Compressors cylinder capacity

The first method is used most frequently. The next preference is for the last method, mostly used in small compressors due to problems with speed control of electrical motors. Other means of capacity control are very seldom utilized due to thermodynamic inefficiencies and design difficulties. Energy losses in a compressor, when capacity regulation is provided by lifting the suc tion valves, are due to fric tion of gas flowing in and out the unloaded cylinder. This is shown in Fig. 11-84 where the comparison is made for ideal partial load operation, reciprocating, and screw compressors. 

Compressor cylinders [dry or water-cooled) of cast iron, nodular iron, or forged steel are engineered to suit required pressures and capacities. 

Reciprocating compressor capacity may easily be adjusted by changing compressor speed, changing compressor cylinder clearance, unloading compressor cylinder inlet valves, recycling gas from unit discharge to unit suction, or a combination of these methods. All these methods may be accomplished either manually by the operator or automatically by the control panel. 

From the manufacturer s specific compressor cylinder tables, select cylinders to give the required [ (PD) (E, ) ] follow the two-stage compression example here. The final actual capacity depends upon this selection of cylinders. 

When multiple compressors are specified, only those required for the initial plant should be purchased. Adequate space must of course be allocated to meet expansion needs. If only one compressor is specified, one large enough to meet expansion needs should be specified. For centrifugal compressors the capacity can easily be varied over a wide range. The designer should, however, check to see that the initial requirements exceed its minimum capacity, which is called the surge point. In the case of reciprocal compressors, either a clearance volume can be added to the compressor cylinders or the suction valve can be removed. See reference 39 for a discussion of this. 

Using this technique, I once discovered that one end of a compressor s two double-acting cylinders had a defective unloader. This failure reduced the capacity of the affected cylinder end by 40% and hence reduced the compressor s capacity by 10% (i.e., 40% x 25%). 

For example, for one machine equipped with plastic poppet valves (i.e., compressor cylinder valves designed for high capacity but low compression ratios), valve losses as measured by a Beta Scan were reduced from 25% to 10% when the compression ratio was reduced from 1.42 to 1.28. This occurred even though the gas volume moved through the compressor increase by 50%. 

The slide is located in the compressor casting below the rotors, allowing internal gas recirculation without compression. Slide valve is operated by a piston located in a hydraulic cylinder and actuated by high-pressure oil from both sides. When the compressor is started, the slide valve is fuUy open and the compressor is unloaded. To increase capacity, a solenoid valve on the hydraulic hne opens, moving the piston in the direction of increasing capacity. In order to increase partload efficiency, the slide valve is designed to consist of two parts, one traditional shde valve for capacity regulation and other for built-in volume adjustment. 

Control of the compressor is achieved by engaging the required number of cylinders. In. say. a 16-cylinder compressor if we engage only four cylinders, the compressor will run at 25 7r capacity, and if w c engage eight cylinders, the compressor will run at. 5()C< capacity. Electrically operated solenoid valves are provided for capacity control. Energy can be conserved by using static controls, as discussed in Section 6.15. 

It would appear obvious for startup, and in some cases full-time operation. that a suction strainer or filter is mandatory. The reason for ilic strainer is to keep Junk and pipe scale out of the compressor. Fines from pipe scale and rust will make short work of the internal bore of a cylinder and are not all that good for the balance of the components. In some severe ca.ses, cylinders have been badly damaged in a matter of a few weeks. The strainer should be removable in service for cleaning, particularly when it is intended for permanent installation. Under all circumstances, provision must be made to monitor the condition of the strainer. Much frustration has been expended because a compressor overheated or lost capacity and no one knew if the strainer had fouled or blinded. 

An additional capacity control method is the unloader. This method can be used in conjunction with clearance pockets to extend the range of control to zero capacity. On double-acting cylinders, unloading the individual sides one at a time will provide a two-step unloading of the cylinder. On multicylinder arrangements, the cylinders can be unloaded one at a time providing as many steps as cylinders operating in parallel. The unloaders can also be used to totally unload the compressor, as is necessary for electric motor driver startup. 

Another variation is the tandem cylinder. The tandem cylindet arrangement again allows two stages of compression on the same compressor throw but uses two double-acting cylinders separated by a second distance piece. This arrangement is usually used in low rod load applications where higher capacity is required. 

Typically, in specifying a unit, the suction and discharge pressures, capacity (MMsefd), inlet temperature, and gas properties are given. The actual sizing of the cylinders is left to the manufacturer from his specific combinations of standard cylinders, pistons, and liners. However, once a proposal is received from a manufacturer, sometimes it is beneficial to check the cylinder sizing and make sure that indeed the compressor will perform. Sometimes it is necessary to size a new cylinder for an existing compressor or to verify that an existing compressor will perform in a different service. 

These figures illustrate what takes place inside the cylinder during the compressor s operation. When specifying performance, the actual capacity at suction and/or discharge conditions must be specified. 

Material Capacity Suction pressure Suction temperature Discharge pressure Discharge temperature Compressor speed Single-cylinder, double-acting... 

The rotary compressor is adaptable to direct drive by the use of induction motors or multi-cylinder gasoline or diesel engines. These compressors are compact, relatively inexpensive, and require a minimum of operating attention and maintenance. They occupy a fraction of the space and weight of a reciprocating machine having equivalent capacity. 

Better balance between cooling capacity and load can be obtained by capacity control of the compressor (s). Targe systems will have a number of compressors, or built-in capacity control on the cylinders. A central condensing unit of this sort may be coupled to several fan-coils, each with its own thermostat and liquid solenoid value. The COP is good at all but the lowest load levels. 

HA A single-stage double-acting compressor running at 3 Hz is used to compress air from 110 kN/irr and 382 K to 1150 kN/nr. If the internal diameter of the cylinder is 20 cm, the length of stroke 25 cm and the piston clearance 5%, calculate (a) the maximum capacity of the machine, referred to air at the initial temperature and pressure, ami power requirements under isentropic conditions. 

Positive-Displacement Compressors Reciprocating compressors are built in different sizes (up to about one megawatt refrigeration capacity per unit). Modern compressors are high-speed, mostly direct-coupled, single-acting, from one to mostly eight, and occasionally up to sixteen cylinders. 

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