CHAPTER X.Control.

When the displacement type of vacuum producer of more than one-sweeper capacity is used with a vacuum cleaning system, some means must be employed to prevent the vacuum rising above that necessary for efficient operation of the sweepers when there are less renovators in use than the capacity of the vacuum producer or when carpet renovators are in use on all outlets.

If the displacement pump be run at constant speed, every change in the quantity of air exhausted will cause a change in the vacuum produced. This will result in inefficient operation and may result in undue effort being necessary to operate the renovator and in excessive wear on the carpets.

The earlier systems were not provided with any control and the first attempt to control the vacuum was by placing a spring relief valve on the pipe line near the separator, which admitted additional air when the vacuum tended to rise. This resulted in full load being thrown on the pump at all times when the same was in use, which does not give economical operation.

The controllers that have been devised for maintaining a constant vacuum without the introduction of air into the system operate on one of three principles:

1. Closing the suction of the vacuum producer.

2. Opening the suction of the vacuum producer and holding vacuum in the system.

3. Varying the speed of the vacuum producer.

FIG. 99. FIRST TYPE OF CONTROLLER INTRODUCED BY THE SANITARY DEVICES MANUFACTURING COMPANY, KNOWN AS THE “UNLOADING VALVE.”

The first type of controller was introduced in the vacuum cleaning field by the Sanitary Devices Manufacturing Company, and was known as the “unloading valve.” It was similar to the unloader which had been used for some time in connection with air compressors. The detail of construction is shown inFig. 99, and consists of a balanced valve, which is connected to a weighted piston, operating in a chamber communicating with the separators by a pilot valve. The pilot valve is operated by an auxiliary piston which is weighted to overcome the lifting effort due to the vacuum desired. When the vacuum in the cylinder becomes great enough to overcome the weights attached to the auxiliary piston, it rises, allowing vacuum to reach the main piston, which is drawn up and the suction valve closed. When this valve is closed the vacuum in the pump at once starts to build up to the maximum possible for the pump to produce, and if the pump used is of the piston type the vacuum will run up to nearly 28 in., resulting in the pump’s taking the least power on which it can be operated. As soon as the vacuum in the separators falls below that which will sustain the weight on the auxiliary piston the valve falls open and the pump again draws air through the system. In actual practice this valve will operate at more or less frequent intervals. The author timed the action of one of these valves connected to the suction of an eight-sweeper piston pump, and its time varied from ²⁄₅ second to 65 seconds. The current taken by the pump when the suction was open was 100 amperes at 220 volts. When the valve was closed for but ²⁄₅ second the current dropped to 75 amperes, there not being sufficient elapsed time for the pump to produce a perfect vacuum. When the valve was closed for2¹⁄₅ seconds, the vacuum reached its maximum value and the current fell to 32 amperes.

Fig. 100is a curve plotted from the results of this test and shows an increase in the power above that necessary to overcome the friction in the moving parts of the pump in direct proportion to the percentage of full load that the pump was serving.

FIG. 100. TEST OF CONTROLLER CONNECTED TO SUCTION OF 8-SWEEPER PISTON PUMP.

This is as near an ideal condition as one could expect to obtain by any means other than stopping the pump or otherwise decreasing the friction load. However, this form of unloader is not suitable for a pump without valves, as the power will increase with an increase in vacuum, and other means must be employed to control such a pump.

The second form of control is adapted to this type of pump. The arrangement of one of these controls is shown inFig. 101. This consists of a single-ported valve opened by the vacuum in the cylinder, M, the action of which is controlled by a pilot or auxiliary control valve actuated by the vacuum in the separator. This auxiliary valve is fitted with two pistons, S andO, which are held together by springs, and when so held the main cylinder is open to the atmosphere through the small ports in the piston, O. When the vacuum in the separator becomes great enough to overcome the compressive strength of the springs, T and P, the pistons, S and O, are drawn apart, closing the port in the piston, O, and opening the port in piston, S, allowing the vacuum to enter the main cylinder, M, and open the main valve. This valve permits the atmospheric pressure to enter the pump suction, the air being prevented from entering the separators by a check valve, not shown. The pump then operates without producing any vacuum, and the power required to operate the pump is reduced. A relief valve of the common vacuum-breaker type is shown at the left of the cut. This valve is provided to prevent overload in case the control fails to operate.

FIG. 101. TYPE OF CONTROLLER FOR USE ON PUMPS WITHOUT VALVES.

This type of control does not effect as great a reduction in the power as the first type of control described, since it requires a greater per cent. of the full load power to operate the pump at no vacuum than at perfect vacuum. No air is moved in the latter case, and the maximum volume of air is moved in the former case.

Either of these controls gives fairly economical results whenthe pump is serving at least a part of the sweepers at all times. However, when the system is used in a building where there may be cleaning done at any time and vacuum must be “on tap” at all times, as in a hotel, there will be many occasions when no sweepers will be in use, and the pump might then be stopped entirely, provided that it could be automatically started when needed.

FIG. 102. REGULATOR FOR MOTOR-DRIVEN VACUUM PUMP, MANUFACTURED BY THE CUTLER-HAMMER MANUFACTURING CO.

Where the steam aspirator is used, the control (Fig. 97) is attached to the steam supply valve. When the valve is closed no steam is consumed by the aspirator. This is the ideal condition where we must keep vacuum “on tap,” and is a characteristic of the aspirator system which has led to its introduction in many instances.

The same economy can be obtained with a steam-driven pump by inserting a throttle valve, controlled by the vacuum in the separators, which will start and stop the engine driving the pump and vary its speed in accordance with the quantity of air required by the system.

Several appliances for varying the speed of a motor-driven vacuum pump have been placed on the market, the simplest and probably the best of these appliances being that manufactured by the Cutler Hammer Manufacturing Company, illustrated inFigs. 102and103.

FIG. 103. INSPIRATOR TYPE VACUUM CONTACTOR, USED TO CONTROL PILOT MOTOR OF CUTLER-HAMMER CONTROLLER.

The object of the apparatus shown inFig. 102is to automatically start a motor-driven vacuum pump and control the speed of the motor so that the vacuum is maintained at the desired degree, irrespective of variation in the number of sweepers in use. This control of the degree of variation is accomplished in a more efficient manner than if the pump were to be driven at its maximum speed at all times and the pressure kept at the desired point by means of a blow-off or by-pass valve. With this system a motor is used having a control, by shunt field weakening, of approximately 3:1 in order that the control of the speed may be as efficient as possible.

Referring toFig. 103, a small pilot motor is mountedon brackets at the side of the panel, driving directly, through an insulating coupling, a screw shaft which carries a traveling cross-head. This cross-head is shown in the photograph at the extreme right of its travel, which corresponds to the maximum speed of the motor, the left-hand end corresponding to zero speed of the motor. In this position the motor circuit is opened by the clapper type magnetic switch. Assuming that the cross-head is in the extreme left-hand position and the knife switch is closed, the pilot motor will be started in such a direction as to move the cross-head to the right. A slight movement in this direction completes a connection to the magnetic switch, which thereupon closes the motor circuit through all of the resistance, starting the pump motor.

Inasmuch as the pilot continues to move the cross-head toward the right, the speed of the pump will be gradually increased until, at a point about midway of its travel, all of the resistance in the armature circuit of the motor will have been cut out upon the upper segments and further movement then serves to weaken the field. This is accomplished by means of the contact buttons shown just below the screw shaft.

As soon as the cross-head has weakened the field to its minimum value and thus speeded the motor up to its maximum point, a limit switch stops the pilot motor and thus prevents further motion in that direction. As soon as the pump working this at its maximum speed has produced a vacuum in the cleaning system of, say, 12 in. of mercury, the cross-head will begin to move backward and reduce the speed to a point corresponding with the air required.

This control of the pilot motor is accomplished by means of what is termed “inspirator type vacuum contactor.” This apparatus is shown more in detail inFig. 103, and consists of a diaphragm closing one side of a chamber. The diaphragm is pressed outward by an internal spring whose tension may be adjusted by means of a hexagonal head cap screw, visible in the photograph of the complete regulator.

The diaphragm is coupled to a pivoted arm carrying insulated conical-pointed silver screws, so located that they enter holes in small silver plates mounted on opposite sides, respectively,of the upper and lower contact posts. These contact posts are hollow and communicate with the diaphragm chamber, which latter is connected by piping to the vacuum system.

Normally, the internal spring forces the diaphragm over so that the lever makes contact with the lower post. This serves to drive the pilot motor in a direction to move the cross-head to increase the speed of the pump. When the degree of vacuum for which the apparatus is adjusted is reached the lever starts to move toward the left hand, and in so doing stops the pilot motor. This maintains the pump speed at that particular value. Should the vacuum increase to a sufficient degree the lever will be drawn further over toward the left and contact will then be established with the upper post, which will cause the pilot motor to move the cross-head to the left, and thus decrease the pump motor speed.

Inasmuch as the motion of the diaphragm lever is very gradual, destructive arcing would take place at the pilot motor contacts were it not for the small openings in the silver contact plates, which, as the pointed screw leaves the hole, immediately sucks the arc inward and extinguishes it.

This method of preventing arcing is exceedingly unique and is subject to patents now pending.

It is possible to adjust the high and low limits by changing the setting of the pointed silver screws, the usual adjustments being such as to maintain the vacuum within 2 in. of mercury. The speed of the pilot motor may be adjusted by means of the small link shown in the upper left-hand corner of the panels to correspond with the capacity of the system, it being found that systems of large capacity require a slower motion than those in which the amount of piping, etc., is less for the same size of pump. In practice, the regulator will very quickly find the position corresponding to the proper speed for the number of outlets in use, and only moves a slight amount either side of this particular position.

With this regulator it is possible to employ remote control permitting the establishment of vacuum in the piping system by the turning of a pilot switch located at any point in the building. If desired, several such switches may be placed inparallel, and, under these conditions, the turning on of any switch will establish the vacuum supply which will be maintained until all of the pilot switches are turned off. By this means it is possible to have several janitors working at the same time on different floors of the building, and each will be independent of the others in his control of the vacuum; although one man may finish and turn off the switch on his floor, the pump will not be stopped if the vacuum is still required by workers on other floors.

When the total size of one installation becomes greater than 25 H. P., it is found desirable to provide two pumping units, and, in this case, the same system is applicable. The cross-head is then arranged to start first one pump and increase its speed to a maximum. If this does not supply the necessary amount of air, the cross-head continues to move, and starts the second pump, which will then be run at a necessary speed to supply the remaining amount of air.

The first pump always remains in motion at its point of highest efficiency. It is evident that this duplex arrangement is more efficient than one large pump when only a very few sweepers are in operation, since, for this condition, the very large pump would have to be run at such a slow speed that the armature resistance would be in circuit, while the single smaller pump would be running at a more efficient speed and with less proportionate motor losses.

In duplex outfits switches are provided for disconnecting either motor in case of its being necessary to clean or repair either unit. When so disconnected the other unit may be operated and maintain the same degree of vacuum within the limits of its capacity.

While this type of control is more economical in current consumption than either of the former types described, its cost is much higher, and it is seldom used unless specifically ordered.

When the centrifugal type of vacuum producers is used no control is necessary, as the inherent feature of this type of apparatus insures a practically constant vacuum at all air quantities within the capacity of the machine.

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