Oil systems for compressors - Flexible Learning Toolboxes

plantcalicobeansUrban and Civil

Nov 29, 2013 (3 years and 9 months ago)

137 views


© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License


Compressors


2

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License


TABLE OF CONTENTS

Compressor styles

................................
................................
................................
................

3

Hermetic Compressors

................................
................................
................................
......

3

Semi
-
Hermetic compressors

................................
................................
..............................

4

Open
-
Drive compressors

................................
................................
................................
...

4

Crankshaft Construction Types

................................
................................
.............................

6

Crank throw type


combination reciprocating and rotating action
................................
......

6

Eccentric type


off centre disc is mounted on the shaft

................................
.....................

6

Scotch Yoke Mechanism

................................
................................
................................
...

6

Swash plate type


disc mounted on an angle on the shaft, common on automotive
compressors

................................
................................
................................
......................

8

Crankcase seal

................................
................................
................................
.....................

8

Reciprocating compressor heads and valve plates:

................................
.........................

10

Oil systems for compressors

................................
................................
...............................

11

Reciprocating compressors

................................
................................
..............................

11

Rotary compressors

................................
................................
................................
.........

11

Centrifugal compressors

................................
................................
................................
..

11

Helical screw compressors

................................
................................
..............................

11

Scroll compressors

................................
................................
................................
..........

11

Promoting

oil return

................................
................................
................................
.........

12

Mufflers

................................
................................
................................
............................

12

Compressor Efficiency

................................
................................
................................
.....

13


3

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License


COMPRESSOR STYLES

Hermetic

Compressors

Fea
tures:



electric motor is inside the compressor



totally sealed and not accessible



suction vapour is used to cool the motor windings



do not run in a vacuum, windings resistance is
a
ffected



windings are
a
ffected by contaminants in the system



more common in sm
aller applications
.

Diagram 1:

Hermetic compressor










(
Source:
Danfoss
)


4

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License


Semi
-
Hermetic

compressors

Features:



electric motor is inside the compressor



mechanical part only is accessible not motor



suction vapour is used to cool windings
.

Diagram 2:

Semi
-
hermetic compressor










Open
-
Drive

compressors


5

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License

Features:



external belt or direct drives



less prone to problems from contaminants



shaft seal integrity very important
.

Diagram

3
:

Cutaway view of small, external
-
drive, two
-
cy
linder reciprocating co
mpressor

























6

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License

CRANKSHAFT CONSTRUCT
ION TYPES

Several types of linkages may be used to connect the connecting rod to the crankshaft:

Crank throw type



combination
reciprocating and rotating action

Diagram

4
:
Crank throw type
crankshaft










In reciprocating
compressors, the crankshaft
converts rotary
motion from the motor to
reciprocating
motion for the pistons.
Crankshafts rotate within
the
main bearing, which must firmly support the crankshaft and
resist e
nd loads placed on the shaft by the motor and connecting rods. The exact amount of
the endplay would be specified in
the
manufacturer
’s

literature.

Eccentric type


off centre disc is mounted on the shaft

The eccentric crankshaft has an off
-
centre, circula
r boss on the crankshaft to create the up

and down motion. This system eliminates the need for caps or bolts on the connecting rod.
Instead, the one
-
piece rod end is fitted to the crankshaft before final assembly.

Diagram

5
:

Eccentric type crankshaft











Scotch Yoke Mechanism


7

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License

The
S
cotch yoke uses no connecting rod. Instead, the lower portion of the piston contains a
groove, which accepts the throw of the crankshaft. The groove permits the crankshaft throw
to travel lateral
ly and to drive the piston only up and down. Both the Scotch yoke and the
eccentric
mechanisms
are found primarily on domestic and automobile systems.

Diagram

6
:

Scotch yoke mechanism









8

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License


Swash plate type

Diagram

7
:


Cross section through a swash

p
late reciprocating compressor



a
s
the
drive
shaft and swash plate revolve, double
-
end piston is moved back and forth in
the
cylinder

















Diagram

8
:

Auto compressor








CRANKCASE SEAL


9

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License

Introduction

In open
-
drive systems, the seal between t
he crankshaft and the crankcase is
a
common
source of problems. The seal is subjected to a great deal of pressure variation and must
operate and seal whether the crankshaft is rotating or is stationary. Clearance

between the
rotating and stationary surface
s

is minute and must be accurate with

lubrication fill
ing the

tiny gap. The seal is commonly made of hardened steel
,

bronze, ceramic
s

or carbon. The
absence of the crankshaft seal is the major advantage of the hermetic design.

The rotary
-
type seal is a sim
ple, common seal that rotates on the shaft in operation. A
spring, in combination with internal pressure, forces the seal face against a stationary seal
face.

The major source of problem with crankcase seals is leakage due to misalignment.

Care
must be tak
en when aligning the motor shaft to the compressor shaft so
the
seal will be not
stressed during the operation. The close tolerance specified by

the

manufacture of the
compressor must be observed in both horizontal and angular directions. In most cases, th
e
seal is lubricated by
a
compressor oil pump. Make sure that the compressor is operated
occasionally during the long shutdowns to keep the seal lubricated. A slight leakage after
start up, during which a dry seal is lubricated with oil, may be normal.

A l
eaking seal can be detected with a refrigerant leak detector.


Diagram 9:

Crankcase

seal


10

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License


Reciprocating compressor heads and valve plates

Compressor cylinder heads are generally made of cast iron and are designed to hold the
gaskets in place to provide
positive seal between the valve plate, the cylinder block and
head. Cylinder heads must have passages to admit suction gas into the cylinder. The head
is generally affixed to the block with cap screws.

Intake valves are designed to admit refrigerant during

the intake stroke and close during the
compression stroke. Discharge valves are closed during the intake stroke and open at the
end of the compression stroke. The valve plate is the assembly holding both valves tightly in
place.

Valves are usually made of

spring steel and designed to make a tight seal until the pumping
action of the piston opens them. The mating surfaces of valves must be perfectly flat, and
defects as small as
0.0254 mm

can cause unacceptable leaks. In service, the valve must
open about 0
.254 mm. Large
r

openings will cause valve noise, while smaller openings will
prevent enough refrigerant from entering and exiting the cylinder.

Operating temperature has
a
great effect on valve durability. Intake valves operate in a
relatively cool environ
ment and have constant lubrication from oil vapours. Discharge valves
are the hottest component in a refrigeration system, hotter than the discharge line, so they
are more commonly a source of trouble than intake valves. Discharge valves must be fitted
wit
h special care. Heavy molecules of oil tend to accumulate on them, causing carbon build
-
up and interfering with valve performance. Discharge valves and oil will be damaged by
temperature hotter than
the range
163

°C

to
177

°
C. In general, the discharge lin
e
temperature should be kept
in the range
107

°C

to 121

°C
.

Diagram

10
:

Reciprocating
compressor valve plate assembly








Discharge valves may have relief springs to enable them to open abnormally wide if
a
slug of
liquid refrigerant or oil enter
s
the
compressor piston from the suction line or the compressor
crankcase.


11

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License

OIL SYSTEMS FOR COMP
RESSORS

Reciprocating compressors


There are t
wo types of lubricating systems:

1.

A s
plash
f
eed

system uses the crankshaft to splash oil; oil reaches the main bearing by
flowing through bearing channels. Bearing may be noisy because this system produces
a small oil cushion.

2.

A f
orced
f
eed

(
oil pressure
)

system uses an oil pump driven by gears in the crankcase;
oil is forced into channels in the connecting rods, main bearing
s and piston pins.

The o
il
pump system does a better job of ensuring lubrication and quiet operation. The pump
must have an overload relief valve to prevent development of dangerous pressures in
the
compressor lubrication circuit. A safety switch is usuall
y used to monitor oil pressure
and shut down the compressor if the oil pressure drops below a safe level.

Rotary compressors

Rotary compressors r
equire a film of oil on the cylinder, blades and roller. Some machines
propel the oil by
a

sliding action; oth
ers use an oil pump.

Centrifugal compressors

Centrifugal compressors o
perate at high speed and may have elaborate oil control systems,
with a pump, oil separator, reservoirs

(
to lubricate bearings during cast
-
down
)
, oil filter, relief
valve and oil cooler.

Helical screw compressors

Helical screw compressors n
eed oil to cool, seal, and silence the rotors; they generally have
a forced lubrication system. A positive displacement pump may operate independently of the
compressor, ensuring complete lubrication at

the compressor start up. Oil is separated,
piped to an oil sump (reservoir)
, c
ooled and delivered to the bearings and ports for injection
into the compression chamber. The oil sump (reservoir) has a heater to prevent oil dilution
by refrigerant during the

off
-
cycle.

Scroll compressors

Scroll compressors r
equire oil to cool and seal between orbiting and stationary scroll. Oil is
driven to the scrolls by centrifugal action through
a
hole in a shaft of the motor and orbiting
scroll.

Three devices are generall
y used in industrial refrigeration system
s

to control system oil:



oil separator
s



oil level regulator
s



oil reservoir
s
.


12

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License

Other elements, such as oil strainers, solenoid and isolating valves may be needed to
complete the system.
Oil should be tested regularl
y
to detect damaging acidity in the
refrigeration compressor oil.

Promoting oil return

Oil in direct expansion or dry evaporator systems must be swept back to the compressor by
the flow of refrigerant. The velocity in the evaporator tubes must be sufficien
t to carry the oil
back.

A

velocity of about 214m per minute
is

required in horizontal lines and about 457m per
minute in vertical lines.

Several additional measures will help to
ensure

proper oil return to the compressor. Slope
the refrigeration lines tow
ard the compressor. Ensure adequate refrigerant velocity in the
suction line by making it
the
proper size, not oversize. High viscosity oil (as measured in
evaporator condition) is more resistant to return by refrigerant flow. Oil that readily dissolves
in

refrigerant remains more fluid than oil without refrigerant. The amount of refrigerant
dissolved in the oil varies according to pressure and temperature conditions in various parts
of the evaporator, and the nature of the two fluids.

Oil return is more di
fficult in low
-
temperature evaporators, because oil becomes more
viscous as the temperature and pressure of the refrigerant becomes low. High compression
ratio also decreases oil return, because the suction gas is less dense. Thus adequate
suction line vel
ocity is especially important in low
-
temperature evaporators.

Oil will not be swept back to the compressor in a flooded evaporator, so an oil return line is
required. In some systems, a special chamber is connected to the evaporator to allow
refrigerant to

be boiled from the oil before the oil is returned to the compressor.

Muffler
s

A muffler is used to reduce the transmission of reciprocating compressor discharge pulsation
and noise to the piping system and to the condenser. A muffler is a cylinder with ba
ffle plates
inside. In general, mufflers which create a large pressure drop, are more effective than those
with less restriction. Both the volume and density of the gas flow
ing

through the muffler
will
affect muffler performance
.


13

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License


COMPRESSOR
E
FFICIENCY

Al
l of the mechanical energy put into a compressor should be used to move or pump gas. If
any of the compressed gas backs through into the compressor on its suction stroke, energy
is wasted.

Likewise, if energy is used to overcome friction or inertia of the

parts, the compressor cannot
be 100% efficient.

All compressors have frictional losses.

Volumetric efficiency

is the term used to measure the overall efficiency of a compressor
and is defined as the ratio of actual volume of gas pumped by the compressor (
actual) to the
volume displaced by the compressor pistons (theoretical). It is expressed as a percentage
and can be determined from the equation:

E
v

=
V
a


x 100


V
p

Where:

E
v =

T
otal volumetric efficiency (%)



V
a

=

Actual volume of
suction vapour

c
ompressed per second (L/s)



V
p

=

The

piston displacement of the

c
ompressor per second (L/s)

Worked example:

If a compressor actually

pumped 18 L/s when the displacement volume of the pistons under
the same operating conditions is 30 L/s, c
alculate the volumetric efficiency of the
compressor.

Solution:

Applying
the
equation:


E
v

=
V
a


x 100


V
p



=
1
8

kPa absolute




30


Ev = 60%

T
he efficiency of a compressor can vary over a wide range of operating co
nditions,
depending on the compressor design and

the
compression ratio.

The compression ratio of a compressor is the ratio of the absolute discharge pressure
(kPa
A
) to the absolute suction pressure (kPa
A
). It is always calculated from the absolute
pressure

scale, thus suction and discharge gauge pressures must be converted by adding

100 kPa to the gauge reading.



14

© Commonwealth of Australia 2012, Licensed under Creative Commons Attribution
-
ShareAlike 3.0 Australia License

R =
absolute discharge pressure


absolute suction pressure

Where:

R
=

compression ratio

Worked Example:

Determine the compression ratio of a

compressor that operates at a discharge pressure of
700 kPa g and a suction pressure of 100 kPa g.

Solution:

Apply equation:


R =
absolute discharge pressure


absolute suction pressure


=

800

kPa absolute



2
00 kPa absolute


= 4:1


The f
ive factors that
a
ffect volumetric efficiency
are
:



c
learance between the top dead centre of the piston and the valve plate.

This refrigerant
must be re
-
expanded before any further refri
gerant can enter the compressor



e
ffici
ency of the compressor valves


s
pring tension and valve weight and inertia will
throttle the amount of ref
rigerant through the compressor



v
apour entering a hot compressor


hot cylinders will make the entering vapour absorb
and expand the refrigerant.

This reduces the amount of suction

v
apour entering the
compressor



p
iston fit


bypass of refrigerant past the piston through the cylinder to the sump rather
than into the discharge



c
ompression ratio


as the compression ratio increases the volumetric efficiency
decreases
.