Compressors, Blowers, Vacuum Pumps and Their Performance Characteristics

Blowers and compressors these are actually air handling devices or machinery which one will come across in many plants and industries. So the working fluid in blowers and compressors is air or gas see this fluid handling devices are very similar in they are out say principle or in philosophy in comparison with electrical machines

Compressor:

A compressor is a machine which reduces the volume of gas or liquid by creating a high pressure. We can also say that a compressor simply compresses a substance which is usually gas. A compressor is the most important and often the costliest component (typically 30 to 40 percent of total cost) of any vapour compression refrigeration system (VCRS). The function of a compressor in a VCRS is to continuously draw the refrigerant vapour from the evaporator, so that a low pressure and low temperature can be maintained in the evaporator at which the refrigerant can boil extracting heat from the refrigerated space. The compressor then has to raise the pressure of the refrigerant to a level at which it can condense by rejecting heat to the cooling medium in the condenser.

Blowers:

Blower is a machine to move air at a moderate pressure. Especially industrial air blowers. The basic difference between the above three devices is the way they move or transmit air/gas and induce system pressure. Compressors, Fans & Blowers are defined by ASME (American Society of Mechanical Engineers) as the ratio of the discharge pressure over the suction pressure. Fans have the specific ratio up to 1.11, blowers from 1.11 to 1.20 and compressors have more than 1.20.

Blowers can achieve much higher pressures than fans, as high as 1.20 kg/cm2 . They are also used to produce negative pressures for industrial vacuum systems. Major types are: air blower and positive-displacement blower. Centrifugal blowers look more like centrifugal pumps than fans. The impeller is typically gear-driven and rotates as fast as 15,000 rpm. In multi-stage blowers, air is accelerated as it passes through each impeller. In single-stage blower, air does not take many turns, and hence it is more efficient. Centrifugal blowers typically operate against pressures of 0.35 to 0.70 kg/cm2 , but can achieve higher pressures. One characteristic is that airflow tends to drop drastically as system pressure increases, which can be a disadvantage in material conveying systems that depend on a steady air volume. Because of this, they are most often used in applications that are not prone to clogging. Positive-displacement blowers have rotors, which “trap” air and push it through housing. Positive-displacement blowers provide a constant volume of air even if the system pressure varies. They are especially suitable for applications prone to clogging, since they can produce enough pressure – typically up to 1.25kg/cm2 – to blow clogged materials free. They turn much slower than centrifugal blowers (e.g. 3,600 rpm), and are often belt driven to facilitate speed .

Vacuum pumps

A compressor that takes suction at a pressure below atmospheric and discharges against atmospheric pressure is called a vacuum pump. Any type of blower or compressor, reciprocating, rotary or centrifugal can be adapted to vacuum practice by modifying the design to accept very low density gas at the suction and attain the large compression ratios necessary As the absolute pressure at suction decreases, the volumetric efficiency drops and approaches zero at the lowest absolute pressure attainable by pump. Usually the mechanical efficiency is also lower than for compressors. The required displacement increases rapidly as the suction pressure falls, so large machine is needed to move much gas. The compression ratio used in vacuum pumps is much higher thane. in compressors, ranging up to 100 or more, with a correspondingly high adiabatic discharge temperature. Actually however , the compression is nearly isothermal because of the low mass flow rate and the effective heat transfer from the relatively large area of exposed metal.

Characteristic curve of compressors

Performance of reciprocating compressors For a given evaporator and condenser pressures, the important performance parameters of a refrigerant compressor are: a) The mass flow rate (m) of the compressor for a given displacement rate b) Power consumption of the compressor (Wc) c) Temperature of the refrigerant at compressor exit, Td, and d) Performance under part load conditions.

Centrifugal Compressor Performance characteristic curve is the curve between Polytropic Head and Inlet Volume Flow. In a characteristic curve the terms like Compressor Surge line, Anti surge line, Choke or Stone wall point are the important phenomenon to be understood. When the inlet flow to the centrifugal compressor decreases beyond a limit an unwanted and unstable cyclic phenomenon occurs called Surge. When the compressor loses its ability to develop the pressure head due to low inlet flow, the reversal of gas flow from discharge to suction takes place. Again the compressor tries to develop the head and this cycle is repeated till the desired head is achieved. This cycle develops roaring noise and causes severe damage to compressor and even foundations. Centrifugal compressor surge line is the limit for the inlet flow below which it should not be operated.

The reasons for compressor surge might be plant trips, Power failure or Utility failure, Compressor Suction Heat exchanger leaks, Suction filter choke, Intercooler leaks, Compressor Discharge Valve closure, Molecular Weight changes due to process upsets etc.

Centrifugal compressor characteristic curve

The most common way of creating a vacuum is to pump the gas out of a vessel that is initially at atmospheric pressure. … The backing pump is used to extract residual gases from the main pump to keep it at low enough pressure to operate. The pressure of the backing pump is called the backing pressure.

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