Part 1

RECIPROCATING COMPRESSORS


Description The reciprocating compressor is a positive displacement, intermittent-flow machine and operates at a fixed volume. One method of volume variation, however, is by speed modulation. modulation. Another, more common method is to use clearance pockets with or without valve unloading.


Description 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 configurations. A unique feature of the reciprocating compressor is the possibility of multiple services on one compressor frame.


Description On a multistage frame, each cylinder can be used for a separate gas service. Lubrication of compressor cylinders can be tailored to the application. The cylinders may be designed for normal hydrocarbon lubricants or can be modified for synthetic for synthetic lubricants. lubricants. The cylinder may also be designed for self lubrication, generally referred to as nonlubed nonlubed..


Description A compromise lubrication method which uses the nonlubed design but requires a small amount of lubrication is referred to as the mini-lube system.


Description An unusual nonlubed compressor is a labyrinth piston compressor.


Description The piston does not touch the sides of the cylinder because it is equipped with a series of circumferential by labyrinths operating with a close clearance to the cylinder wall.

What is the effect of that ?


Description The piston does not touch the sides of the cylinder because it is equipped with a series of circumferential by labyrinths operating with a close clearance to the cylinder wall. Efficiency is sacrificed (due to gas by-pass) in order to obtain a low maintenance cylinder. This design is mentioned primarily due to its being unique, as it is not widely manufactured.


Description Another feature necessary to the reciprocating compressor is cylinder cooling. Most process compressors are furnished with water jackets as an integral part of the cylinder. Alternatively, particularly in the smaller size compressors, the cylinder can be designed for air-cooling.


Classification Reciprocating

compressors can be classified into several

types. One type is the trunk or automotive piston type . The piston is connected to a connecting rod, which is in turn connected directly to the crankshaft.


Classification This type of compressor has a single-acting cylinder and is limited to refrigeration service and to smaller air compressors. Most of the smaller packaged refrigeration system compressors are this type.


Classification The more common type of compressor used in process service is the c rosshead rosshead type .


Classification The piston is driven by a fixed piston rod, which passes through a stuffing or packing box and is connected to a crosshead.


Classification The crosshead, in turn, is connected to the crank-shaft by a connecting rod. In this design, the cylinder is isolated from the crankcase by a distance piece.


Classification A variable length or double distance piece is used to keep crankcase lubrication from being exposed to the process gas.


Classification This design has obvious advantages for hazardous materials. The cylinder can be either single or double acting.


Classification Except for very small compressors, most reciprocating compressors furnished to the process industry use the double acting configuration.


Arrangement Most multicylinder arrangement is in pairs in the form of a V usually at 45o from the vertical. The few single-acting crosshead compressors are normally single-stage machines with vertical cylinders. The more common double-acting type, when used as singlestage, has horizontal cylinders. The double-acting cylinder compressor is built in both the horizontal and the vertical arrangement.


Arrangement From

a ring wear consideration the more logical orientation is vertical, however, taking into account size and the ensuring physical location as well as maintenance problems, most installations normally favor the horizontal arrangement. The most common multistage configuration being the horizontally opposed. Other variations include V, Y angle or L or L type.


Arrangement


Arrangement


Principle of work Compression W ith

the piston moving towards the closed end (compression (compression stroke), stroke ), the original volume of air is reduced and the pressures increases until the cylinder pressure exceeds the receiver discharge pressure.

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Principle of work Discharge At this time, the cylinder pressure exceeds the receiver pressure forcing the discharge valve to open to pass gas to the receiver. The flow continues until the piston reaches the end of the stroke at point 3 and the pressure declines as the piston reverses its direction. The inlet valve is closed during discharge.

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Principle of work Expansion The gas expansion takes place during the next part of the stroke. Both the inlet and exhaust valves 3 are closed, the piston is moving towards the point of beginning and the pressure is decreasing to the inlet pressure. During the expansion stroke, the compressed gas left in clearance volume area from the discharge expands to the cylinder inlet pressure.

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Principle of work Intake During this time, the inlet valve is open and the piston completes the intake stroke to a fully expanded position. The piston movement creates a partial vacuum causing the inlet valve to (loaded) for the next cycle.

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Principle of work Intake The effectiveness is influenced by numerous factors such as clearance volume,, inlet pressure, volume pressure, receiver pressure,, valve performance, pressure performance, pistoncylinder leakage, leakage, nature of gas, gas, temperature,, etc. temperature The mass flow will remain constrant for dry gas although pressure, volume and temperature will change. If condensate is removed from the gas the mass flow will reduce.

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Principle of work Actual capacity Is the quantity of gas actually compressed and delivered to the discharge system by the machine at rated speed and under rated inlet and discharge conditions (and under rated inter-stage conditions in a multi-stage machine). Actual capacity is usually expressed in cfm or m3/hr referred to first stage inlet temperature and pressure.


Principle of work Actual capacity

Actual Capacity Piston

VE=

displacement

Actual Capacity Piston displacement


Compressor make-up Cylinder ± serves as a container for the gas. It must be strong enough to withstand whatever pressures are to be generated. iston P iston

± a circular object usually made of metal, which fits inside the cylinder. It causes the cylinder volume to change when moved, (compression).

Inlet valve ± Permits gas to enter the cylinder on the intake stroke and closes when the piston starts compression. Several inlet valves may be needed.


Compressor make-up Discharge

valve ± Opens at the early part of the compression stroke allowing the gas to pass out of the cylinder into a discharge vessel. The discharge pressure keeps the valve open until the pressure drops at the end of the stroke. Other essentials include provision for lubrication, removing heat from compressor and power to move the piston.


Compressor make-up Cylinders


Compressor make-up Cylinders Separable Distance piece Piloting

Cooling (Water or Air) Steel for the smaller, high pressure cylinders Nodular or Ductile iron

Frame

Cylinder Distance piece


Compressor make-up Cylinders On larger cylinders, there is normally enough space for clearance pockets. An additional location is the head casting on the outboard end of the cylinder . On smaller cylinders, this feature must be provided external to the cylinder.


Compressor make-up Cylinders Clearance volume is the volume present in one compressor cylinder or one compressor in excess of the net volume displaced by the piston during one cycle.


Compressor make-up Cylinders When applied to double acting piston compressor, the volumes are referred to both the head end (HE) and the cylinder end (CE) . It may not be the same for the two ends in a double acting cylinder. An average is usually used.


Compressor make-up Piston and rods


Compressor make-up Piston and rods The The pist piston on must must tran transl slat ate e the energy from the crankshaft to the gas in the cylinder.

The piston is equipped with a set of sliding seals referred to as piston rings. Rings are made of a material, which must be reasonably compliant for sealing, yet slide long the cylinder wall with minimum wear.


Compressor make-up Piston and rods Different rings are used for lubricated or nonlubricated service, with the rings in the nonlubed cylinders needing good dry lubricating qualities. For

lubricated service, metallic rings such as cast iron or bron bronze ze as well well as nonm nonmet etal alli lic c mate materi rial als s such such as filled nylon are used.


Compressor make-up Piston and rods For

nonlubricated service, the ring material is nonm nonmet etal alli lic, c, rang rangin ing g from from carbon to an assortment of fluorocarbon fluorocarbon compounds.


Compressor make-up Piston and rods Horizontal cylinder pistons feature the addition of a wear band, some someti time mes s refe referr rred ed to as a ride rider r ring . Piston

may be of segmented cons constr truc ucti tion on to perm permit it the the use use of one piece wear bands.


Compressor make-up Piston and rods eightt Weigh

in a pist piston on cont contri ribu bute tes s directly to the compressor shaking forces and must be controlled. For

this reason aluminum pistons are often found in large low pressure cylinders. Hollow Holl ow pi pist ston ons s are used but can pose a hazard to maintenance personnel if not properly vented. If trapped, the gas will be released in an unpredictable and dangerous manner when the compressor is dismantled. dismantled.


Compressor make-up Piston and rods The piston rod is threaded to the piston and transmits th e reci recipr proc ocat atin ing g moti motion on from from the the crosshead to the piston. The piston rod is normally constructed of alloy steel and must have a hardened and polished surface particularly wher where e it pass passes es thro throug ugh h the the cy cyli lind nder er pack packin ing g (dou (doubl bleeacting cylinders).


Compressor make-up Piston and rods Rod

loading must be kept within the limits set by the compressor vender because overloading can cause excess run out of the rod resu result ltin ing g in prem premat atur ure e pack packin ing g wear. This in turn leads to leakag leakage, e, reduce reduced d effic efficien iency cy and increased maintenance expense.


Compressor make-up Piston and rods In unloaded or part load operation, rod reversals must be of sufficient magnitude to provide lubrication to the crosshead bearings. The bearings are lubricated by the pumping action of the opening and closing of the bearing clearance area.


Compressor make-up Piston and rods Tail rods are dummy rods, which protrude from the head end of the cylinder cylinder .


Compressor make-up Piston and rods The purpose of the rod is to pressure-balance a piston or to stabilize a particular piston design.


Compressor make-up Piston and rods In a tandem cylinder arrangement, the outboard cylinders are driven with a rod similar to the tail rod.


Compressor make-up Valves


Compressor make-up Valves Of all the many components in a reciprocating compressor, none works harder nor serves a more important compressor component function than the suction and discharge valves. In fact, compressor efficiency is determined by the performance of the valves more than any other component.


Compressor make-up Valves The com compress resso or cy cyli lind nde er valves are of the spring load loaded ed,, gas gas actu actuat ated ed type type in all but a limited number of portable of portable compressors. This kind of valve is used in contrast to the cam actuated poppet type normally found in piston engines.


Compressor make-up Valves The The recip recipro roca cati ting ng comp compre resso ssor r uses uses auto automa mati tic c spri spring ng load loaded ed valves that open only when differential pressure exists across the valve. Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure in the cylinder is slightly above discharge pressure.


Compressor make-up Valves Reciprocating

compres ressors generally use one of three basic valve configurations: configurations: Plate Channel Feather type


Compressor make-up Valves Plate valve



This type of valves employs two additional spring-loaded discs, discs , which effectively reduce flutter and cushion the impact of opening and closing .



The built-in dampening action of these valves makes them particularly suitable for compressors operating at high speeds (750 rpm or more), and in applications with either a high pressure differential (greater than 17 atm differential) or varying pressure conditions within the same compressor .


Compressor make-up Valves Channel valve

In operation, gas trapped between the spring and channel provides a cushioning or dampening effect to minimize pounding and wear, and permits a somewhat larger lift than other types.



In operation, gas trapped between the spring and channel provides a cushioning or dampening effect to minimize pounding and wear, and permits a somewhat larger lift than other types. Each channel and spring operates individually as an independent valve



Channel valves are good for medium to low pressure service,, with a maximum service pressure differential of 34 atm. atm. They are tolerant of dirty environments,, and offer a environments particularly advantageous flow pattern in sizes up to 133 mm diameter .




Compressor make-up Valves Feather valve

The feather valve is most common on the higher speed compressors because of its light weight . Due to the valve design and the fact that there are no springs, springs, these valves also provide a good flow pattern through the centre of the valve. Feather valves are normally used with a maximum pressure differential of up to 25.0 atm. atm.

Part 1

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