Air Compressors

plantcalicobeansUrban and Civil

Nov 29, 2013 (3 years and 11 months ago)

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Air Compressors


Graph of Pressure against volume in a reciprocating compressor

Volumetric efficiency Vh = Actual suction volume Vx/



Theoretical suction
volume Vs

For greater efficiency air compression should be isothermal as this requires the minimum
work input. In practice Isothermal compression is not possible, an ideal Isothermal cycle
requires sufficient time to all
ow all the required heat to be transferred out of the cylinder,
practicality dictates that the piston must have a relatively high speed to give a reasonable
output,

Cylinder cooling on a single stage compressor gives better efficiency but there is a
limit
ation in the surface area to cylinder volume that can be used for cooling effect, but
multistage compressors with an efficient extended surface interstage cooler gives cycle
improved compression efficiency better approaching that of the isothermal. In theo
ry the
greater the number of stages the closer the curve will approach the ideal isothermal
compression curve, however there is an increase in cost, complexity, and the law of
diminishing returns limit the number.

Compression in stages has the following a
dvantages;

1.

The compression ratio at each stage is lower and so the final temperature is lower.
This reduces problems with lubrication

2.

. The machine is smaller and better balanced

3.

water can be drained off at each stage

4.

Compression better approaches the i
deal isothermal

It is important that the compressor clearance volume is kept small as possible in order to
improve overall volumetric efficiency as the air trapped in this space must expand to
below suction pressure before new air can enter, this is an ef
fective loss of stroke.

A clearance is required in order to prevent the piston striking the cylinder cover when
starting or stopping off load. The clearance volume is sometimes referred to as the 'Bump
Clearance'.

Crankcase lubrication

Lubrication of th
e crankcase in a compressor does not pose any specific problems and
normally consist of splash lubrication with pressurised oil being fed to shell bearings.
Where drip cylinder lubrication is used, this should be kept to a minimum conducive with
liner wear
. A standard mineral oil similar to that used in the main engine may be used,
although due to carbon deposits, higher quality oils are generally used with the most
effective being specifically designed synthetics which have allow a considerable
reduction i
n maintenance but are costly.

Mineral oils contain a blend of lighter elements such as paraffin's, and heavier elements
such as asphaltenes. During compression the lighter elements are vaporised leaving the
heavy ends, these coat the piston rings and disc
harge valves in combination with oxidised
oil deposits. These deposits also coat passage ways and coolers resulting in higher
interstage air temperatures. Deposits on discharge valves cause them to become sticky
and leak resulting in hot air being drawn ba
ck into the cylinder for recompression. This
increases the temperature and hence causes greater oxidation and deposits, and so the
condition deteriorates with increasing rapidity.

Temperature can become very high, this may result in oily deposits at the d
ischarge
valves carbonising. Eventually this carbon could glow red and cause detonation. It is
more likely, however, that oily deposits will be carried over to the air receiver and air
start manifold to be ignited by blowpast at the cylinder air start valv
e.

Deposits at piston rings cause leakage allowing oil to enter the cylinder from the
crankcase thus increasing the danger it is essential that crankcase lubrication be kept to a
minimum compatible with an acceptable wear rate. Regular maintenance will mi
nimise
oily deposits build up and hence the risk of explosion

Materials and design of a reciprocating compressor

The compressor casing, cylinder covers and piston rings are generally of cast iron.
Pistons may be of cast iron, steel of aluminium. Aluminiu
m being the preferred material
for use on the LP piston due to its larger diameter. Valves are usually made so that parts
can be interchanged between the suction and discharge valves. Seats are of mild steel
with small diameter air passages to prevent the
fragments of broken valve plate from
entering the cylinder. Valve plates are of vanadium steel heat treated and ground to
provided the required hardness and surface finish. Springs should be arranged such that
they lift and seat squarely. Uneven spring for
ce or deposits on the seat cause valves to
bend resulting in fatigue cracking.


For compressors designed for starting air requirements a water jacket relief
valve is fitted.


Rotary Compressor


The rotary compressor may be o
f the impeller type similar to that used in the
turbocharger , scroll, twin rotating lobes or of the sliding vane type similar to the one
shown above. In practice there would be several more vanes than shown.

Rotary compressors are capable of handling lar
ge quantities of low pressure air much
more efficiently than a reciprocating compressor. In order to produce increased pressures
it is possible to stage rotary compressors but leakage problems increase at higher
pressures as well as stress on the vanes.

T
he sliding vane compressor consists of a slotted rotor with its axis offset from that of the
cylindrical casing. Vanes fit in the slots and have contact with the casing

On the suction side the space contained between the casing, the rotor and an adjacent
pair
of vanes is gradually increasing allowing air to be drawn in.

On the compression side this same space is gradually reduced causing the pressure
increase. When the leading vane uncovers the discharge port air will flow to outlet.
Larger compressors of

this type are water
-
cooled, smaller compressors tend to be air
cooled.

The main problems related with sliding vane compressors concern wear at the vane tips
and sealing of the ends

Rotary/reciprocating Compressor

Rotary compressors in general do not re
quire internal lubrication but they are not suitable
alone for providing air at a pressure for starting duties. They can, however, be linked to
reciprocating stages to produce a hybrid compressor.

The compressor is lighter, more compact and better balance
d than an equivalent all
reciprocating unit. In basic terms the rotary first stage supplies air to the reciprocating
second and subsequent stages. All stages being driven by the same shaft


Safety Valve


Materials

Cast iron
-
Casing, Liners, Pistons( the LP piston is sometimes made from an aluminium
alloy, Cylinder cove
rs
Steel
-

Crankshaft, Conrods, Pistons, Valve seats
Vanadium Steel
-

Valve plates

Starting air compressor circuit

Starting and stopping sequence is adjustable, the magnetic valves are open when the
compressor is stopped so any residual pressure is blown o
ff. On starting the magnetic
valve are sometimes delayed to close so as to allow the compressor motor to reach full
speed before the compressor is loaded up.

The non
-
return valves prevent HP air leaking back from the receiver on which the filling
is also
of the non return type.


Calculation of required cylinder compression for a multistage
reciprocating compressor

r = stage pressure ratio

R = compression pressure
ratio

for a two stage

r = R1/2

for a three stage

r = R1/3

for example, a 3 stage compressor requiring a final pressure of 64bar would have the
following interstage pressures 1st stage 1bar compressed to 4bar

2nd stage 4bar compressed to 16bar

3rd stage
16bar compressed to 64bar

It would appear that most of the work is being carried out in the final stage, this is untrue
as with the increase in pressure is a complimentary reduction in volume, if the
temperature conditions remain the same then work will be

equally divided between the
stages.

By reducing the suction pressure, the cylinder is required to do more work on the air
before the discharge valve opens. This means that the air will be delivered at a higher
pressure. The higher temperature can lead to

problems with the cylinder lubrication as
well as a drop in efficiency as well as carbonising of the oil and increased deposits on the
valves and piston rings and interstage passages. In the extreme it can lead to seizure and
possible diesel detonation of

the oil laden air.

The reduction in pressure at the suction can be due to a partially blocked suction filter or
partially choked suction valve. The lower pressure conditions in the cylinder at the start
of compression can cause oil laden air to be drawn
from the crankcase up the liner. This
oil can lead to increased deposits in the compressor as well as further downstream in the
distribution system

(P1.V1)/ T1 = (P2. V2)/T2

and

(P1/P2).(T2/T1) = (V2/V1)

P1.V1
g

= P2.V2
g


and

P1/P2 = V2
g
/V1
g

From these w
e get;

T2 = T1. (P2/P1)
(g
-
1)/g

g = 1.4

and if we take for and example

P1 = 0.4 bar

P2 = 1 bar

Pf = 5 bar

Tinitial = 300 K we end with final temperatures for the two compression's of

T1= 617 K and T2 = 475 K

from the graph it can be clearly seen that loss
es due to the bump clearance has increased
and the period of constant pressure delivery has been reduced.

Coolers



Plain Tube
-


o

-
easy to clean

o

-
very effective due to large surface area of large number small diameter
tubes

o

-
plugging of failed tube allos
cooler to continue in service with little loss
in efficiency

o

-
must allow for thermal expansion by having one tube plate floating



'U' tube
-


o

-
suitable for higher pressures than plain tube

o

-
self compensating for thermal expansion

o

-
efficent due to large n
omber small diameter tubes

o

-
failed tubes may be plugged

o

-
more expensive than plain tube and diificult to clean



Coil tube

o

-
self compensating for expansion

o

-
suitable for high pressures

o

-
difficult to clean

o

-
inefficicent due to large tube diameter

o

-

no
t easy to plug

o

-
expensive