Double-acting

Hydraulic The design of typical hvdraiilic actuators is similar to double-acting piston pneumatic types. One kev advantage is the high pressure (yvpicallv35 to 70 bar [500 to 1000 psi]), vvFich leads to high thrust in a smaller paclcage. The incompressible nature of the... 

Piston Pumps There are two ordinary types of piston pumps, simplex double-ac ting pumps and duplex double-acting pumps. 

Simplex Double-Acting Pumps These pumps may be direc t-acting (i.e., direct-connected to a steam cyhnder) or power-driven (through a crank and flywheel from the crosshead of a steam engine). 

FIG. 10-48 Double -acting steam-driven reciprocating pump. 

Dupl ex Double-Acting Pumps These pumps differ primarily from those of the simplex type in having two cyhnders whose operation is coordinated. They may be direct-acting, steam-driven, or power-driven with crank and flywheel. 

Occasionally phmger pumps are constructed with opposed cylinders and plungers connected by yokes and tie rods this arrangement, in effect, constitutes a double-acting unit. 

FIG. 10-86 Two -stage double-acting compressor cylinders with intercooler. 

One method of volume variations is by speed modulation. Another, more common method, is the use of clearance pockets, with or without valve unloading. With clearance pockets, the cylinder performance is modified. With valve unloading, one or more inlet valves are physically open. Capacity may be regulated in a single- or double-acting cylinder with single or multiple cylinder configuration. 

After all the previous statements, it would seem very difficult to select a piston. speed. For someone without direct experience, the following guidelines can be used as a starting point. Actual gas compressing experience should be solicited when a new compressor for the same gas is being eonsidered. These values will apply to the industrial process type of compressor with a double-acting cylinder construction. For horizontal compressors with lubricated cylinders, use 700 feet per minute (fpm) and for nonlubricated cylinders use 600 fpin. For vertical compressors with lubricated cylinders, use 800 fpm and for nonlubricated cylinders use 700 fpm. 

To calculate Pd, use Equation 3.1 for single-acting cylinders, Equadofi 3.2 for double-acting cylinders without a tail rod, and Equation 3.3 foi double-acting cylinders with a tail rod. 

Calculate the suction capacity, horsepower, discharge temperature, and piston speed for the following single-stage double acting compressor. 

A packing is required on double-acting cylinders to provide a barrier to leakage past the rod where it passes through the crank end cylinder closure. The same arrangement is needed at the head end if a tail rod or tan-... 

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. 

Double-acting cylinders are those where compression occurs twice per cranksheet revolution. 

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. 

Figure T1 -8. Valve-in-piston double-acting compressor cylinder. Couriesy of Dresser-Rand Company.)...

Double acting cylinder (sum of head end and crank end displacement)... 

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