The Flooded Rotary Srew Compressor

7/30/2019 The Flooded Rotary Srew Compressor

1/35

THE OIL-FLOODED ROTARY SCREW COMPRESSOR

1

HCM CITY UNIVERSITY OF TECHNOLOGY

REFRIGERATION THERMAL TECHNOLOGY

INDUSTRIAL REFRIGERATION

Lecturer: Dor. BI NGC HNG

Reporter: TRN XUN AN

CHAPTER: 14

THE OIL-FLOODED ROTARY SCREW

COMPRESSOR


2/35


CONTENTS GENERAL

TYPES OF COMPRESSORS

HELICAL ROTORS

ADVANTAGES OF THE ROTARY SCREW COMPRESSOR

APPLICATIONS FOR THE ROTARY SCREW COMPRESSOR

VAPOR RECOVERY

SIZING A ROTARY SCREW COMPRESSOR

2


3/35


General

The rotary screw compressor has attracted increasedattention in the gas industry during the last decade as an ideal

compressor for low pressure/high capacity op-erations, with

low pressure defined as suction pressure at or near zero psig

and discharge pressure at less than 400 psig.The rotary screw is a positive displacement compressor.

The rotary screw gleans honors for its simplicity, low

cost, easy maintenance and almost pulsation-free flow (dng

chy xung ng)

3


4/35


Benefits offered by rotary screw compressors include:

Simple maintenance Low maintenance costs

Long compressor life

Full use of driver horsepower

Low operating expense

Low purchase price

High compression ratios (Rc) up to 16 Rc per stage

Operation at low suction pressure up to 26 inches of vacuum Light weight

Compactness

4


5/35


5


6/35


6


7/35


14.1 TYPES OF COMPRESSORS

Rotary compressors may be either positive displacement ordynamic compressors

The positive displacement rotary utilizes either vanes, lobes or

screws to literally pack the gas into the discharge line.

The dynamic compressor works on transfer of energy from arotating set of blades to a gas, and then discharges the gas into

a diffuser(ngkhuych tn) where the velocity is reduced andits kinetic energy is converted to static pressure.

7


8/35


8


9/35


9


10/35


Reciprocating compressors consist of a piston acting within a

cylinder to physically compress the gas contained within thatcylinder.

They may be either single acting or double-acting, and can be

designed to accommodate practically any pressure or capacity.

=> For this reason, the reciprocating compressor is the mostcommon type found in the gas industry.

Each compressor is designed to handle a specific range ofvolumes, pressures, and compression ratios.

10


11/35


Compared to the rotary compressor, the reciprocating compressor is

more complex, and may cost more to maintain. However, its higherefficiency and ability to handle greater pressures outweigh thesedisadvantages.

Generally, small rotary compressors are driven by electric motors,

while the larger rotary compressors are usually turbine driven.

Reciprocating compressors may be driven by electric motors,turbines (gas or steam) or engines (gas or diesel).

In some types of reciprocating compressors, the power cylinders and

compression cylinders are integrated (tch hp) into one unit, andshare the same frame and crankshaft. These compressors are

referred to as integral units. The power and compression cylinders

of an integral unit may be either horizontally opposed or in a V-

configuration with the power cylinders on one bank and thecompression cylinders on the other.

11


12/35


Another type of reciprocating compressor is the separable unit.

In this type unit, the prime mover is separate from thecompressor, thereby allowing the user to choose the driver best

suited to the application. Although this design may beslightly

more complex than that of the integral unit, its inherent

flexibility often gives the separable unit an advantage over theintegral unit.

A wide variety of compressor designs can be used on the

separable unit including horizontal, vertical, semi-radial and V-

type. However, the most common design is the horizontal,

balanced-opposed compressor because of its stability andreduced vibration.

12


13/35


HELICAL ROTORS

13


14/35


14


15/35


The rotary screw compressor consists of two intermeshinghelical rotors contained in a housing. Clearance between therotor pair and between the housing and the rotors is .003 in to.005 in.

The main rotor (male rotor) is driven through a shaft extensionby an engine or electric motor. The other rotor (female rotor) is

driven by the main rotor through the oil film from the oilinjection; there is no metal contact. The length and diameter of the rotors determine the capacity

and the discharge pressure. The longer the rotors, the higherthe pressure; the larger the diameter of the rotors, the greater

the capacity. The helical rotor grooves are filled with gas as they pass the

suction port. As the rotors turn, the grooves are closed by thehousing walls, forming a compression chamber.

15


16/35


16

ROTOR SCREW AIR COMPRESSOR PRINCIPLE


17/35


17



18/35


18



19/35


Lubricant is injected into the compression chamber after the

grooves close to provide sealing , cooling and lubrication . Asthe rotors turn to compress the lubricant /gas mixture, the

compression chamber volume decreases, compressing the gas/

lubricant toward the discharge port.

The gas/ lubricant mixture exits from the compressor as the

compression chamber passes the discharge port. Each rotor is

supported by anti-friction bearings held in end plates near the

ends of the rotor shaft. The bearings at one end, usually the

discharge, fix the rotor against axial thrust (lcdctrc) ,

carry radial (ly tm) loads, and provide for the small axialrunning clearances necessary.

19


20/35


20

TYPICAL LUBRICATION CIRCUIT OF A SCREW COMPRESSOR


21/35


After compression, the gas/ lubricant mixture enters a multi-

stage separator which removes the lubricant from the gas.From the separator, the gas flows to the aftercooler Thelubricant is also cooled, returning to the compressor through a

thermostatically-controlled valve.

Oil is the lifeblood of a rotary screw compressor. The limited

clearance inside the rotary screw means that without proper

lubrication, the screw may experience higher than normal wear

(hao mn). Rotary screw compressor operators use a synthetic

(tnghp) hy-drocarbon oil of ISO 100, 150 or 220 viscosity

(nht). Viscosity is selected based on the specific gravity ofthe gas. The gas analysis is very important in oil selection.

21


22/35


During the initial start-up (khing ban u) of a unit, gas

will dilute (pha long, gim) the viscosity of the oil. It shouldnot be allowed to drop below the minimum recommendedvalue.

Since the rotary screw has a closed oil system, it should use

minimal oil. Most packagers design the package with oil

carryover(cun theo) offive (5) parts per million. If a rotaryscrew compressor loses oil and no leak can be found, then the

oil is going down the sell line. These conditions mean a

scavenging line orifice is plugged (ng lm schdubtc)

or a coalescing filterhas collapsed (bindng).

22


23/35


The oil filterfor the oil injected into the rotary screw is a 10-

micron filter. The fine mesh (lnh) is needed to protect thebearings and shaft seal. An oil change is recommended onlyevery 8,000 operating hours, unless the oil is contaminated

(nhimbn).

A regular oil sampling will help the operator determine when

an oil change is needed.

Many rotary screw models are available with internal capacity

control and variable volume ratio systems, which permit

efficient variable load and pressure operation.

Such systems are particularly desirable when constant speed

electric motors are used and varying pressure conditions exist.

23


24/35


24


25/35


25CAPACITY CONTROL SLIDE VALVESCREW COMPRESSOR


26/35


26


27/35


ADVANTAGES OF THE ROTARY SCREW COMPRESSOR

In many applications, the rotary screw compressor offers significantadvantages over reciprocating compressors:

1. Its few moving parts mean the elimination of maintenanceitems such as compressor valves, packing and piston rings, andthe associated downtime for replacement.

2. The absence of reciprocating inertial forces (lc qun tnh)allows the compressor to run at high speeds, which results inmore compact units.

3. The continuous flow of cooling lubricant permits much highersingle-stage compression ratios.

4. The compactness (chcgn) tends to reduce package costs. 5. Rotary screw technology reduces or eliminates pulsations (sxung ng, va p), resulting in reduced vibration. 6. Higher speeds and compression ratios help to maximize

available production horsepower.

27


28/35


APPLICATIONS FOR THE ROTARY SCREW COMPRESSOR

A rotary screw compressor package is ideal for numerouscompression applications including:

1. Fuel gas boosting

2. Casing head gas boosting

3. Vapor recovery 4. Landfill and digester gas compression

5. Propane/butane refrigeration compression

6. Compression of corrosive (n mn) and/or dirty processgases

28


29/35


A rotary screw compressor package can also be used to

upgrade existing reciprocating compressor installations. Byboosting low suction pressure, capacity may be increased at

minimum cost with continued use of existing reciprocating

equipment.

If an application requires large volume/ low suction pressure,

but discharge pressures are greater than the screw can provide,

a combination screw/ reciprocating unit with a common driver

can be the solution

29


30/35


VAPOR RECOVERY

One example of rotary screw utilization (ng dng) is forvapor recovery. Vapor recovery is the gathering of stock tankvapors and the compression of these vapors into the gas sales

line. Capturing (thu hi) these vapors is profitable andenvironmentally advisable.

The gas vapors are gathered into a common header (hp phnphi) and fed into a vapor recovery unit (VRU), which usuallyincludes a suction scrubber (b lc kh), a compressor, adriver, a discharge cooler and separator, and controls for

unattended (ko gim st) operation.

The vapors are usually rich and wet, conditions which lead to

condensate and the washout (ra tri) of lubricant. Washoutcauses excessive wear (lm mn) in vanes or piston rings

30


31/35


A rotary screw compressor does not suffer (gpphi) from

these problems for two reasons: there is enough lubricantinjected to take care of washout; and the rotary screw does not

require the rotors to make contact with the stator.

The sizing of a rotary screw compressor for vapor recovery is

extremely(cck) important. An oversized unit will operate inthe partial load stage or shut down and start up too often . An

undersized unit will not be able to keep up and the vents (lthot) will emit (pht) vapors into the atmosphere, defeating

economy and the effort to maintain clean air.

31


32/35


SIZING (X/ K/T) A ROTARY SCREW COMPRESSOR

To size a rotary screw compressor, one needs to know thesuction and discharge pressures, the desired capacity, the gas

analysis, temperature and the elevation. (See Eq. 1 for formula

to determine capacity and Eq. 2 for formula to determine horse

power.)

32


33/35


Figures 14.5A and 14.5B show adiabatic (on nhit)

efficiency at different pressure ratios,hile Fig. 14.6 showsefficiency change with variable volume ratio.

33


34/35


34


35/35


THANK MUCH FOR YOUR ATTENTION

Q & A

35

The Flooded Rotary Srew Compressor

Documents

Transcript of The Flooded Rotary Srew Compressor