GB2411695A - Multi-stage compressor with intercooler - Google Patents

Abstract

A multi-stage compressor arrangement comprises a rotodynamic compressor stage 10 in series with, and upstream of, a water injected screw compressor stage 14. A water spray intercooler 12 is located between the two stages to reduce the temperature of gas entering the screw stage 14. The intercooler 12 and the water injected screw stage 14 share a common water supply. The rotodynamic compressor stage 10 is electrically driven, variable speed and may be an axial flow or a centrifugal flow stage. The screw compressor stage 14 may be fixed or variable speed. A separator 16 may be used to separate water from the discharged gas, and the water supply for the screw compressor stage 14 and the intercooler 12 may be drawn from the separator 16 via a conditioning unit 18.

Description

MULTI-STAGE NO-OIL GAS COMPRESSOR

Field of the invention

The present invention relates to a multi-stage no-oil gas compressor with an intercooling arrangement.

Background to the Invention

0 There are many techniques available to compress gases and each has its merits in terms of the total pressure rise, the volume flow rate that can be achieved and the efficiency at which the process can operate.

A rotodynamic compressor, which term includes axial flow compressors and centrifugal compressors, achieves gas compression by using a high speed rotor to increase the momentum of the gas, the momentum being converted to a pressure head. This type of machine is ideal when a high volumetric flow is required with relatively low inlet pressure. To increase the outlet pressure, higher rotational speeds are required which can become difficult to achieve whilst maintaining an acceptable efficiency relative to other approaches. These machines can generally be considered oil free machines as the working elements are not in contact with oil lubricants. This is important for processes that require pure air, such as chemical and food related industries where any oil contamination can affect the main process.

Another known form of compressor is the positive displacement screw compressor which compresses a volume of gas by driving it through a continually reducing volume between two contra rotating screw profiles. The profile of the screw elements determines the fixed "internal pressure ratio" of the compression stage. However, in this device there is also an element of "external compression" of the l - 2 - gas generated around the discharge port which enables the machine to significantly increase the compression ratio.

To help seal the system, lubricate the contacting surfaces and take away heat from the compression process, the screw mesh may be injected with oil or water, this liquid eventually being recovered later in the process. To effectively remove any oil from the air requires significant investment and maintenance of filtering systems. The major lo benefit of water injected screw compressors is that water is not generally considered as a contaminant, is easily removed and environmentally friendly. The water injection enables near isothermal compression of the gas to take place, resulting in a highly efficient compression stage. To this end, most water injected compressors have a closed loop water circuit, including reverse osmosis and filtration to condition the water, to continuously lubricate and cool the compression elements and the compressed gas flow simultaneously.

A further variant of screw compressors is one where no liquid is injected into the mesh and this is commonly referred to as a "dry screw" compressor. In this type of compressor, the additional external compression includes both the isentropic temperature increase inherent in any adiabatic compression process and the non-isentropic temperature increase resulting from the pulsating flows at the discharge port. In this type of machine, the lack of cooling, in the form of oil or water lubricant, can result in mechanically critical temperature rises and possible seizure of the rotating elements.

The combination of two fundamentally different compressor systems to optimise the performance of the coupled arrangement has been understood for some years, as described in patent GB 2 034 818, although the technology at that time was orientated around large centrifugal compressors, generating up to 50,000 m3/h of compressed gas.

Conventional intercooler and aftercooler arrangements were used to reduce the temperature of the lower pressure gas before it entered the subsequent stage or exited the system.

These typically take the form of shell and tube heat exchangers with an external supply of water passing through the tube sections and taking away the heat of compression.

The conventional heat exchangers exhibit a pressure drop in the system unless they are substantially oversized and then lo are prohibitively expensive. They therefore present a source of poor efficiency and are a substantial component of the overall cost of the machine.

In both cases, to achieve higher pressures, more compression stages can be introduced and two stages are typical. In the case of screw compressors, because of the nature and complexity of the screw profile, the size of the compressor is limited by the machining accuracy achievable that ensures low leakage and high compression efficiencies.

This makes screw compressors less suitable for high volume flows. Conversely, for centrifugal machines, large volume flows and hence sizes are preferred. Difficulties occur at small volumes where smaller machines are required that run at increasingly higher rotational velocities to ensure adequate momentum exchange. This presents balancing and bearing related problems as balancing accuracies need to be increased and speeds can be in excess of conventional bearing capabilities.

Summary of the invention

According to the present invention, there is provided A multi-stage compressor comprising a variable speed electrically driven rotodynamic compressor stage connected in series with and upstream of a water injected screw compressor stage and an intercooler arranged between the two compressors to reduce the temperature of gas entering the - 4 - screw compressor stage, characterized in that the intercooler is a water spray intercooler, and the water spray intercooler and the water injected screw compressor stage share a common water supply.

The invention combines the benefits of each type of no oil compressor configuration into one unit, exploiting the potential compactness of a high speed electrically driven rotodynamic compressor first stage with a screw compressor lo element second stage, as has been previously disclosed. The high speed rotodynamic stage can provide a high volumetric flow in a relatively compact arrangement, thus making it suitable for the inlet stage, whereas the screw stage has the ability to accommodate variable inlet conditions with it remaining able to achieve constant pressure delivery. In a comparable two stage screw machine, the inlet stage is large and bulky and its substitution with a high speed rotodynamic unit offers cost as well as performance benefits.

This invention proposes to utilise water to cool the gas directly via a spray cooler system. In this instance, water is injected via spray nozzles into the gas stream exiting the first centrifugal stage compressor. By using this approach, high thermal exchange rates can be achieved with negligible pressure drop, as the gas is in intimate contact with the water. The absence of plates and tubes in the intercooling stage eliminates any thermal resistance from this part of the process. It also reduces cost, complexity and size.

The main significance of this arrangement is that the subsequent screw stage can accept a gas/water mixture, which can result in a further compression efficiency improvement.

By adopting this approach, half the water is effectively injected into the screw at inlet and the remainder at some intermediate point in the compression process within the - 5 - screw. This has the effect of introducing the liquid when it is needed rather than all at one position.

By careful regulation of the water flow rates and droplet size of the spray, the heat of compression is accepted by the water.

In a preferred embodiment, the invention proposes the use of a variable speed high speed centrifugal stage in lo combination with a conventional fixed speed water injected screw compression stage (although a variable speed second stage may be accepted).

Water injected screw compressors require a water is filtration and purification process to ensure deposits and fines do not build up in the machine. The closed loop water levels may be added to or detracted from by varying humidity levels in the compressed gas. Therefore most conventional compressors of this type have a make up and drain off system that continuously conditions the water present in the machine. The spray intercooling stage is therefore compatible with this requirement and the system can be readily extended to accommodate this additional feature.

The water screw stage, because of the water injection process, does not require an aftercooler. The gas exiting the screw stage is nominally at 50 C and a simple centrifugal separator and refrigerant drier ensures that all the water can be recovered and the delivered gas is free of contaminants and particularly oil free.

Brief description of the drawing

The invention will now be described further, by way of example, with reference to the accompanying drawing which shows schematically a two stage compressor of the present invention. - 6

Detailed description of the preferred embodiment

In the single figure, the conduits along which gas flows are represented by double lines whereas the pipes, numbered 24, that carry water are shown as single lines.

Gas to be compressed enters at 22 into an electrically driven centrifugal compressor 10. Once compressed by the centrifugal compressor 10, the hot gas flows through a water spray intercooler 12.

The water spray intercooler 12 is effectively a canister which by virtue of its increased cross sectional area slows down the gas. At the same time, water is injected at high pressure into the canister through nozzles or jets 20. These cause the water to atomise into a fine mist which cools down the now slower travailing gas giving the water more time to absorb heat from the gas. The water flow rates and droplet size of the spray in the intercooler are regulated in order to ensure the heat of compression of the gas is accepted by the cooling water. This method of cooling avoids the use of heat exchangers which are bulky, expensive, introduce a pressure drop and are susceptible to damage by virtue of their use of thin metal fins and propensity to blockages due to the accumulation of scales and other chemical deposits.

In the next stage of the process, the cooler high pressure gas enters the inlet port of a water injected screw compressor 14 where it is further compressed. Screw type compressors such as this, consist of two counter rotating intermeshing screws. As they turn gas trapped between them is forced down the length of the screws, the further along the screw the gas is pushed, the greater the compression. In water injected screw type compressors, water is injected under pressure at some intermediate point along the screw in - 7 order to cool the gas as it is heated by the compression process.

In many applications which require the use of such a compressor, it is important for the water to be free of impurities such as minerals and any particles which may promote growth of bacteria.

In the illustrated embodiment of the invention, the lo water which interacts directly with the gas stream remains within a closed loop defined by the water pipes 24 and air lines pre and post screw compressor 14 and separator 16. In particular, upon being discharged from the screw compressor 14, the gas passes through the separator 16 in order to separate the majority of water from the gas for recycling.

The separated water is cooled by a heat exchanger 26, to extract from it the heat absorbed from the gas during the intercooling and compression processes and it is passed through a conditioning unit 18 before it is recycled to the intercooler 12 and the screw compressor 14. Cooling of the water within this closed loop may be achieved by conventional external water or coolant circulation systems which may include blast coolers, cooling towers or even river water passing through the heat exchanger 26.

The illustrated system provides water from the same water supply 18 both to the intercooler spray nozzles 20 and the working elements of the screw compressor 14. The pressure used to drive the water out of the water spray intercooler nozzles 20 and into the screw compressor is provided by the pressure of the gas at the outlet of the screw compressor 14. - 8 -

Claims (8)

1. A multi-stage compressor comprising a variable speed electrically driven rotodynamic compressor stage (10) connected in series with and upstream of a water injected screw compressor stage (14) and an intercooler (12) arranged between the two compressors to reduce the temperature of gas (22) entering the screw compressor stage (14), characterized in that lo the intercooler is a water spray intercooler (12), and the water spray intercooler (12) and the water injected screw compressor stage (14) share a common water supply (18).

2. A multi-stage compressor as claimed in claim 1, wherein the first stage is a high speed centrifugal stage and the second stage is a fixed speed water injected screw compressor.

3. A multi-stage compressor as claimed in claim 1 or 2, wherein the first stage is a high speed centrifugal stage and the second stage is a variable speed water injected screw compressor.

4. A multi-stage compressor as claimed in any preceding claim, wherein the discharged gas and water from the intercooler (12) flow directly into the screw stage (14).

5. A multi-stage compressor as claimed in any preceding claim, further comprising a separator (16) to remove substantially all the water from the gas discharged from the multi-stage compressor stage.

6. A multi-stage compressor as claimed in claim 5, wherein the water supply draws water from the separator (16) and includes a conditioning unit (18). - 9 -

7. A multi-stage compressor as claimed in any preceding claim wherein the water flow rates and droplet size of the spray in the intercooler are regulated in order to ensure the heat of compression of the gas is accepted by the cooling water.

8. A multi-stage compressor substantially as herein described with reference to and as illustrated in the accompanying drawing.