2. • A typical oil flooded twin screw compressor
consists of male and female rotors mounted
on bearings to fix their position in a rotor
housing which holds the rotors in closely
toleranced intersecting cylindrical bores
3. • The rotors basic shape is a screw thread, with
varying numbers of lobes on the male and
female rotors
• The driving device is generally connected to
the male rotor with the male driving the
female through an oil film
4. • four or five lobed male rotors generally drive
six or seven lobe female rotors to give a
female rotor speed that is somewhat less than
the male speed
5. • All common refrigeration screws use oil injection into the
compression area for lubrication, sealing of leakage paths,
and cooling. Injected oil quantities are approximately 10-
20 gal/min per 100 HP. The use of such large oil quantities
transfers most of the heat of compression to the oil and
allows discharge temperatures to be very low even at high
compression ratios. Running single stage at 20:1
compression ratio on ammonia would yield 650° F
discharge temperatures in non-flooded types of
compressors. With oil flooding, the screw discharge
temperature does not exceed 200°F. Running screws at
20:1 or even higher, single stage, though not energy
efficient compared with two stage systems, will not harm
the compressor
8. Temperature Effect
• This increase of the gas temperature can be a
problem.
• It can cause a compressor to overheat.
Overheating decreases lubricant viscosity and
can result in the breakdown of lubricant films
in bearings.
• It also causes expansion of components that
may result in damage to bearings, seals, etc
9. Compression Ratio
• The compression ratio of a compressor
tells us how much the pressure of a gas
increase as it travels from the suction to the
discharge of the compressor
Compression ratio = Discharge pressure / Suction pressure
10. Compressor Capacity
• The capacity of a compressor is the volume flow rate of gas
through the compressor
• Because gas volume changes as its pressure and
temperature change, the volume flow rate must be given
for the gas at a fixed pressure and temperature. If not, you
would get different capacity values at suction and
discharge of the same compressor
• Typically, standard flow rate measured in normal meters
cubed/hour (Nm3/h) to measure compressor capacity
• Normal means that the flow rate is calculated after
converting the gas volume to what it would be if it
was at atmospheric pressure (1.1013bara) and at a
temperature of 15oC
11. Compressor Connections
• Parallel connection increases the capacity because the gas
flowing through each compressor is added together
– give the same pressure increase as a single compressor
– give double the flow rate (capacity) of a single compressor
12. Compressor Connections
• Series connection increases the pressure because all the gas is
compressed by the first unit, then compressed again by the
second
– give a greater pressure increase than a single compressor
– give the same flow rate (capacity) as a single compressor
16. • The cylinders may be arranged in a number of
different ways
– in-line (vertical or horizontal)
– balanced-opposed
– L-type (or angle-type)
– V-type
– W-type
