Saturday, December 6, 2008

Reciprocating Compressors

Introduction
Air compressors of various designs are used widely throughout DOE facilities in numerous applications. Compressed air has numerous uses throughout a facility including the operation of equipment and portable tools. Three types of designs include reciprocating, rotary, and centrifugal air compressors.

*Reciprocating Compressors
The reciprocating air compressor, illustrated in Figure 1, is the most common design employed today.
The reciprocating compressor normally consists of the following elements.

a. The compressing element, consisting of air cylinders, heads and pistons, and air
inlet and discharge valves.

b. A system of connecting rods, piston rods, crossheads, and a crankshaft and
flywheel for transmitting the power developed by the driving unit to the air
cylinder piston.



c. A self-contained lubricating system for bearings, gears, and cylinder walls,
including a reservoir or sump for the lubricating oil, and a pump,
or other means of delivering oil to the various parts. On some compressors a
separate force-fed lubricator is installed to supply oil to the compressor
cylinders.

d. A regulation or control system designed to maintain the pressure in the
discharge line and air receiver (storage tank) within a predetermined range of
pressure.

e. An unloading system, which operates in conjunction with the regulator, to
reduce or eliminate the load put on the prime mover when starting the unit.

A section of a typical reciprocating single-stage, single-acting compressor cylinder is shown in Figure 2. Inlet and discharge valves are located in the clearance space and connected through ports in the cylinder head to the inlet and discharge connections.


During the suction stroke the compressor piston starts its downward stroke and the air under pressure in the clearance space rapidly expands until the pressure falls below that on the opposite side of the inlet valve (Figures 2B and 2C). This difference in pressure causes the inlet valve to open into the cylinder until the piston reaches the bottom of its stroke (Figure 2C).

During the compression stroke the piston starts upward, compression begins, and at point D has reached the same pressure as the compressor intake. The spring-loaded inlet valve then closes.

As the piston continues upward, air is compressed until the pressure in the cylinder becomes great enough to open the discharge valve against the pressure of the valve springs and the pressure of the discharge line (Figure 2E). From this point, to the end of the stroke (Figures 2E and 2A), the air compressed within the cylinder is discharged at practically constant pressure.

5 التعليقات

High engineer! Nice blog !

Please could you help me on the following problem or give me the address of a good forum where I could get some help ?

I try to calculate the energy efficiency (from electric plug to the output of the compressor) of this compressor when it produces air@300bars :
BAUER V 500-11-5 F

See specifications :
http://www.alliedsolutions.com/pdf/compressors/verticus_f_en.pdf

FAD : 500 l./min @ 200 bars
Power : 11kW
Filling time for 1 liter@300bars : 0.60 min.

Please, could you validate/correct the following calculation ?

Efficiency = output energy / input energy

* Output energy :
Isothermal expansion from 1 bar to 300 bars:
W = 30 MP x 0.001 m^3 x ln 300 = 0.1711 MJ = 47.5 Wh

* Input energy:
11000 W x 0.6/60 = 110 Wh

Efficiency = 47.5 / 110 Wh = 43%

It seems quite low... Perhaps the motor doesn't use the max.specified power... but if I take let's say 10500W, I get efficiency = 45%.

What is today the best efficient HP compressor @300 bars or 420 bars we can find on the market ?
What is its efficiency in % ?

Thanks by advance,

Best regards,

Steve.

Hi again !

Perhaps I can simplify the problem:

Does a multi-stage high pressure compressor only filling a buffer tank usually work at constant torque, power and speed ?

...or does it work harder while pressure goes up in the buffer tank because, by example, it's harder to open the output valve of the last cylinder ?

In other words, can I evaluate its electricity consumption like this:

energy used = power of the motor x running time

In fact, I guess the motor is not used at its maximum power... but what % ? 90% ?
The best thing to do is of course to measure the used energy at the plug with a wattmeter by it's not easy before buying !

I'd like to evaluate the electricity cost of a compressor before buying...
Any suggestion ?

Thanks by advance !

Regards,

Steve.


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