An
expansion engine is an essential equipment used in gas plants. It is a
well-known fact that gases can be cooled by expansion from high pressure to
low pressure and this is accomplished by using an expansion engine. A
vertical, single acting type of engine, an expansion engine produces the
cold required for the gas plant.
Working of an Expansion Engine
At the start of downward stroke of the piston, high-pressure air enters
through inlet valve. On further moving the piston down, the inlet valve
closes and entrapped air expands. During upward stroke, inlet valve remains
closed while the outlet valve remains open. Therefore, in downward stroke
air enters the cylinder. This in turn leads to the expansion of the air. In
the upward stroke the expanded air is pushed outside. An electric motor is
required to start the machine. After that the engine is moved by the air
pressure itself and during this process, the motor retains the speed by
acting as a brake. Since work is done by air in moving the flywheel, it
loses its heat content and thus the air gets cooled.
Components of an Expansion Engine
The expansion engine consists of three major units:
- The cylinder unit used for air expansion: The cylinder unit is used
for processing the air and it consists of cylinder, inlet and outlet
isolation valve assemblies and a piston. The ball valves are activated
using a push rod in accordance with the timing transmitted by the cam.
The push rods are fixed in a stuffing box to avoid air leakage and are
activated by the hydraulic system.
- The drive unit: The drive unit is similar to any reciprocating
machine with usual parts, such as, crank case, crankshaft, connecting
rod, crosshead, etc. The expansion engine has an extended crankshaft
enabling the accommodation of the cam box. The moving parts are
lubricated through a hole in crankshaft. Oil scrapper rings are provided
to prevent oil escape to cylinder unit.
- The hydraulic system used for operating the valves: The hydraulic
system is the control system of the expansion engine. The hydraulic oil
in the engine is fed by a pump to:
- Lubricate the rolled holders.
- For valve actuation, pressurize the oil container.
- Lubricate crankshaft and drive unit parts.
Types of Expansion Engines
- Reciprocating type: The reciprocating type expansion engine
with liquefied gas is run by pistons. There is a heat source which
vaporizes the liquid which then expands over the piston and produce the
desired work. Also known as a piston engine, a reciprocating engine is a
heat engine that uses one or more pistons to convert pressure into a
rotating motion.
- Turbine type: Using a rotatable shaft attached to the turbine
blades, the turbine type of expansion engine is used to convert pressure
drop in the inlet gas flowing through the turbine vanes into mechanical
energy.
A typical expansion engine used in oxygen gas plants.
Features of an Expansion Engine
Though different manufacturers may come up with different features to
improve the efficiency of expansion engine, some of the common properties
are:
- Low flow rate
- Constant efficiency is maintained over a wide range of operating
conditions.
- High expansion ratio
- Resistance to contamination
- Less sensitive to power fluctuation
- Control of speed/flow rate is easier
- Oil-free expansion engine
- Explosion-proof design, for natural gas and other combustible gases.
- Modular design available
- Depending on expansion pressure ratio, single- and multi-stage
design.
- Optimum output efficiency
- Robust and safe design
- Reduced wear due to low piston speeds
Buying Tips
Characteristic values for expansion engine to be considered are as follows:
- Gas volume flow rates
- Intake pressure
- Back pressure
- Output
- Nominal speeds
- Variable speeds
Expansion
Engine
