mechanical engineering — Oil Injected Air Compressors

Order Description

General background
The separation of liquid and gaseous fluids is not a trivial problem and occurs in many manufacturing processes. Droplet size is critical to the separation efficiency with the smaller droplets presenting the most difficulty in separation. Typical examples of separation to provide dry gasses as in the production of “dry” steam for manufacturing processes and for the cleaning of crankcase “blow –by” gasses in internal combustion engines. The latter is now becoming more critical as Exhaust Gas Recirculation (EGR )into the inlet manifold for low NOx emissions together with closed crankcase ventilation places this oil mist in contact with hot gasses resulting in significant intake deposits.
This project will concentrate on one aspect of this issue by considering an oil / water and air separator for use in a modern air compressor, where oil is used to undertake a number of functions as well as to protect the moving components in the compressor.
This technical specification is for the air oil and water separation unit required for modern high compression oil injected compressors which can be either sliding vane or screw type. (Look up Mattei, Hydravane, VMAC and Rotocomp websites for examples amongst many manufacturers)
For this project the compressors can be considered to be oil flooded compressors of differing types and manufacture but both basically provide an output of mixed oil and air. The oil is used to perform three major functions :-
To Lubricate the moving parts and provide a hydrodynamic oil film support to prevent metal to metal contact of the moving parts be they sliding vane, or driven screw.
This oil film also provides a seal to reduce back leakage and increase the effective compression ratio
The flow of oil also acts as a thermal mass thus reducing compressor outlet temperatures. (from isentropic compression equations estimate the uncooled temperature for a 12 bar compressor)
This oil, having completed the three primary tasks, has to be separated from the air mass to provide better than BS ISO 8573-1 class 3 air whilst allowing the separated oil to be cooled and re-circulated back to the compressor inlet. ( the oil cooler is NOT part of this project task)
The separation system will contain a spin on oil filter capable of the full oil flow with a pressure drop of not more than 2 bar, as well as bosses fixtures and fittings to allow oil aerosol inlet, separated oil outlet(s), clean air outlet and water condensate drain, plus tapings for instrumentation and safety valves, with a method of ease of oil drain & filling. Full details to be agreed during the design reviews of the development project as presented at the various gate reviews. ( dates as provided in blackboard and in the main presentation )
Failure modes for the design must be considered and a full DFMEA analysis must be undertaken and presented as part of this project.
All pressure tank design directives must be adhered to and the system must be fully CE marked ref BS ISO 4126 & BS ISO 13445 including Pressure vessels directives 97/23/EC & 87/404/EEC.
Compressed air is not capable of holding all of the water present in humid air at atmospheric pressure (app 1) and any water in the system can freeze as the compressed air is expanded in doing work. (see gas law thermodynamics for temperatures on isentropic compression and expansion and estimate the exit temperature for 12bar at initially at 30®C expanding to atmospheric pressure). The system should therefore include provision for the removal of as much water as possible to achieve the required standard, and preferably with features to avoid icing of compressed air tools in winter conditions
Performance is to be outlet air purity to BS ISO 8573-1 class 3 or better.
Screw fittings must be to the metric international standard eg
ISO/BS 2779-NF EO3-005 DIN 3852 Pg. 2 (Parallel Threads)
BS21-IS07, DIN 3852 (Taper Threads)
ISO 6149 DIN 3852
.

Outline Technical Specification
Technical Performance
Two systems are to be considered:- small & large.

Outline technical Specification small large
Flow FAD @ STP Cubic Feet /min (CFM) 44 75
Flow FAD @ STP m3/min 1.28 2.18
Mass Flow kg/min 1.55 2.64
Oil flow litres/ min ( l/min) 20 30
Oil: air ratio m/m 12.9 : 1 11.36 : 1
Oil air compressor exit temperatures 95⁰C 110⁰C
Total oil volumes max litres 4 5

Oil Aerosol droplet size not fully determined
For initial design work consider the following log normal distribution curve for droplet size.

(Ref paper for this source USA military )
Maximum working pressures for each system is 12 bar, with normal operation at 8 to 10bar.
Air : Oil separation to be better than :- ISO8573-1 Class 3 and preferably up to class 2 to minimise oil consumption to achieve a service life of over 2,000 hours.

 

Water separation to be better than :- ISO8573-1 Class 3 and preferably up to class 2 to minimise icing on compressed air tools in winter conditions.

 

 

Max air pressure drop (dP) across the separation units at full flow 50mbar
Air purity will be dictated by the inlet air filter but care must be taken to ensure that metallic particle is the ISO 8573-1 class 3 quality.

 

 

Pressure Vessel Design Safety standards

The pressure vessel will be E or CE marked to BS ISO 13445 or the simple pressure vessel directives which ever applies. The tank will be drained of compressed air when not in use and hence transport pressure directives do not apply

Pressure vessel safety legislation
The design of the pressure vessel must meet also ISO and safety standards particularly BS ISO 13445 Pressure Vessel design, Pressure vessels directives 97/23/EC & 87/404/EEC and BS ISO 4126 Safety Devices.
It is expected that the vessel will exceed the 50 bar. Litres pressure capacity limit thus placing the design in a category 1 class of system to Pressure vessels directive 97/23/EC. ( design max 170 bar.litres and note that larger than this places the design in Class II with a higher inspection classification and hence manufacturing cost)
In category 1 Form A applies, namely the internal production control of the manufacturer.
Suitable quality standards ie ISO 9001 will therefore be required to ensure that legislative quality has been achieved and records such as Conformity of Production (CoP) can be produced for inspection.
A CoP report template should be provided as part of this project work.

Pressure Vessel Safety Devices
The tank will be fitted with at least two pressure relief valves, one of which will be electrically controlled for system overpressure and one will be a mechanical device to prevent gross overpressure. Both devices will be designed to meet BS ISO 4126 standards and may be purchased items.
These devices will be under vehicle body mounted or marine applications and hence must be corrosion resistant (Internal & External) and capable of operating with water ingress as well as with the ambient operating conditions noted in section 2). (Ref:- salt spray testing).

Ambient conditions
Since this unit is to be mounted under vehicle or for marine engine space applications temperatures of up 70⁰C can be expected since engine convective heat can be expected together with tropical ambient applications.
Air intake to the system is not expected to exceed 60⁰C
Humidity max possibility 100% @ 35C {= 26g H2O per kg Air }
Small system = c 2kg H2O per hour
Large system = c 3.4 kg H2O per hour
Minimum working temperature -15⁰C
Minimum storage temperature -35⁰C
The unit will be mounted to the vehicle or boat under frame and hence may experience low amplitude vibrations in the 50 to 200Hz range.
The separator system and all fittings must have corrosion protection for 10 years service life.
Maximum shock load (impact) = 30g
Tilt :- +/- 30⁰ from horizontal in all planes with compounded angles
Note the separation system must be capable of providing both full oil flow whilst maintaining Class 3 air under all tilt and ambient conditions.
This condition must be maintained at full oil capacity (4 or 7 litres)
Size and weight constraints
Under vehicle and marine engine space packaging dictates that size is a major issue, hence the total unit should be no bigger than 150mm dia x 800mm length. Incorporating mounting fittings not greater than 25mm depth.
The oil filter housing should allow for ease of servicing and free access for the removal of the filter.
The unit would be bracket mounted to specific vehicles or marine applications so provision must be made in the design for these additional supports.
Minimum weight is required to avoid any impact on the load carrying capacity of the vehicle or boat stability.
Special considerations
Cost is a critical issue and manufacturing methods should be considered around 5,000 to 10,000 units per annum.
Parts to be produced to ISO9001 QA standards
Corrosion protection (Internal & External) to suit under vehicle and marine use for at least 10 years. (Look for salt spray testing)

Oil Filtration
Oil flow 20 l/min and 30 l/min.
Filtration level is to be 5μm or better (99% cut off point) (Ref Fram, K&N, & other manufacturers)
The filter will be a spin on cartridge design, suitable for 2,000 hours operational life. ( This can be a purchased item)
Oil filter temperatures 90⁰C and 110⁰C max
Max working oil pressure 12 bar, (175 psi) (note safety & burst pressures are typically 2 to 3 times higher than MWP)
Elastomer seal used must be capable of > 4,000 hr life under these conditions. Seals must meet oil compatibility (seal swell and hardening) standards >4,000 hrs.
Oil 100% Synthetic PAG and POA types

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