CHAPTER IX.Vacuum Producers.

The next portion of the cleaning system is that which produces the motion of the air through the system and that to which the motive power is applied, namely, the vacuum producer.

—Vacuum producers can be divided into general classes: 1. Displacement type, in which a constant volume of air is displaced during each complete cycle of operations of the machine, and 2. Centrifugal type, in which the volume of air passing the producer during each complete cycle of operations varies with the resistance to the passage of such air through the system.

—Under this head the piston and rotary pumps are classed, and they are subdivided according to construction into reciprocating and rotary, valved and valveless, air cooled and water cooled.

—Under this head the fan type of vacuum producers are classed. They may be divided, according to construction, into single stage and multi-stage, horizontal and vertical.

—In order to ascertain the efficiency of the various types of exhausters to be discussed in this chapter it is necessary to ascertain the actual power necessary to move one cubic foot of free air at any degree of vacuum.

As nearly all machines tested by the author were driven by electric motors and the power was, therefore, indicated in watts, the curve C-D inFig. 78showing the actual power necessary to exhaust one cubic foot of free air at the vacuum noted in the lower margin, assuming no clearance and adiabatic compression, is used as a basis for calculation of efficiency. This showsthat to produce a vacuum of 8 in. mercury there will be required an expenditure of 16 watts for each cubic foot of free air exhausted, and to produce a vacuum of 12 in. mercury will require an expenditure of 27 watts. If these quantities be divided by the efficiency of the machine the actual power required will be determined.

FIG. 78. POWER CONSUMPTION AND EFFICIENCY OF AIR COMPRESSOR USED AS A VACUUM PUMP.

—The reciprocating pump was used on the majority of the earlier vacuum cleaning systems. The most common form in early use was a commercial air compressor which was used as a vacuum pump without any change in its construction. It was usually fitted with mechanically-operated induction and poppet type of eduction valves of heavy pattern, fitted with cushions of the dash pot principle, the same as are used on air compressors working against terminal pressures as high as 100 lbs. per square inch. The cylinders were water jacketed to remove the heat of high compression. The valves inthese compressors were heavy and required considerable pressure to open them and the friction of the valve gear and other moving parts, which were made heavy enough to withstand the strains of high compression, was excessively high for a machine where the compression did not exceed 8 or 9 lbs. per square inch. Their efficiency, therefore, is lower under actual operating conditions than if they were working against pressures for which they were designed. A curve of the power consumption of a 14-in. × 8-in. Clayton compressor is shown onFig. 78, the abscissae being the vacuum in inches of mercury and the ordinates of curve “AB” the watts required to exhaust one cubic foot of free air. Curve “cd” represents the theoretical watts required to do the same work. These compressors were used in connection with systems operating with 1-in. hose and the vacuum usually carried was 15 in. mercury. They require approximately 77 watts per cubic foot of free air at this vacuum and the efficiency, shown in curve “ce” (Fig. 78) is 46%.

FIG. 79. MODIFICATION OF RECIPROCATING PUMP MADE BY THE SANITARY DEVICES MANUFACTURING COMPANY.

Were this compressor used in connection with a system operating through 1¹⁄₄-in. hose and a vacuum of 8 in. mercury maintained, the efficiency would drop to 31%.

A modification of the reciprocating pump was manufactured by the Sanitary Devices Manufacturing Company in whichlight-weight poppet valves placed in the heads of the cylinder were used, as indicated inFig. 79. Curves of the watts per cubic foot and efficiency of this type of compressor are shown inFig. 80. It will be noted that this compressor shows a better efficiency than the air compressor at all degrees of vacuum and it is the best reciprocating pump that the writer has ever tested.

This pump was made for several years without water jacket and no trouble was ever experienced with overheating. However, owing to the commercial air compressors being jacketed, the makers using same made this a talking point and this company was obliged to jacket its pumps.

FIG. 80. POWER CONSUMPTION AND EFFICIENCY OF MODIFIED RECIPROCATING PUMP.

The Vacuum Cleaner Company used a Clayton pump on its smaller plants which was fitted with a semi-rotary valve in each end serving as an induction and eduction valve, while the heavy poppet eduction valve of the air compressor was dispensed with. The increase in efficiency that should have resulted from this change was not realized. The reason for this can be more readily seen by inspection of the indicator cards,Figs. 81and82.

Fig. 81is a card taken from one of the Clayton compressors fitted with combined induction and eduction valves, andFig. 82a card from a compressor with light steel induction and eduction valves of the poppet type.

It will be noted that the compression line, a-d,Fig. 81, extends above the atmosphere line, the pressure at the time of opening the eduction valve being 4 lbs. per square inch above the atmosphere. This is due to the failure of the mechanically-operated valve to open soon enough. This valve being also the induction valve, it is necessary for the eduction port to be closed before the induction port can be opened, in order to prevent a short circuit of air from the atmosphere into the separators. This fact is responsible for the sudden increase in the pressure at b, the eduction port having closed before the completion of the stroke and the air in the clearance space being compressed to 6¹⁄₂ lbs. above atmosphere. The induction port is not opened until after the beginning of the suction stroke resulting in the high degree of vacuum at c.

FIGS. 81 AND 82. INDICATOR CARDS FOR CLAYTON AND MODIFIED PUMPS.

Compare this with the card,Fig. 82. Here the compression does not extend above the atmosphere line more than ¹⁄₄ lb. per square inch and the eduction valve does not close until the end of the stroke so that the vacuum at the beginning of the suction stroke is no lower than during the entire stroke.

These pumps were working under the same conditions,i. e., 15-in. vacuum in the separator. The M. E. P. forFig. 81is 7.05 while that inFig. 82is 6.7 and is higher than is usually the case with this pump, due to the fact that the exhaust pipefrom this pump was very long and crooked, a condition which should be avoided whenever possible. Also, the pump from which this card was taken is one of the older pattern and the clearance was greater than in the later models. The point at which the eduction valve opens inFig. 81is 53% of the stroke and it closes at 95% of the stroke and is, therefore, open 42% of the stroke, while inFig. 82the eduction valve opens at 46% of the stroke and remains open to the end of the stroke, and, therefore, is open for 54% of the stroke. Thus the pump with the poppet valves will move more air at the same vacuum with less expenditure of power than the pump with the mechanically-operated valves.

Another type of reciprocating pump has been introduced in the past two or three years in which a single valve which rotates continuously in one direction is used for induction and eduction valve, for both ends of the cylinder. This valve is a plain cylindrical casting having ports cored through to alternately connect the cylinder ports with the intake and exhaust ports.

By rotating this valve 180° on its stem the vacuum pump is changed to an air compressor. This arrangement is adopted in order to discharge the contents of the separator into the sewer as was explained inChapters IandVIII. In this pump there must be points at which both the induction and eduction valves are closed at the same time and results similar to those found with the semi-rotary valves of the Clayton pump will naturally be in evidence. The author has endeavored to obtain an indicator card from one of these pumps but has been unable to do so. The effect of simultaneous closing of both induction and eduction ports would naturally be more marked in this pump than in the Clayton, as the motion of the valve in this case is uniform at all times while the motion of the valve gear of the Clayton pump is so arranged that the valve moves very fast at the time that both ports are closed. One of the two pumps of this type which was recently installed in the New York Post Office is illustrated inFig. 83. These pumps have a displacement of 1,200 cu. ft. each and are the largest reciprocating pumps in use for vacuum cleaning at this writing.

An interesting property of the piston pump which lends itself to the economical control of the vacuum in the system is illustrated by the curve at the top ofFig. 78which shows the total power required to operate the Clayton type air compressor, the efficiency of which is indicated by the lower curves on this figure. The compressor was operated at constant speed and the air volume varied to give various degrees of vacuum from atmospheric pressure to a closed suction and the power to operate the compressor read at intervals of two inches. The current input to the motor in amperes is indicated by ordinates and the vacuum in the separator by the abscissae. This indicates that the piston pump requires the maximum power to operate at about 15-in. vacuum and that the least power is required when the vacuum is at the highest point possible to obtain. The method employed in utilizing this characteristic of a piston pump will be discussed in a later chapter.

FIG. 83. ONE OF THE PUMPS INSTALLED IN CONNECTION WITH THE VACUUM CLEANING SYSTEM IN THE NEW YORK POST OFFICE, THE LARGEST RECIPROCATING PUMP USED FOR THIS PURPOSE UP TO THE PRESENT.

—The Garden City rotary pump is a good example of the single-impeller type of pump and is or has been used to some extent by at least two makers of vacuum cleaning systems. Its interior arrangement is shown inFig. 84. A solid cylindrical impeller, A, is mounted eccentrically in the cylindrical outer casing, the impeller being fitted with four sliding vanes which are provided with distance pieces, E, andwearing faces, B. The oil reservoir is provided with a needle valve which is automatically opened as soon as there is any vacuum produced and closes automatically when the machine is shut down. The rate of feed of oil is adjusted by the screw I. This type of pump offers a large surface in rubbing contact with the case and becomes very hot when in operation. It requires liberal lubrication in order to prevent heating and cutting of the surface of the casing. End wear in these pumps causes leakage, and, as usually constructed, there are no means provided for taking up this wear. It can be provided for, however, by using metal shims on the ends of the cylindrical casing.

FIG. 84. INTERIOR ARRANGEMENT OF THE GARDEN CITY ROTARY PUMP.

FIG. 85. POWER REQUIRED TO OPERATE GARDEN CITY TYPE OF ROTARY PUMP.

The power required to operate this type of pump (Curve a-b,Fig. 85), is nearly the same as that required to operate a piston pump for vacuum less than 12 in. mercury, but when the vacuum becomes higher, the power required becomes much greater than that required by the piston pump. The efficiency (Curve c-e,Fig. 85), is identical with that obtained with the light-weight poppet valve pump (Curve c-e,Fig. 80) from 0 to 11 in. vacuum, but for higher vacuum the efficiency of this type of pump falls off, while the efficiency of the piston pump becomes greater as the vacuum becomes higher. This difference in the characteristics of the two types of pumps is due to the presence of valves in one case and their absence in the other. With the piston pump the atmospheric pressure reaches the cylinder only while air is being discharged, the eduction valves being closed at other times and a partial vacuum exists on both sides of the piston. The higher the vacuum produced, the less time there is atmospheric pressure on the piston until, when no air is discharged, the air contained in the clearance space of the cylinder is compressed and expanded, the compression and expansion lines being coincident. The indicator card will have no area, and the only power expended is that required to overcome the friction in the moving parts. With the rotary pump there are no discharge valves to hold the atmospheric pressure from the discharge side of the impeller and the compression of the rarified air is accomplished by the atmospheric pressure admitting air through the eduction port into the chamber. Asit comes opposite the eduction port there is no difference in the time during which the impeller is subject to atmospheric pressure, no matter what the quantity of air being discharged. The higher the vacuum in the spaces containing rarified air, the greater the difference in pressure on the opposite sides of the sliding vane and, therefore, the greater total power required to turn the rotor.

FIG. 86. ARRANGEMENT OF DOUBLE-IMPELLER ROOT TYPE ROTARY PUMP FOR VACUUM CLEANING WORK.

Another type of rotary pump which is fast becoming the most popular is the double-impeller type. This is generally known as the Root blower, as the firm of this name was the first to manufacture same. They have been in use for many years as blowers for gas works, and as vacuum producers for various purposes, mainly the operation of pneumatic tube systems.

Why this form of vacuum producer was not earlier adopted in vacuum cleaning systems, instead of the sliding-vane type, is hard to understand. This pump contains two impellers or cams which are mounted on shafts geared together and revolve in opposite directions inside of a case, always being in close proximity to the case and to each other, but never touching. They are, therefore, frictionless in operation and the introductionof a small amount of water renders them practically air tight. There being no metallic contact between the moving parts, internal lubrication is unnecessary and there is no wear on either the impellers or the casing and no means of taking up wear are necessary.

The arrangement of the impellers and the method of providing water to seal the parts is shown inFig. 86. A reservoir containing water is provided on the discharge side of the pump and a small pipe leads from this reservoir to the suction side of the pump. The vacuum lifts water from the reservoir and discharges same in a spray into the suction chamber. This water passes through the pump and is separated from the air in the discharge chamber to be returned to the suction chamber by the vacuum. This operation will start automatically as soon as any degree of vacuum is formed and will cease as soon as the pump is shut down.

FIG. 87. ROTARY PUMP ARRANGED WITH DOUBLE-THROW SWITCH FOR REVERSING PUMP.

Any of these rotary pumps having no valves can be changed to an air compressor by reversing the direction of rotation. This is adapted by the American Rotary Valve Company in connection with their wet separators to discharge the contents of the separator into the sewer, on all of their smaller-sized plants.Fig. 87shows one of these plants arranged with double-throw switch for reversing the electric motor used to operate the pump and also shows the arrangement of the rotary brushwhich is used to clean the screen in the wet separator, as has been explained inChapter VIII.

FIG. 88. POWER CONSUMPTION AND EFFICIENCY ROOT TYPE OF PUMP.

FIG. 89. THE ROTREX VACUUM PUMP, USED BY THE VACUUM ENGINEERING COMPANY.

The power consumption and efficiency of this type of pump are shown inFig. 88. The watts per cubic foot of free air (Curve a-b) show a much lower consumption of power at the lower vacuum than any of the pumps already tested. This is probably due to the fact there is no internal friction. It willbe noted that the power to operate at no vacuum is but 10 watts per cubic foot of free air, while all the others require from 24 to 34 cubic feet. This also results in the efficiency curve (c-e,Fig. 88) reaching its maximum value at a lower vacuum than in the case of the sliding vane pump (Fig. 85).

FIG. 90. LATE TYPE OF CENTRIFUGAL EXHAUSTER MADE BY THE SPENCER TURBINE CLEANER COMPANY.

The efficiency is fairly constant between 6-in. and 10-in. vacuum and is much higher than is obtained with any of the other types of pumps at these vacua. When they are operated at higher vacuum the efficiency is about the same as obtained with the sliding vane pumps and lower than that obtained with the reciprocating pumps. The best efficiency of this pump is at the vacuum necessary to operate a cleaning system provided with 1¹⁄₄-in. hose.

A slight modification of this type of pump is that used by the Vacuum Engineering Company, known as the Rotrex. Thispump has but one impeller, of nearly the same form as the impellers in the Root blowers and has a follower driven by crank and connecting rods which is always in close proximity to the impeller but does not touch same. The arrangement of this pump is illustrated inFig. 89which also shows the saturation chamber and screens used instead of a separator, as explained inChapter VIII.

FIG. 91. POWER AND EFFICIENCY CURVES FOR THE SPENCER MACHINE.

The author has never tested the economy of these pumps but would infer that their economy should be about the same as that of the Root blower.

—This type of exhauster has always taken the form of a fan. The first stationary fan type of exhauster was manufactured by the Spencer Turbine Cleaner Company. Their latest type is illustrated inFig. 90. It consists of a series of centrifugal fans mounted on a vertical shaft, stationary deflection blades being provided between the wheelsto conduct the air from the periphery of one wheel to the center of the next.

FIG. 92. INTERIOR ARRANGEMENT OF INVINCIBLE MACHINE, MANUFACTURED BY THE ELECTRIC RENOVATOR MANUFACTURING COMPANY.

These centrifugal exhausters do not have a positive displacement, as do all those already described, and therefore the variation of the vacuum is not as much as in case of the positive displacement machines. The vacuum produced when the machine is moving no air is slightly less than the maximum that the exhauster can produce and there is very little variation in the vacuum with air quantities which can be moved without exceeding the capacity of the motor or other means producing the power. The curves showing the power required to operateand the efficiency of this type of vacuum producer are, therefore, plotted with abscissae representing the air moved in cubic feet per minute. The vacuum produced and the power required to operate are plotted as ordinates. The curves for the Spencer machine are shown inFig. 91. This curve is taken from a four-sweeper machine and the vertical lines numbered 1 to 4 represent the conditions when that number of sweepers are in operation; that is, bare floor renovators, with 50 ft. of hose or 80 cu. ft. of free air per minute. The maximum efficiency is reached at full load and is approximately 42%. The vacuum at this efficiency is 5¹⁄₂ in. mercury, a drop of ³⁄₄-in. from the maximum which was obtained at one-fourth load.

FIG. 93. POWER CONSUMPTION. VACUUM AND EFFICIENCY OF FIRST TYPES OF INVINCIBLE MACHINE.

These machines have rather large clearances and a preliminary separator is all that is required. They operate at a speed of about 3,600 R. P. M. and the peripheral speed of the fans varies from 15,000 to 22,000 ft. per minute. This producessome noise and considerable vibration and care must be exercised in mounting the machine. In order to insure quiet running the usual method is to place the machine on a felt pad of considerable thickness.

FIG. 94. POWER CONSUMPTION, VACUUM AND EFFICIENCY OF INVINCIBLE MACHINE AFTER VALVE WAS FITTED TO DISCHARGE.

The machines made by the Electric Renovator Manufacturing Company are horizontal and have much smaller clearances than the Spencer machines. They operate at approximately the same rotary and peripheral speed and are, therefore, as noisy. However, the center of gravity of these machines is lower and the vibration is not so great. The Spencer Company is now making a horizontal machine which it furnishes only when required, the claim for their vertical machine being that the weight of the moving parts counteracts the thrust of the atmospheric pressure against the fans and relieves the work of the thrust bearings, at the expense of greater vibration. With ball bearing thrusts, the author does not consider this to be of great importance.

A view of the interior arrangement of the Invincible machine, as manufactured by the Electric Renovator Manufacturing Company, is shown inFig. 92.

These machines, when first made, were without valves and the power consumption, vacuum and efficiency are shown inFig. 93. It will be noted that the vacuum produced, when the machine is operated at or below one-half load, is considerably lower than is obtained at greater loads. This characteristic produces a disagreeable noise when the machine is not handling any air, evidently due to air rushing back through the outlet when the vacuum tends to build up to the maximum which occurs at intervals of about one-half second.

FIG. 95. FOUR-SWEEPER INVINCIBLE PLANT INSTALLED IN THE UNITED STATES POST-OFFICE AT LOS ANGELES, CAL.

In order to overcome this trouble a valve has been fitted to the discharge, as indicated at 4,Fig. 92. With this valve in place the power consumption, efficiency and vacuum are asshown inFig. 94. It will be noted that the vacuum is as high at no load as at any load up to full load and is practically constant. The efficiency at light loads is the same as before but it is slightly lower at full load, being 50% without the valve and 47% with the valve. This is due to the power being expended in opening the valve for large quantities of air and to friction in the valve passage.

A four-sweeper plant of this manufacture is shown inFig. 95. This plant is installed in the United States Post Office at Los Angeles, Cal. The separate centrifugal separator, shown at the left of the cut, is not used in the regular equipment and was added in this case to fulfill the specification requirements.

A centrifugal pump with a single impeller is manufactured by The United Electric Company and is known as the Tuec system. A phantom view of the pump and separator is shown inFig. 96. It will be noted that the shaft is vertical. However, the vacuum is under the impeller in this case, and the thrust due to the atmospheric pressure is down instead of up, as in the case of the Spencer machines. This throws the weight of the parts, plus the thrust due to atmospheric pressure, on the thrust bearing. These machines do not produce a vacuum greater than 3-in. mercury, and the additional thrust is not as great as in the case of the machines producing higher vacuum, the impeller being 24 in. in diameter, its area 450 sq. in. and the thrust, with a vacuum of 3-in. mercury, 675 lbs., which is worth considering. This downward thrust is partially counterbalanced by mounting the armature of the electric motor used to operate the fan, slightly below the magnetic center, thereby causing an upward magnetic pull. These machines are intended to be used with large hose and pipe lines to reduce the friction to a very low point. When operating carpet renovators the vacuum at the renovator rises to 1-³⁄₄-in. mercury and the type of renovator used by them passes approximately 50 cu. ft. of air, while the bare floor renovators pass approximately 95 cu. ft. They are extensively used where bare floor work is required, their first cost being low.

The results of tests of two of these machines of four-sweeper capacity, driven by alternating and direct-current motors,respectively, are shown inFig. 96a. These curves indicate a considerably higher efficiency with the alternating than with the direct-current motor. This is due to the low efficiency of the special high-speed direct-current motors used with all centrifugal fan-type exhausters. The alternating-current motors are not so affected, in fact, the speed at which these fans are operated is fixed by the requirements of the alternating-current motors.

FIG. 96. CENTRIFUGAL PUMP WITH SINGLE IMPELLER, MANUFACTURED BY THE UNITED ELECTRIC COMPANY.

The efficiency of the other types of centrifugal exhausters(Figs. 91,93and94) is in every case accomplished with direct-current motors. This machine has an efficiency about the same as the Spencer machine. It will be noted that the vacuum produced does not fall off as the load increases, as in the case of the multi-stage fans. This characteristic is probably due to the fact that there is no wire drawing in the diversion vanes, as in the case of the multi-stage exhauster.

FIG. 96a. TEST OF CENTRIFUGAL PUMP WITH SINGLE IMPELLER.

—The steam aspirator as a vacuum producer in connection with vacuum cleaning systems was first used by the American Air Cleaning Company, and has been used to a limited extent by the Sanitary Devices Manufacturing Company. The type of apparatus used by the American Air Cleaning Company is illustrated inFig. 97. A single partial separator is used with this system and the lighter dust is allowed to passthrough the aspirator, where it is mixed with the steam and sterilized. The aspirator is in the form of an ejector, with a specially designed nozzle, and is always fitted with an automatic device for cutting off the steam when the vacuum in the separator reaches the degree desired.

FIG. 97. STEAM ASPIRATOR USED BY THE AMERICAN AIR CLEANING COMPANY.

The steam consumption required to exhaust 1 cu. ft. of free air at various vacua, as determined by actual test of four different nozzles, is shown inFig. 98, the steam being the actual weight of dry and saturated steam at the gauge pressures noted. The American Air Cleaning Company used to guarantee a steam consumption of 250 lbs. per hour from and at 212° F., assuming that the feed water temperature was 32° F., the vacuum to be maintained at 9 in. mercury at the aspirator.

Taking the results of the test of the three-sweeper nozzle as an average, 0.066 lbs. of steam will be required to exhaust 1 cu. ft. of free air at 9 in. vacuum. The total heat in 1 pound of dry steam at 110 lbs. gauge is 1187 B. T. U. and at212° F. the latent heat is 970 B. T. U. The factor of evaporation, therefore, is 1.235, and the weight of steam at 110 lbs. allowed by the guarantee is 202 lbs. This amount of steam will exhaust 3,060 cu. ft. per hour, or 51 cu. ft. per minute, which is more than sufficient to operate a carpet renovator, and is a little less than will pass through a bare floor brush attached to the end of 50 ft. of 1 in. diameter hose, if the hose is attached directly to the aspirator. With a line of pipe between the hose cock and the aspirator, the air quantity will be somewhat less, and this guarantee will undoubtedly be fulfilled in every case.

FIG. 98. STEAM CONSUMPTION OF STEAM ASPIRATOR.

The advisability of using an aspirator will depend on the conditions to be met at the building in each case. Three typical cases are cited below:

1. When there is a Generating Plant in the Building, and a Plant Using 1¹⁄₄-in. Hose and 8-in. Vacuum is Desired.—A Root blower will require 27 watts for each cubic foot of air exhausted (Fig. 88), and the three-sweeper aspirator, 0.065lbs. of steam. Then the pounds of steam required by the aspirator to do the same work as one K. W. hour at the motor of the Root blower will be

0.065 × 600.027= 146.3

The generating plant will produce a kilowatt hour at the switchboard with not exceeding 60 lbs. of steam, and if the transmission loss is 10% there will be required by the Root blower not over 66 lbs. of steam to do the same work that takes 146 lbs. with the aspirator. This case would require that the Root blower, driven by an electric motor, be used.

2. When there is High Pressure Steam Available, but no Generating Plant.—Then we may use either the aspirator or a Root blower driven by a steam engine. This engine should have an economy of 60 lbs. per indicated horse power, with not over 15% friction loss, which will require 69 lbs. per brake horse power. This will be equivalent to 69 × 0.776 = 90¹⁄₂ lbs. per K. W. hour, which is still much better than 146 lbs. required by the aspirator.

3. When Steam is Generated on the Premises with Coal Costing $3.00 per ton and all Machinery Must be Driven by Electricity Purchased for 5 Cents per K. W. Hour.—Cost of steam to do the same work in the aspirator that 1 K. W. hour will do in a motor driving a Root blower is:

146 × 3007 × 2240= 2.8 cents

as against 5 cents that would have to be paid for current. In this case there would be a saving in using the aspirator, which would not require as much attention as the motor, and at loads less than full load, the steam used by the aspirator would be in direct proportion to the load, as the control would shut the steam off entirely during a portion of the time, while the motor would require some current as long as it was in operation, even if no air was being exhausted. On the other hand, the steam which is exhausted from the aspirator is not suitable for use in heating, as it is mixed with air and fine dirt, and must be thrown away, a condition that must always be considered where there is an opportunity to use exhaust steam for heating or other purposes.

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