Document 7MEa0Qx5ZVJnnXg6mVzq77LpB

HEATING VENTILATING AIR CONDITIONING CUIDE 1942 tion weighing 2 to 3 times the weight of the motor and compressor, in order to maintain the machine movement below 0.03 in. Compressor and motor............................ 4,500 lb. Concrete foundation........................................................................ 9,000 lb. Total .......................................................... 13,500lb. Practical application dictates the number of springs to be used, which is based on the design of the machine foundation and the supporting floor structure. However, it is desirable to design for at least 8 springs and one or two spares for cases of unknown weights. As many as 50 springs have been used on one installation. The distribution of the springs must be balanced against the masses to be supported, otherwise the foundation design and supporting structure determine the location of the springs. The choice of the material used in the design of the resilient mounting is also important. For the slow-speed type compressor a common speed found in practice is 360 rpm. For speeds below this, isolation should not be attempted except under careful supervision. Referring to Fig. 6, it is found that for 360 rpm the static deflection required for a ratio of w/wn of 3 to 1 (line EF) is 2.5 in. and for a ratio of 5 to 1 (line GH) it is 7 in. For these values of deflection the only choice of material is the coil spring. This is also true for speeds up to about 700 rpm. In consideration of the transverse spring constant (so as to maintain good ratios among the various degrees of freedom) experience has shown that the spring should be designed with a working height equal to 1.0 to 1.5 times the outside diameter. A long spring of small outside diameter has very low transverse rigidity and therefore requires some additional means of preventing side drift of the unit and on very sensitive applications this may tend to destroy the isolation efficiency. For speeds of 700 to 1200 rpm the required deflections range from 0.22 in. to 1.75 in. For these conditions rubber in shear serves as a rather satisfactory material if protected from oil. For speeds higher than 1200 rpm cork can be applied with good results. These limitations are by no means absolute because with careful and well engi neered installations, especially, in consideration of all six degrees of freedom, certain liberties may be taken and still good results accomplished. When a machine unit is properly isolated it will Have a definite amount of movement which is determined by the ratio of the unbalanced forces to the total mass of the machine. If this resultant machine movement is too great for the necessary connections or the satisfaction of the customer it can be reduced only in two ways without destroying the quality of the isolation; first, adding mass or dead weight to the machine (such as concrete) common in the application of low speed, partially balanced machinery; second, accurately balancing (both statically and dynamically) all moving parts so as to eliminate the vibration at the source. This latter method is the best engineering practice and is the modern trend. However, even with well balanced machinery, installed in the vicinity of quiet offices it is usually necessary to properly isolate the equipment to prevent the transmission of vibration likely to cause complaints. Where limitation of machine movement is desired during the starting and stopping periods, the application of friction or hydraulic damping will serve without seriously interfering with the efficiency of the isolation. 646 Chapter 34 AUTOMATIC CONTROL Purpose of Automatic Control, Types of Control, Central Fan Systems, Limit Controls, Static Pressure Control, Unit Systems, Control of Automatic Fuel Appliances, Residential Control Systems, Control of Refrigeration Equipment THIS chapter is prepared with the purpose of acquainting the engi neer with the principles underlying the use of automatic control, the general types and varieties of control equipment available and their application. Automatic control, properly applied to heating, ventilating and air conditioning systems, makes possible the maintenance of desired con ditions with maximum operating economy. A properly designed and complete control system has the ability to interlock and coordinate the various functions of heating, ventilating and air conditioning in a manner impossible to accomplish with manual regulation. Automatic control is an integral and essential part of a heating, venti lating or air conditioning installation and cannot be regarded as an acces sory. In order to insure satisfactory results, the control should be designed with and incorporated in the heating, ventilating or air conditioning system. The control equipment should be given careful consideration in the planning of any installation in order that the entire system may operate together with satisfactory results. In order that proper selection and application of controlling devices may be made it is important that a broad understanding exist as to the types of control available and their principles of operation. Improper selection and application of control equipment will result in unsatis factory and inefficient operation. Specific control devices and systems are described in the Catalog Data Section. PURPOSE OF AUTOMATIC CONTROL Automatic control is normally applied to heating, ventilating or air conditioning systems: 1. To insure the maintenance of certain desired or required conditions of temperature, pressure, humidity, air motion or air distribution. 2. To serve a safety function, limiting pressures or temperatures within predetermined points, or preventing the operation of mechanical equipment unless it may function without hazard. 3. To produce economical results and thereby insure operation of the system at a minimum of expense. 647