Reciprocating / Piston Compressor Mineral Oil (5 Litres) SCS

8.3.3.1 Outlet temperature

A screw compressor compresses gas. The process can be assumed to be adiabatic. That means that Eq. (8.8) should have some role in predicting the discharge temperature. If we have a compressor that has a suction pressure of 12 psia (82.7 kPaa) at 80°F (26.7°C) and a discharge pressure of 112 psia (772 kPaa), Eq. (8.7) would predict the outlet temperature would be 405°F (207°C) with k=1.28. That temperature would be a very difficult problem for metallurgy and for operations in general. The injected oil must have a role to play in that temperature. First you should use Eq. (8.8) to calculate the heat of compression. Then you have to convert the flows to mass-flow rate. Eq. (8.15) is used to determine the thermal energy that the act of adiabatic compression added to the process.

(8.15)Qgas=m˙gas⋅cpGas⋅(TdischTheo−Tsuct)

The discharge temperature can now be determined using Eq. (8.16).

(8.16)Tdisch=Qgas+TsuctGas⋅m˙gas⋅cpGas+ToilIn⋅m˙oil⋅cpOilm˙gas⋅cpGas+m˙oil⋅cpOil

Example. A screw compressor has the conditions in Table 8.7.

Table 8.7. Screw compressor temperature example

This data is used to calculate the temperature of the oil out of the compressor (Eq. (8.17)).

(8.17)m˙gas=qgas⋅ρgasStd=300,000SCF/day⋅0.046lbm/SCF=13,770lbm/daym˙oil=qoil⋅ρwater⋅SGoil=40gal/min⋅8.34lbm/gal⋅0.81⋅1440minday=389,000lbm/dayTdisch-gas=(110+460)(100+120+12)(1.28−1)/1.28=929RQgas=13,770lbm/day⋅(0.52669BTUlbm⋅R)⋅(929R−570R)=2.60×106BTU/day2.60×106BTU/day+570R⋅13,770lbm/day⋅0.52669BTUlbm⋅RTdisch=+640R⋅389111lbm/day⋅0.45BTUlbm⋅R13,770lbm/day⋅0.52669BTUlbm⋅R+389,000lbm/day⋅0.45BTUlbm⋅R=652R=192°F

This example is plotted on an extract of the McKetta–Wehe Chart in Fig. 8.11. In this example, the ability of the gas to hold water vapor at outlet conditions is less than its ability at inlet conditions, so 836 lbm/MMSCF (13 g/SCM) remain in the oil—increasing the volume of the oil reservoir by 30 gal/day (113 L/day).

8.3.3.2 Oil temperature control

The “standard” way to control oil temperature is shown in Fig. 8.12. In this common scheme, the 3-way “constant temperature valve” looks at the temperature into the screw and bypasses the cooler to try to maintain a constant input temperature. In the example in Table 8.7 the 3-way temperature control valve is set to maintain 180°F (82.2°C) into the process. You can see from the example, this setting (and the oil flow rate) result in a 12°F (6.6°C) ΔT, and 192°F ( 89°C) outlet temperature which causes considerable water to collect. Changing the set point would help for these exact conditions, but if gas-flow rate increases, or discharge pressure decreases, etc., it will no longer be correct. The long and the short of the story is that controlling the inlet temperature is a ineffective practice that has come to us from plant compressors (where, if you’ll recall, water-vapor in the gas is much lower) and really should have stayed there.

The packagers of the first screw compressors that I deployed failed to understand field use to the extent that the 3-way valve was set for 140°F (60°C) into the screw and the oil flow rate was expecting about three compression ratios (we had 12 compression ratios) so the oil was flowing so quickly that temperature out of the compressor was 142°F (61.1°C) and we had a considerable number of serious issues and very high oil-replacement/replenishment costs.

These skids were so basic that they didn’t even have a temperature gauge anywhere on the compressor, let alone on the compressor outlet.

After many months of fighting with controlling the wrong thing, I realized that life gets easier if you control the right thing (I’ve always been a slow learner). The “right thing” is to control the temperature out of the compressor in the face of frequently changing conditions. In Fig. 8.13, there is a temperature sensor looking at the temperature out of the screw. Based on that temperature, the PLC controls (it goes through the following steps in sequence with a 2-minutes pause before going to the next step)

1.

Adjust the speed of the glycol pump (can be adjusted to a minimum, and then on the next time the PLC looks at the glycol pump it can be turned off).

2.

Adjust the setting on the thermostatic valve on the oil pump discharge (this valve can go fully shut, the orifice in the bypass is sized for required lubrication and minimum oil-injection).

3.

Adjust the speed of the fan (can go to a minimum, but not zero).

4.

Adjust the speed of the oil pump (can go to a minimum, but not zero).

5.

Repeat.

A group of skids designed to this temperature control scheme was able to run for 3 years with zero unscheduled downtime and nearly zero added oil. This skid had variable speed drives on all electric motors. Before this skid was designed, I was reluctant to set an electric-motor driven compressor on a well site because that puts the weakest link under someone else’s control—a hard concept for a well-site engineer. After reviewing the results of this compressor, I am prepared to set a genset to run site facilities with electric power. Genset’s combined with variable speed electric motors has a high capital cost, but low-operating cost and the cross-over happens in the first couple of years.

8.3.3.3 Oil pressure management

As mentioned above, plant machines historically do not have oil pumps, but rely on the differential pressure across the skid to move oil. This can work in steady-state operations, but well-site and gathering operations are rarely steady state. Experience has shown that field compressors without oil pumps will tend to be on the top of down-time lists and failure reports. On engine-driven compressors oil pumps can be run off the pony shaft. On electric skids the oil pump should have its own variable speed drive.

8.3.3.4 Coalescing element

We often call the big lump of steel on the backside of the skid a “coalescing filter.” It is not a filter. The coalescing elements look somewhat like filter elements, but they serve a different function. The coalescing element is intended to force small droplets (that are buoyant in the gas stream) to crash into other droplets and coalesce into larger drops that are not buoyant in the gas. As is normal with any piece of equipment, there is a range where they are more effective. It is recommended to try to get the same magnitude of velocity in a coalescing element as the target velocity for in a separator mist pad (see Chapter 5: Well Site Equipment).

12.7.1 Refrigeration compressor lubricants

Refrigeration compressors deserve special mention in lubricant technology. Refrigerator oils are used in refrigerators, air conditioners, dehumidifiers, cold-storage chests, freezers, automatic vending machines, cooling units in chemical plants, etc., serving as a hydraulic control and functional fluid. In a domestic refrigerator or freezer compressor, motor bearings require lubrication. Refrigeration oils should help to ensure that the compressor and system components receive the proper lubrication for years of trouble-free operation. Since motor and compressor are sealed inside the refrigeration circuit, the lubricant cannot be changed for the lifetime of the appliance, typically 10–20 years. The lubricant is not exposed to atmospheric oxygen and water; consequently, oxidative and hydrolytic stability are of less importance than thermal stability. Refrigeration lubricants must have very good electrical insulation properties, must be stable to electrically induced decomposition and exhibit very good low-temperature fluidity characteristics with no tendency to deposition of waxes. High dielectric strength allows the oil/refrigerant mixture to serve as an insulator between the motor windings and the body in a compressor. The requirements of today’s refrigeration and air conditioning compressor lubricants are complex (see Table 12.25).

Table 12.25. General requirements for refrigeration lubricants

a

Much lower PP than mineral oils.

b

Typically CFCs, HCFCs, HFCs, CO₂, hydrocarbons, ammonia.

c

Typically BR, NR, NBR, HNBR, nylon-6,6, Teflon, EPDM, Neoprene, etc.

d

For retrofitting.

The principal functions of refrigeration oils are lubrication of pistons or rotors, sealing of valves and dissipation of heat. High-quality refrigeration oils are required in view of the longevity expected of such compressors, interaction with other substances and low and high temperatures. A mixture of lubricant and air-conditioning fluid must have sufficient lubricating properties to protect the air-conditioning pump, but should not be aggressive towards materials making up the system. Refrigeration oils should maintain high film strength even when diluted with refrigerant. High chemical stability minimises the reaction with refrigerants and other materials (e.g. elastomers) that are part of the system. Important factors are the lubricity of the refrigeration oil, interaction with the refrigerant, evaporation behaviour, solubility and mixture behaviour. Refrigerants and lubricants should be fully miscible and have high mutual solubility (at all temperatures that the system will experience), as in use of hydrofluorocarbon (HFC) refrigerants and POE lubricants. Lubricants with a thermal stability of > 175 °C generally work well in refrigeration systems. Ultra-low temperature refrigerants (as low as − 100 °C) require specialty lubricants (e.g. EMKARATE RL ‘H’). Obviously, for such applications all the components of the lubricant, including additives, need good solubility and low-temperature flow characteristics.

The most important parameter for determining the lubricity of oils or oil–refrigerant mixtures is viscosity. Selection of an optimum refrigeration oil depends on the specifications of the compressor, the system as a whole and the refrigerant. Refrigeration lubricants must have a wide variety of viscosities to achieve optimal effectiveness in the many different types of refrigeration equipment. For conventional home refrigerators, equipped with low power compressors, lubricants with relatively low viscosities at normal operating temperatures are generally satisfactory. Low-viscosity lubricants are preferred for economy of operation. By contrast, automobile air-conditioners and industrial refrigeration systems require relatively higher viscosity lubricants by reason of the more extreme conditions of operation. A high VI is an indication that their effective viscosity will not change drastically with the wide swings in temperature seen by most typical refrigeration and air conditioning systems. This means that energy consumption will be minimised at low temperatures and lubrication will be maximised at elevated temperatures.

Moisture can enter the refrigeration systems by improper evacuation of the system, system leaks, system components, improper handling of the refrigerant or of the lubricants. The moisture content for refrigeration lubricants should be < 100 ppm. Hygroscopicity of refrigeration lubricants ranks (in descending order) as PAGs > PVEs > POEs > alkyl benzenes (ABs) > mineral oils. The rate at which POEs pick up moisture is dependent on temperature, relative humidity, exposure time, and relative surface area. PAG lubricants are typically used in automotive applications and PVE is used sparingly in certain regions of the world.

Refrigeration oils differ according to the refrigerants being compressed (ASHRAE Standard 34–1992) [172]. A refrigeration lubricant must be miscible with the refrigerant gas. Modern refrigerants are halohydrocarbons (e.g. HFCs), such as HFC-134a and HFC-152a, as substitutes for the ozone layer destructive chlorofluorocarbons (CFCs), now in controlled use. Under the terms of the Montreal Protocol CFCs were phased out in 2000; HCFCs should be completely phased out in 2030. Hydrofluorocarbon (HFC) gases, which are currently the most widely used non-flammable, zero ozone depletion refrigerants, are relatively polar and not miscible with mineral oils.

Typical refrigerator oils are generally naphthenic or paraffinic mineral oils, ABs, polyglycolic oils, ester oils or their (additivated) mixtures with kinematic viscosity of 10–200 cSt at 40 °C. Mineral oils are still the most significant group of oils for traditional refrigeration compressors using ammonia along with CFCs and hydrochlorofluorocarbons (HCFCs), but ABs or PAOs are also being used. For instance, Glova [173] has disclosed a refrigeration lubricating oil composition based on branched-chain alkylbenzenes. These oils are not sufficiently soluble in the new chlorine-free substitute refrigerant mixtures (fluorocarbons (FCs) and HFCs) such as R 134a, R 404a and R 507. R 134a is currently the most commonly used new refrigerant. Modern synthetic refrigeration oils should not only have high lubricity, low hygroscopicity and high thermal and chemical stability, but also excellent compatibility with the alternative halohydrocarbons to ensure lubricant suitability. The tribological performance of POE and PVE synthetic lubricants operating with HFC-134a refrigerant has been reported [174]. Biodegradable POEs are the lubricants of choice for use with HFC gases [175].

Honeywell and DuPont are to produce a new eco-friendly refrigerant for use in car air conditioning systems. The refrigerant, dubbed HFO-1234yf (2,3,3,3-tetrafluoropropene), has a global warming potential (GWP) of 4; cf. GWP of 1 for CO₂. The currently used refrigerant HFC-134a (1,1,1,2-tetrafluoroethane; main supplier: Mexichem) has a GWP of 1430, which is above the EU Mobile Air Conditioning Directive (2006/40/EC) that states that by 2011 all new vehicle models must use a refrigerant with a GWP below 150 [176]. Notice that HFC-23 (CHF₃, fluoroform) is an extremely potent greenhouse gas with GWP of 14 800.

In the past, the widespread commercial use of chlorine-free refrigerant heat transfer fluids such as 1,1,1,2-tetrafluoroethane (formerly Klea 134a, ICI) has been hampered by the lack of commercially adequate lubricants. Examples of refrigeration oils that are miscible with HFCs include oxygencontaining compounds such as esters, ethers and carbonates. In particular, ester-based oils now constitute a most important product group. Various recent patents describe esters useful as refrigerator oils (see Table 12.26). Schnur [177] discloses an ester blend, including an ester based on NPG and/or PE and 2-ethylhexanoic acid. As the solubility of NPG and PE esters in non-chlorinated HFCs is often only fair, improved polyol-based esters are required. US Patent No. 6,582,621 to Sasaki et al. (to Nippon Mitsubishi Oil) [178] describes the development of mono- and dicarboxylic acid polyol esters with high insulating properties, whereas US Patent Appl. No. 2004/0046146 to Ankner et al. (to Neste Chemicals OY) [179] claims polyol esters comprising a sterically hindered diol esterified with mono- and dibasic carboxylic acids. In US Patent No. 6,831,045 [180] and US Patent No. 7,045,490 [181] Shimomura and Takigawa describe alicylic di- and polycarboxylic acid ester compounds. In US Patent No. 6,656,891 B1 (to Idemitsu Kosan Co.) Sakanoue et al. [184] disclose a refrigerating machine oil composition composed of a blend of PVE or POE with a PAG alkyl ether and an AB, suitable for HFC type, HC type, ether type, CO₂ or ammonia-type refrigerants. Polyether-based lubricant oil compositions for refrigerators have also been disclosed by Enna et al. (to Asahi Glass Co.) [185] and Kaimai and Takahashi (to Japan Energy Corp.) [186]. Reflo™ is an inherently biodegradable ammonia-type refrigeration compressor oil (Petro-Canada Lubricants, Inc., Mississauga, ON).

Table 12.26. Selected refrigeration oil compositions

a

EG, NPG, PE, TME, etc.

b

CHDM, 1,4-dimethylcyclohexane; ETHD, 2-ethyl-1,3-hexanediol.

c

EMKARATE RL™ product line.

Uniqema (formerly ICI)’s EMKARATE RL™ POE line is specifically designed for use with HFC and HCFC refrigerants and is widely accepted by major OEMs. These POE refrigeration lubricants cover a wide viscosity range from 7 cSt (35–40 SUS) to 220 cSt (1100 SUS). EMKARATE RL™ is designed specifically for use with environmentally friendly HFC refrigerants. The superior performance has been achieved with base fluids that are specifically formulated to deliver optimum performance with minimal additive levels. EMKARATE RL™ lubricants are compatible with all CFCs, HCFCs, HFCs, CO₂ and hydrocarbons (e.g. R 290, R 600a) refrigerants, but not with ammonia (R 717) [183]. HFC refrigerants and the POE lubricant have both polar molecular structures, which attract polar water molecules. The solubility of water in HFCs, such as R 134a, is many times greater than in the CFCs they replace. POEs, such as EMKARATE RL™, are also considerably more hygroscopic than traditionally used mineral oils. Some physical properties of EMKARATE® RL 9HPlus are as follows: viscosity at 40/100 °C, 8.6/2.5 mm²/s; VI, 120; PP, − 49 °C; FP, 195 °C; AV, 0.01 mg KOH/g; no foaming [183].

Branched synthetic POEs have been used extensively in non-biodegradable applications, such as refrigeration lubricants, and have proven to be quite effective if more than 25% 3,5,5-trimethylhexanoic acid is incorporated into the molecule. However, trimethylhexanoic acid is not biodegradable (OECD 301B) and its incorporation into the lubricant base oil would drastically lower the biodegradation of the polyol ester due to the quaternary carbons contained therein. Also incorporation of trialkyl acetic acids (or neo acids) into POEs produces very useful refrigeration lubricants. Again, these acids do not biodegrade and cannot be used to produce POEs for biodegradable applications. POEs of all branched acids can be used in refrigeration oils, but they do not rapidly biodegrade. Although POEs made from linear C₅ and C₁₀ acids for refrigeration application are biodegradable under the Modified Sturm test, they would not be adequate as a lubricant in hydraulic or two-cycle engine applications because the viscosities would be too low and wear additives would be needed.

Recently, Chevron Corp. [182] has developed various diester and triester lubricating base oils for refrigeration application with molecular mass from 340 to 780 a.m.u. (Fig. 12.3) having kinematic viscosity of at least 3 cSt at 100 °C and PP below − 20 °C. Diester lubricants may be prepared according to Fig. 6.41 in which the C₈–C₁₆ olefin used is a reaction product of a Fischer–Tropsch process; the carboxylic acid can be derived from alcohols generated by Fischer–Tropsch and/or it can be a bio-derived FA [187]. Figure 12.4 shows the synthetic strategy for conversion of a mono-unsaturated fatty acid (MUFA; C₁₀–C₂₂) to a triester derivative. The demand for polyol esterbased synthetic refrigeration oils is increasing. It is essential to shield these ester oils from water in the compressor.

In subcritical CO₂-based car air-conditioning systems special synthetic polyglycols (PAGs) or ester oils with AW/EP additives are to be used to guarantee the lifetime of the compressor under the severe conditions of the CO₂ transcritical process.

Reducing electrical energy consumption through lubricant base fluid design and optimisation of lubricant performance are of considerable economic interest. Some 5–10% of the UK’s domestic electricity consumption is used to power refrigerators and freezers (up to 15% worldwide). A similar proportion of non-domestic power consumption is shared by industrial and commercial refrigeration and air conditioning [46].

Small appliance refrigeration compressors are designed to operate under conditions of hydrodynamic lubrication, where metal-to-metal contact is avoided by ensuring that a full-fluid film separates the surfaces. The energy required for shearing the entrained hydrodynamic film increases with the viscosity of the lubricant. Therefore, the energy requirement of an appliance compressor can be reduced by reducing the lubricant viscosity. However, if the lubricant viscosity is reduced too much, a full-fluid film is no longer ensured and the system will move into the boundary lubrication regime. Here, metal-to-metal contact occurs; wear reduces efficiency of the operation and limits the lifetime of the appliance. As friction coefficients in mixed and boundary lubrication greatly exceed those in the hydrodynamic regime, also increased frictional losses occur. Higher friction leads to overheating of the fluid and contacting surfaces, again compromising the expected life-time. Where possible, a periodic refrigerant analysis is important to detect and control contaminants (moisture, acid, particulate/solids, organic matter – sludge, wax, tars – and non-condensable gases) in the refrigerant, which can result in degradation/failure of the various components, and cause inefficient operation of the unit.

Structure–property relationships (see Table 10.15) have been used for optimisation of the energy efficiency of POEs in refrigeration lubrication applications [188]. Optimisation of POEs has permitted a gradual reduction in the industry standard lubricant viscosity from 18 cSt at 40 °C when polyol esters were first introduced to 10 cSt today. This translates into real benefits in energy consumption (about 10%) for a standard domestic refrigerator, as shown in Fig. 12.5. The new generation optimised POEs are also largely based on renewable materials and readily biodegradable (Fig. 12.6).

52.13.1 Quality and safety

Over the years, the progressive improvements in compressor lubricants have kept pace with developments in compressor technology, and modern oils make an impressive contribution to the performance and longevity of industrial compressors. More recently, a high proportion of research has been directed towards greater safety, most notably in respect of fires and explosions within compressors. For a long time the causes of such accidents were a matter of surmise, but it was noticed that the trouble was almost invariably associated with high delivery temperatures and heavy carbon deposits in delivery pipes. Ignition is caused by an exothermic (heat-releasing) oxidation reaction with the carbon deposit, which creates temperatures higher than the spontaneous ignition temperature of the absorbed oil.

Experience indicates that careful selection of base oils and anti-oxidation additives considerably reduce such deposits. Nevertheless, the use of top-class oil is no guarantee against trouble if maintenance is neglected. For complete safety, both the oil and the compressor system must enjoy high standards of care.

52.13.2 Specifications

The recommendations of the International Standards Organization (ISO) covering mineral-oil lubricants for reciprocating compressors are set out in ISO DP 6521, under the ISO-L-DAA and ISO-L-DAB classifications. These cover applications wherever air-discharge temperature are, respectively, below and above 160°C (329°F). For mineral-oil lubricants used in oil-flooded rotary-screw compressors the classifications ISOL-DAG and DAH cover applications where temperatures are, respectively, below 100°C (212°F) and in the 100–110°C range. For more severe applications, where synthetic lubricants might be used, the ISO-L-DAC and DAJ specifications cover both reciprocating and oil-flooded rotary-screw requirements.

For the general performance of compressor oils, there is DIN 51506. This specification defines several levels of performance, of which the most severe – carrying the code letters VD-L – relates to oils for use at air-discharge temperatures of up to 220°C (428°F).

The stringent requirements covering oxidation stability are defined by the test method DIN 51352, Part 2, known as the Pneurop Oxidation Test (POT). This test simulates the oxidizing effects of high temperature, intimate exposure to air, and the presence of iron oxide, which acts as catalyst – all factors highly conducive to the chemical breakdown of oil, and the consequent formation of deposits that can lead to fire and explosion.

Rotary-screw compressor mineral oils oxidation resistance is assessed in a modified Pneurop oxidation test using iron naphthenate catalyst at 120°C (250°F) for 1000 hours. This is known as the rotary-compressor oxidation test (ROCOT).

52.13.3 Oil characteristics

11.1 General description of plant

Compressed air at various pressures and flow rates is produced by compressors, normally electrically-driven, to satisfy the requirements of control and instrumentation, dust plant jet-blowers and dust pumps, boiler soot-blowing, automatic boiler control, boiler blowdown, turbine forced cooling, breathing apparatus and reactor purging as appropriate to the type of power station.

The main building complex of a power station is served by two primary compressed air systems — ‘general services air’ and ‘control and instrument air’. Figures 10.38 and 10.39 show typical control and instrumentation and general service compressed air systems for a power station. The ‘general services’ system is designed for a flow rate of 0.4–1.0 m³/s at pressure of 7.2 bar, the ‘control and instrument air’ system for 0.2 m³/s at 8.5 bar. Certain plants remote from the main complex, such as those producing hydrogen and sodium hypochlorite, are provided with independent air compressor plant.

Control and instrumentation air system requirements are most stringent in respect of the quality of the compressed air produced and its integrity. A typical system consists of two or three 100% duty electrically-driven air compressors, each complete with air coolers and oil/water separators, feeding two independent air receivers and air dryer systems via a manifold.

A typical general services air system consists of two 50% duty electrically-driven air compressors, each complete with air coolers and oil/water separators, supplying a common air receiver.

11.2 Air compressor drive motors

The drives are usually squirrel-cage induction motors, suitable for direct-on-line starting on 11 kV, 3.3 kV or 415 V three-phase 50 Hz supplies, of totally-enclosed fan cooled construction, as described in Section 2.2 of this chapter. They drive the compressors through either V-belts or flexible couplings and gearboxes. The V-belts and flexible couplings protect the motor from shock loading and vibration.

11.3 Heaters

Heaters in compressed air driers and compressor oil sumps are either connected in balanced three-phase banks and supplied through electrically-held contactors at 415 V three-phase 50 Hz or, for ratings up to approximately 3.5 kW, at 240 V single-phase 50 Hz. Heaters are equipped with dual thermostats, one for normal temperature control and the other acting as a safety device in the event of failure of the control system. The latter is manually reset and initiates remote and local alarms to warn of failure.

11.4 Automatic and safety controls

The compressed air systems are designed to operate automatically, all operating conditions, such as air pressures and temperatures, being continuously monitored. Plant start-up, shutdown and duty selection are performed in the central control room and plant failure alarms are displayed there. Control panels situated local to the compressors house all the alarms, indications and controls necessary for the maintenance and local control of the plant.

2.20.1 Compressor specification

The compressor should be of the dry type cylinder and should supply oil-free air, and be complete with non-return valves, intercoolers, aftercoolers, condensate draining facilities, etc.

The compressors and their drives should satisfy the requirements for running equipment as specified by the user.

2.20.2 Compressor piping

The inlet of the compressors should be so located that the instrument air is free from toxic, obnoxious, or flammable gases, and is free from dust.

The inlet opening should be fitted with a wire mesh cage. The cage should be of adequate size to prevent flying papers, etc. from completely blocking the compressor inlet; the wire mesh should be adequate to prevent flying objects from entering the compressor and to prevent plugging by frost or hoar-frost.

Where the compressor inlet cannot be located in a completely dust-free area, consideration may be given to dust filters in the inlet piping.

To reduce the load on the air drier, the air from the compressors should be cooled to a temperature of 5–10°C above the cooling medium inlet temperature. Where the aftercoolers supplied as an integral part of the compressors are suspected of having only marginal capacity, the installation of additional aftercoolers should be considered.

2.20.3 Compressor controls

Each compressor should have the facilities for manual and automatic starting in the case of failure of the other compressor(s).

The automatic starting system should be so arranged that stopping of a compressor is only possible by manual control.

Automatic starting of the stand-by compressor(s) should be as quick as possible; in addition to an initiator on the piping downstream of the air drier. Initiators should be provided at each compressor discharge upstream of the non-return valve and/or on the compressor oil system. Starting of each stand-by compressor should be indicated by an alarm on the main panel.

The electric motor(s) should have local start/stop controls and be protected against repetitive starting. Electric controls supplied as integral parts of the compressor, as for oil filter, oil pump, oil heater, should be interlocked with the start/stop controls and should be located in a weatherproof housing on, or close to, the compressor.