gases passing through large flues on their way from economizers to chimney (when artificial draft is employed) to
gether with live and exhaust steam can be used successfully. Direct steam heating coils are usually located so that condensation can be returned to the boilers. Heaters using exhaust steam should be so arranged that the cool air passes through or over them in the opposite direction to the current of steam and enters at the cool or discharge end.
It is customary to arrange the fan to draw the air through the coils and force it to the various rooms.
As any blower can usually give more efficient results when placed near the air supply it would seem preferable to install it at the inlet and handle the air before it is heated.
It is important that the openings from air trunks into the rooms should be of moderate size and well distributed, and that the size and capacity of the fan and heaters should be such that the required volume of supply can be furnished at a moderate velocity without causing too much of a current. In many cases this system has not given satisfaction from the failure to give attention to the above points.
Rooms in exposed locations or on the north side of a building always require more heat than those more fortunately situated. These can be accommodated without making any change in the heating plant proper, by placing auxiliary coils in the main ducts leading to the points where extra service is required.
It is often possible to obtain very economical results with this system by using the air more than once, or taking warm air from one department where there may be a surplus of heat from the nature of the work, to a cooler department. This necessitates a return duct or by-pass back to the suction side of fan and is not feasible unless the rooms and heating plant are quite near together.
The advantages of this system are many, and for some kinds. of manufacturing, particularly where a moderate heat and plenty of fresh air is needed, is perhaps the best in use. It is quite simple, and the parts requiring care are compact, with no heating pipes located where freezing is possible, or leakage can do much. damage. Results can be obtained very quickly, and in many
cases the warm air need not be turned into the rooms until within
a few minutes before starting time. In some parts of textile manufacture this constant changing over of the air is not desirable, having a tendency to make the rooms too dry, and unless the incoming air is moistened by some means before entering, conditions are too much dependent on the weather. In rooms filled with revolving shafting and pulleys the current set up by the fan should not be as strong as in those without machinery.
THE HOT WATER OR FORCED CIRCULATION SYSTEM.
In order to obtain satisfactory results with this system the speed of the water through the circulating pipes must be high to prevent any too sluggish movement in the shunts or isolated. coils that offer a little more frictional resistance than the more direct lines, and also the temperature should not be allowed to fall very much during its passage through the entire system to secure uniform efficiency throughout. In ordinarily cool. weather it should be sent out as warm as 200 degrees and returned at not less than 150 degrees.
In some plants this system has been installed with a view of utilizing the heat of the exhaust steam from condensing engines on its way to the condenser, by placing a heater between and running the engines with a vacuum more or less reduced.
As has been mentioned, the temperature of the outgoing water should be about 200 degrees, and that entering the heater 150 degrees, it is evident that to enable it to absorb any heat from the exhaust the vacuum would have to be adjusted to make the temperature of said exhaust somewhat above 150 or at least 160 degrees; this corresponds to a vacuum of about twenty inches, or five pounds absolute pressure.
At a temperature of 180 degrees we have only twelve inches, and at 200 degrees six inches, or a little less than one fourth of the usual efficiency of the condenser. The choice is offered then to either run a low vacuum and sacrifice a portion of the benefit derived from the condenser, or add live steam in a secondary heater to make up for what the low pressure steam
cannot do. For efficient results with this method of heating the water while passing through the exhaust chamber, the size of engines and capacity or requirements of the circulating system should be so proportioned that the passing water will absorb the entire amount of available heat in the exhaust steam.
To illustrate, we will suppose we are running the engines with a vacuum of twelve inches to give the exhaust a temperature of about 180 degrees, our circulating water enters the heater at 150 degrees, thus we have 30 degrees available.
If the quantity of water exposed in the heater is sufficient to absorb all the heat passing in the exhaust (that is, heat above 150 degrees) all is well, but as the temperature of this heating medium is governed wholly by the condition of the vacuum, regardless of the quantity of exhaust going by, any excess will pass through without giving up its available heat, and the impaired vacuum will have to be made up by steam. from the boilers for a part of which there will be no return. In other words, we will have to add enough extra heat in form of boiler steam to raise the mean effective pressure to the point it would be with regular conditions in condenser, and to pay for this we have only what heat has been absorbed by the circulating water.
Every 2.04 inches taken from the vacuum decreases the mean effective pressure one pound. It will be remembered that back pressure or vacuum, as the condition on the discharge side of an engine piston may be called, applies to the entire length of stroke the same as mean effective pressure. It is therefore evident that the raising or lowering of this pressure, even a few pounds, on a large engine cylinder means quite a serious difference in the important work done by the condenser and the power output of the engine.
If the quantity of exhaust steam is insufficient at times, care must be taken to avoid lowering the vacuum too much, as it will spoil regulation, and complicate things generally, especially with compound engines. The line between waste and economy in this proposition is so finely drawn that to use the minimum
temperature of the exhaust with also the minimum amount of direct steam, and realize the required results, the adjustments should be watched very closely. In installations where the engines and condenser systems are not considered and direct steam used entirely, the heaters are usually placed in the boiler house enough above the water line of boilers to insure a gravity return for the condensation. Provision must of couse be made for operating the circulating pump at night and during holidays and Sundays.
In nearly all equipments of this kind a supplementary heater or section of one of the heaters is used for the exhaust from the pump engine or turbine.
As has been spoken of before, the most severe duty on the system comes when the plant is shut down, particularly early mornings. At such times the water must be used at the maximum temperature, and as the exhaust from engine is not over 210 degrees unless back pressure is made use of, the steam will pass by the water coils, transferring little or no heat to them. In places where condensing water is available, it is sometimes better to install a small condenser for the pump engine and do away with the inefficient exhaust heater.
The circulating system is sometimes connected with the economizers to use the waste heat passing to chimney during nonrunning hours. While the theory of this is excellent, in practice it is hardly ever desirable, for the reason that the economizers during working hours are handling water quite hot, and at a high pressure, and if cooler water at a much lower pressure is introduced it contracts the joints and produces a change in conditions which usually develops more or less serious leaks and perhaps the loss of a gasket when the high pressure is again admitted. The repairing of economizer joints is not a pleasant task, to say nothing of the trouble and expense of having the machine out of commission when needed.
This system cannot be hurried to any extent, and it is consequently advisable to avoid letting a building cool down very much during cold weather by running the circulation continually.