CHAPTER 14

THE OIL-FLOODED ROTARY SCREW

COMPRESSOR

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Weatherford Compression

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The rotary screw compressor has attracted increased attention 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. Developments are underway by screw
compressor manufacturers to go to higher discharge pressures.
The rotary screw is a positive displacement compressor, like its better known
relative, the reciprocating compressor. In a comparison between the two, the rotary
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screw gleans honors for its simplicity, low cost, easy maintenance and almost pul-
sation-free flow. It takes a back seat to the reciprocating compressor, however, in
handling high pressure (see Fig. 14.1.)
Benefits offered by rotary screw compressors include:
• Simple maintenance
• Low maintenance costs
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• Long compressor life

• Full use of driver horsepower
• Low operating expense
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• 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
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• Compactness

14.1 TYPES OF COMPRESSORS (see Fig. 14.2)

Rotary compressors may be either positive displacement or dynamic compressors.

The positive displacement rotary utilizes either vanes, lobes or screws to literally
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14.2 CHAPTER FOURTEEN

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FIGURE 14.1 Reciprocating / rotary screw compressors.
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pack the gas into the discharge line. Dynamic compressors, on the other hand,
operate on an entirely different principle. Instead of reducing the volume of the
gas to increase its pressure, the dynamic compressor works on transfer of energy
from a rotating set of blades to a gas, and then discharges the gas into a diffuser
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where the velocity is reduced and its kinetic energy is converted to static pressure.
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FIGURE 14.2 Compressor types.

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 THE OIL-FLOODED ROTARY SCREW COMPRESSOR 14.3

Reciprocating compressors consist of a piston acting within a cylinder to phys-

ically compress the gas contained within that cylinder. They may be either single-
acting or double-acting, and can be designed to accommodate practically any pres-
sure or capacity. For this reason, the reciprocating compressor is the most common
type found in the gas industry. Each compressor is designed to handle a specific
range of volumes, pressures, and compression ratios.

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Compared to the rotary compressor, the reciprocating compressor is more com-

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plex, and may cost more to maintain. However, its higher efficiency and ability to
handle greater pressures outweigh these disadvantages.
In the selection of a compressor unit, one of the primary considerations, besides
pressure-volume characteristics, is the type of driver. Generally, small rotary com-

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pressors are driven by electric motors, while the larger rotary compressors are
usually turbine driven. Reciprocating compressors may be driven by electric mo-
tors, turbines (gas or steam) or engines (gas or diesel).
In some types of reciprocating compressors, the power cylinders and compres-
sion cylinders are integrated into one unit, and share the same frame and crankshaft.
These compressors are referred to as integral units. The power and compression
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cylinders of an integral unit may be either horizontally opposed or in a V-
configuration with the power cylinders on one bank and the compression cylinders
on the other.
Another type of reciprocating compressor is the separable unit. In this type
unit, the prime mover is separate from the compressor, thereby allowing the user
to choose the driver best suited to the application. Although this design may be
slightly more complex than that of the integral unit, its inherent flexibility often
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gives the separable unit an advantage over the integral unit.

A wide variety of compressor designs can be used on the separable unit includ-
ing horizontal, vertical, semi-radial and V-type. However, the most common design
is the horizontal, balanced-opposed compressor because of its stability and reduced
vibration.
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14.2 HELICAL ROTORS

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The rotary screw compressor consists of two intermeshing helical rotors contained
in a housing (see Fig. 14.3). Clearance between the rotor pair and between the
housing and the rotors is .003 in to .005 in. The main rotor (male rotor) is driven
through a shaft extension by an engine or electric motor. The other rotor (female
rotor) is driven by the main rotor through the oil film from the oil injection; there
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is no metal contact.
The length and diameter of the rotors determine the capacity and the discharge
pressure. The longer the rotors, the higher the 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 the housing walls, forming a compression
chamber. Lubricant is injected into the compression chamber after the grooves close

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14.4 CHAPTER FOURTEEN

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FIGURE 14.3 Typical rotary screw
compressor.

to provide sealing, cooling and lubrication. As the rotors turn to compress the
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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, carry radial loads, and provide for the small axial running clearances nec-
essary.
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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. The lubricant 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
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higher than normal wear. Rotary screw compressor operators use a synthetic hy-
drocarbon oil of ISO 100, 150 or 220 viscosity. Viscosity is selected based on the
specific gravity of the gas. The gas analysis is very important in oil selection.
During the initial start-up of a unit, gas will dilute the viscosity of the oil. It
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should not be allowed to drop below the minimum recommended value.

Since the rotary screw has a closed oil system, it should use minimal oil. Most
packagers design the package with oil carryover of five (5) parts per million. If a
rotary screw 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 or
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a coalescing filter has collapsed.

The oil filter for the oil injected into the rotary screw is a 10-micron filter. The
fine mesh is needed to protect the bearings and shaft seal. An oil change is rec-
ommended only every 8,000 operating hours, unless the oil is contaminated. 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 vari-
able volume ratio systems, which permit efficient variable load and pressure op-

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 THE OIL-FLOODED ROTARY SCREW COMPRESSOR 14.5

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FIGURE 14.4a Lift valve unloading

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mechanism.

eration. Such systems are particularly desirable when constant speed electric motors
are used and varying pressure conditions exist. (see Fig. 14.4a and 14.4b).
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14.3 ADVANTAGES OF THE ROTARY SCREW
COMPRESSOR

In many applications, the rotary screw compressor offers significant advantages

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over reciprocating compressors.

1. Its few moving parts mean the elimination of maintenance items such as com-
pressor valves, packing and piston rings, and the associated downtime for re-
placement.
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FIGURE 14.4b Slide valve unloading mechanism.

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14.6 CHAPTER FOURTEEN

2. The absence of reciprocating inertial forces allows the compressor to run at high
speeds, which results in more compact units.
3. The continuous flow of cooling lubricant permits much higher single-stage com-
pression ratios.
4. The compactness tends to reduce package costs.

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5. Rotary screw technology reduces or eliminates pulsations, resulting in reduced

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vibration.
6. Higher speeds and compression ratios help to maximize available production
horsepower.

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14.4 APPLICATIONS FOR THE ROTARY SCREW
COMPRESSOR

A rotary screw compressor package is ideal for numerous compression applications,

including:
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1. Fuel gas boosting
2. Casing head gas boosting
3. Vapor recovery
4. Landfill and digester gas compression
5. Propane/butane refrigeration compression
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6. Compression of corrosive and/or dirty process gases

A rotary screw compressor package can also be used to upgrade existing recip-
rocating compressor installations. By boosting low suction pressure, capacity may
be increased at minimum cost with continued use of existing reciprocating equip-
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ment.
If an application requires large volume/low suction pressure, but discharge pres-
sures are greater than the screw can provide, a combination screw/reciprocating
unit with a common driver can be the solution.
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14.5 VAPOR RECOVERY

One example of rotary screw utilization is for vapor recovery. Vapor recovery is
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the gathering of stock tank vapors and the compression of these vapors into the
gas sales line. Capturing these vapors is profitable and environmentally advisable.
The gas vapors are gathered into a common header and fed into a vapor recovery
unit (VRU), which usually includes a suction scrubber, a compressor, a driver, a
discharge cooler and separator, and controls for unattended operation. The vapors
are usually rich and wet, conditions which lead to condensate and the washout of

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 THE OIL-FLOODED ROTARY SCREW COMPRESSOR 14.7

lubricant. Washout causes excessive wear in vanes or piston rings. A rotary screw
compressor does not suffer from these problems for two reasons: there is enough
lubricant injected 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 im-
portant. An oversized unit will operate in the partial load stage or shut down and

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start up too often. An undersized unit will not be able to keep up and the vents

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will emit vapors into the atmosphere, defeating economy and the effort to maintain
clean air.

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14.6 SIZING A ROTARY SCREW COMPRESSOR

To size a rotary screw compressor, one needs to know the suction 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.)
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EQUATION 1
Rotary Screw
EQUATION 2
Rotary Screw
Compressor Capacity Compressor Power
ICFM ⴝ D3(L / D)(GR)(RPM)(Ev / C) K (P / P )K⫺1 / K ⫺ 1
WR ⴝ P1Q ⴛ ⴛ 2 1 ⴙ WL
Rotor Diameter D K⫺1 Ea
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Rotor Length L Pressures P1, P2

Gear Ratio GR Gas Flow Rates Q
Driver Speed RPM Gas Properties K
Volumetric Efficiency Ev Adiabatic Efficiencies Ea
Rotor Profile C Mechanical Losses WL
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Figures 14.5A and 14.5B show adiabatic efficiency at different pressure ratios,
while Fig. 14.6 shows efficiency change with variable volume ratio.
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14.8 CHAPTER FOURTEEN

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FIGURE 14.5a Adiabatic efficiency (pressure ratio to
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FIGURE 14.5b Adiabatic efficiency (pressure ratio to 15).

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 THE OIL-FLOODED ROTARY SCREW COMPRESSOR 14.9

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FIGURE 14.6 Efficiency improvement
with variable volume ratio.
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