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RECIPROCATING COMPRESSORS


DescriptionThe 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. Another, more common method is to use clearance pockets with or without valve unloading.


DescriptionWith 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.


DescriptionOn 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 lubricants.

The cylinder may also be designed for self lubrication, generally referred to as nonlubed.


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


DescriptionAn unusual nonlubed compressor is a labyrinth piston compressor.


DescriptionThe 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?


DescriptionThe 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.


DescriptionAnother 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.


ClassificationReciprocating 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.


ClassificationThis 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.


ClassificationThe more common type of compressor used in process service is the crosshead type .


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


ClassificationThe 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.


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


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


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


ArrangementMost 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 single-stage, has horizontal cylinders. The double-acting cylinder compressor is built in both the horizontal and the vertical arrangement.


ArrangementFrom 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 Ltype.


Arrangement


Principle of work

Compression

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14 1

2

4

3

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


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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14 14

23


Principle of work

Expansion

The gas expansion takes place during the next part of the stroke. Both the inlet and exhaust valves 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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14 1

23

4


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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1414

3 2


Principle of work

Intake

The effectiveness is influenced by numerous factors such as clearance volume, inlet pressure, receiver pressure, valve performance, piston-cylinder leakage, nature of gas, temperature, etc.

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1414

3 2

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.


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

Piston displacement

Actual Capacity

VE= Actual CapacityPiston displacement


Compressor make-upCylinder – serves as a container for the gas. It must be strong enough to withstand whatever pressures are to be generated.

Piston – 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-upDischarge 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-upCylinders


Compressor make-upCylinders

Separable

Distance piece

Frame

Cylinder

Distance piece

Piloting Cooling (Water or Air)

Steel for the smaller, high pressure cylinders Nodular or Ductile iron


Compressor make-upCylindersOn 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-upCylindersClearance 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-upCylindersWhen 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 averageis usually used.


Compressor make-upPiston and rods


Compressor make-upPiston and rodsThe piston must translatethe energy from thecrankshaft to the gas in thecylinder.

The piston is equipped with a set of sliding seals referredto as piston rings. Rings are made of a material, whichmust be reasonably compliant for sealing, yet slide longthe cylinder wall with minimum wear.


Compressor make-upPiston and rodsDifferent rings are used forlubricated or nonlubricatedservice, with the rings in thenonlubed cylinders needinggood dry lubricatingqualities.

For lubricated service, metallic rings such as cast iron orbronze as well as nonmetallic materials such as fillednylon are used.


Compressor make-upPiston and rodsFor nonlubricated service,the ring material isnonmetallic, ranging fromcarbon to an assortment offluorocarbon compounds.


Compressor make-upPiston and rodsHorizontal cylinder pistons featurethe addition of a wear band,sometimes referred to as a riderring .

Piston may be of segmentedconstruction to permit the use ofone piece wear bands.


Compressor make-upPiston and rodsWeight in a piston contributesdirectly to the compressorshaking forces and must becontrolled.For this reason aluminum pistons are often found in largelow pressure cylinders.

Hollow pistons are used but can pose a hazard tomaintenance personnel if not properly vented. If trapped,the gas will be released in an unpredictable and dangerousmanner when the compressor is dismantled.


Compressor make-upPiston and rodsThe piston rod is threaded to thepiston and transmits thereciprocating motion from thecrosshead to the piston.The piston rod is normally constructed of alloy steel andmust have a hardened and polished surface particularlywhere it passes through the cylinder packing (double-acting cylinders).


Compressor make-upPiston and rodsRod loading must be kept withinthe limits set by the compressorvender because overloading cancause excess run out of the rodresulting in premature packingwear. This in turn leads toleakage, reduced efficiency andincreased maintenance expense.


Compressor make-upPiston and rodsIn unloaded or part loadoperation, rod reversals must beof sufficient magnitude to providelubrication to the crossheadbearings.

The bearings are lubricated by the pumping action of theopening and closing of the bearing clearance area.


Compressor make-upPiston and rodsTail rods are dummy rods, which protrude from the headend of the cylinder .


Compressor make-upPiston and rodsThe purpose of the rod is to pressure-balance a piston or tostabilize a particular piston design.


Compressor make-upPiston and rodsIn a tandem cylinder arrangement, the outboard cylindersare driven with a rod similar to the tail rod.


Compressor make-upValves


Compressor make-upValvesOf 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-upValvesThe compressor cylindervalves are of the springloaded, gas actuated typein all but a limited numberof portable compressors.

This kind of valve is used in contrast to the cam actuatedpoppet type normally found in piston engines.


Compressor make-upValvesThe reciprocating compressoruses automatic spring loadedvalves that open only whendifferential pressure existsacross the valve.

Inlet valves open when the pressure in the cylinder isslightly below the intake pressure.

Discharge valves open when the pressure in the cylinderis slightly above discharge pressure.


Compressor make-upValvesReciprocating compressorsgenerally use one of three basicvalve configurations:

Plate

Channel

Feather type


Compressor make-upValves Plate valve



This type of valves employs two additional spring-loaded 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-upValves 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 pressure differential of 34 atm. They are tolerant of dirty environments, and offer a particularly advantageous flow pattern in sizes up to 133 mm diameter.


Compressor make-upValves 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, 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.

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