Table of Contents
- 1 - What are compressors?
- 2 - What are compressors used for in a refrigerated van?
- 3 - Types of compressors
- 3.1 - Positive displacement
- 3.2 - Reciprocating compressors
- 3.3 - Rotary screw compressors
- 3.4 - Ionic liquid piston compressor
- 3.5 - Rotary screw compressor
- 3.6 - Scroll compressor
- 3.7 - Rotary vane compressors
- 3.8 - Centrifugal compressors
- 3.9 - Axial flow compressors
- 3.10 - Applications of gas compressors
What are compressors?
Compressors are used in many industries because of their wide range of application. Similar to pumps, the evaporator supplies gaseous refrigerant at low pressure and increases to a higher pressure. The pressure and temperature of the vapour increase after compression.
Isentropic conditions provide the ideal environment for compression where the process is both adiabatic and reversible. The isentropic efficiency of a compressor is the ratio of work done by the isentropic compressor to the actual compressor.
What are compressors used for in a refrigerated van?
Compressors work as part of the cooling system in a refrigerated van. In a new refrigerated van, the high-density polymer foam is used to insulate the van. Combined with a seal around the door, insulation leads to an atmosphere that prevents heat from escaping the van.
Three mechanisms that work to remove heat that comes in when the door is open or heat that remains inside include the compressor, condenser and an evaporator.
The compressor is mounted to the fridge van’s engine using a compressor mounting bracket. It is driven by a small engine within the reefer unit. The compressor takes in gaseous refrigerant and compresses it. Refrigerant enters as a low-pressure gas and the compressor changes it to high-pressure gas.
Types of compressors
This compressor is a system that compresses air by displacing a mechanical linkage reducing the volume.
Also known as piston compressors, reciprocating compressors are divided into open, hermetic and semi-hermetic. They are pistons driven by a crankshaft and can either be portable or stationary, multi-staged or single. They are driven by internal combustion engines or electric motors.
Where one end of the crankshaft protrudes outside the pump and can be opened, the reciprocating compressor is called open. If the compressor and the motor are in the same casing, it is semi-hermetic. This means it can be opened for servicing and inspection.
Rotary screw compressors
They are used to force gas into small spaces. They are low capacity, low cost and compact dimension equipment used in freezers and home refrigerators.
The introduction of twin compressors and two rotors reduces noise that leads to lower vibration levels. When compared to the single rotary technology, they are more efficient over a wide operating range.
You can achieve a high efficiency of rotary compressor technology by fitting it with brushless DC motors.
Ionic liquid piston compressor
Also known as an ionic liquid piston pump or ionic compressor, it is a hydrogen compressor based on an ionic liquid piston.
An ionic compressor uses two properties of ionic liquids. These are the large temperature window and non-measurable vapour pressures for the liquid phase. They are combined with low soluble gases such as hydrogen.
Service lifespan is 10x longer than a regular reciprocating compressor. During use, it requires fewer energy costs (cut down by up to 20%) and reduced maintenance.
The heat exchangers from a regular piston compressor are not needed because the heat is removed in the same cylinder it is generated from. There is minimum energy waste as it uses almost 100% of all its energy in the process.
Rotary screw compressor
This is the most commonly used industrial compressor model. It provides continuous compressed air for precision tasks. When in use, it is silent and efficient. It uses a rotary type positive displacement mechanism similar to an air compressor.
It is mostly used in industries that have continuous air demand such as automated manufacturing systems and food packaging plants.
It is operated with rotors to compress gas. Timing gears in a dry running rotary screw compressor ensure that the male and female rotors are precisely aligned at all times.
For the mechanism to run efficiently, clearances must be precisely fitted between the helical rotors, the chamber for sealing the compression cavities and the rotors.
Also known as a scroll vacuum pump or scroll pump, it uses two scrolls to operate. One is fixed while the other moves. To be in contact, they are interleaved to form a series of thinner pockets of air near the centre. The interleaved vanes compressor pumps liquid and gas.
Scroll compressors are mostly used in commercial and home air-conditioning systems, air conditioning systems for data centres, telephone applications, and heat pumps.
They are not used in applications with low cooling capacity where hermetic and rotary reciprocation compressors are more efficient. In the early 90s, scroll compressors were used as the supercharger on Volkswagen G40 and G60 engines.
Rotary vane compressors
Rotary vane compressors have two openings of suction and a discharge, a cylindrical casing and one eccentrically positioned rotor. The blades slide in and out of the slots as the rotor turns to keep in contact with the outer wall of the housing.
Compression occurs when refrigerant flows to the chamber where there is a reduction in the desired volume because of eccentric rotation. The benefits of rotary vane compressors include its compact dimensions and low cost.
These compressors revolve at high speed. They use an impeller in a shaped housing. To increase the velocity of gas, they force gas to the rim of the impeller. They are used with refrigerant possessing high cooling capacities and low compression ratios.
Axial flow compressors
These are dynamic rotating compressors that use several airfoils to compress fluid. The array of airfoils is set in pairs where one rotates and one is stationary. Fluid is accelerated by the rotors or blades.
Axial compressors have a high efficiency of 90% polytrophic but are expensive because of their high-quality materials, tight tolerances and numerous components.
Applications of gas compressors
Air conditioner and refrigeration equipment use compressors to transfer heat in refrigerant cycles. These compressors are described as open, hermetic or semi-hermetic.
The difference is that hermetic uses a one-piece welded steel casing. This means you cannot open it for repair; you can only replace with a new unit. A semi-hermetic can be opened and the pump and motor components can be replaced.