Oil Cooler Screw Compressor for Heat Exchanger

With reference to screw compressor oil cooling systems sales & service engineers should be aware of what methods can be used, how & where to apply & design them. There are three basic oil cooling systems.

• Water cooled
• Air cooled
• Refrigerant cooled

1) Water cooled

Are applicable to systems utilizing Evaporative condensers or cooling towers or where an independent water supply is available.

Water cooled oil coolers are shell & tube or plate heat exchanger design. Generally the oil cooling circuit is on the shell side and the water circuit on the tube side for shell & tube coolers. For PHE's the oil cooler cassettes should be welded & the water side gasketted to facilitate cleaning.

The water circuit on a S&T cooler will normally be 2 pass and the heads or bonnets should be able to be removed for tube cleaning & maintenance purposes. For all applications in Saudi Arabia the tubes + tube sheets should be 90/10 Cu/Ni & the channels or bonnets epoxy coated. Tubes must be straight bore thick wall 0.35" thick, internally enhanced tubes should not be used.

The water circuit can be piped from one end of the pan & back to the opposite end of the pan to ensure thorough mixing of the pan water, using optional extra connections provided by the manufacturer + a separate circulating pump & Y type strainer. Alternatively & probably lower cost option, is to request the condenser manufacturer to provide a "Tee" piece in the spray water pump discharge & uprate the spray pump flow rate + head to accommodate the oil cooler. The water is returned to the spray sparge inlet connection on the condenser. Service stop valves must be included in the flow & return to the oil cooler + a bypass valve between the flow & return located in the standard condenser water pump discharge piping. As the oil cooler circuit will always have the highest pressure drop this bypass valve should be a flow regulating type valve so the flow can be adjusted to the condenser spray manifold with the remainder passing direct to the oil cooler.

Care must be taken in selecting the Evaporative Condenser as the  evaporative condenser or cooling tower sees the oil cooling load as an addition to the compressor THR by virtue of the rise in spray water temperature, unless the water source is independent from the compressor condenser. Generally the rejection capacity will need to be increased by approximately 5% to cover the increase in the spray water temperature. Rather than waste time calculating the rise in spray water temperature just simply include the oil cooling load in the THR figure or select the condenser as normal e.g. evaporator load + shaft power only, but ensure you use a selection with say +10% spare capacity or surface area.

2) Air cooled

Can be utilized on all systems where air cooled condensers are utilized or a water supply is not available. For air cooled systems where the condenser is close coupled to the compressor an additional independent row can be added to the main condenser with its own inlet/outlet header. However sales should contact both the condenser manufacturer to ensure they can provide this facility + the compressor manufacturer to check the engineering & whether a full time lube oil pump is required.

Alternatively a packaged air cooled oil cooler can be mounted on the compressor & the compressor manufacturer should be able to provide a quotation for both the air cooled cooler + mounting & piping.

For all remote air cooled oil cooler applications the oil cooling load is not to be included in the compressor condenser selection. Obviously safety relief valves will be required between any valves in the system & a separate DOL starter contactor or relay, fuse & control circuit will be required for the fans.

3) Refrigerant cooled

Refrigerant cooled oil cooling can be

• Liquid injection either low or high or automatic low & high Vi into the compressor rotors.
• Thermosyphon shell & tube or plate heat exchanger
• Pumped circulation

3.1) Liquid injection oil cooling

In the case of liquid injection the oilcooler load must be included in the THR for the condenser selection. It does provide a low cost means of oil cooling on bare compressor units + chillers. If liquid injection is used, automatic dual Hi/Low injection systems must be used which facilitates injecting earlier or later along the length to the rotors depending upon the operating Vi.

However liquid injection should be avoided wherever possible due to

·         Inability to accurately control the liquid injected at various Vi's & condensing pressures during normal operation.

·         Increase in power + decrease in capacity

·         Problems arising in high back pressure applications where the total mass flow is too high for the radial + axial discharge ports, particularly at pull down.

·         Propensity to cause additional wear in rotors & bearings due to overfeeding or overcooling, causing gas to condense in the discharge ports + oil in the separator.

3.2) Thermosyphon oil cooling

Thermosyphon systems provide the optimum oil cooling system in terms of cost, performance & maintenance. However it is important the system is designed properly and a basic understanding of the system requirements is appreciated. Thermosyphon oil cooling systems may utilize either shell & tube or welded Plate heat exchangers. In all cases the oil cooler load has to be included in the condenser total heat of rejection.

·         Sufficient liquid reserve of 2 minutes must be provided to ensure a continous feed to the oil cooler when the compressor is disabled to allow adequate oil cooling during the motor coast down period.

·         Thermosyphon systems must operate with a liquid overfeed rate of minimum 1.5:1 to 4:1.

·         The liquid feed and wet suction returns have to be very carefully sized to reduce friction losses to a maximum of 0.5pfsi/100' (0.035 kg/cm2) for the liquid feed & 0.2pfsi/100' (0.014kg/cm2) for the return. If the line sizes are too small the friction losses  will increase. As the friction losses increase the circulation rate will reduce; the flow will balance out at a new equilibrium corresponding to the static head minus the system pressure drop.

·         Wherever possible the main system HP liquid receiver should be used rather than using pilot receivers.

·         In terms of static head, the liquid feed static head must be high enough to overcome the pressure drop in the liquid feed + oil cooler + return line. Generally the total pressure drop will be around 1.5pfsi which requires the priority vessel to be mounted at least 2mtr above the TSOC/s.

·         For multi TSOC applications a plug type flow regulating valve must be installed at the inlet to each TSOC to enable proper balancing of the flow. Normal service stop valves or ball valves or butterfly valves are unsuitable for flow regulation & should not be used.

·         Where the main system HP receiver is used or where a priority or pilot receiver is used for single or multi condensers in parallel, the vessel must be installed at a height of at least 6mtrs below the condenser outlet drain in order to ensure free liquid drainage as per the sketch provided to you all previously. The priority or pilot or main HP receiver must be vented back to the condenser inlet connection. To calculate the correct trapping height of the condenser liquid drain lines, the pressure drop must be calculated from the condenser hot gas inlet to the vessel. This averages out at around 5-6psi & @ 40degC liquid temperature (1ft head = 4psi @ 40degC) the drain line trapping height will approximately 6mtr.     Too much height is preferable to little height. If insufficient height is allowed, liquid will back up in the condensers until sufficient static head is available to overcome the drain line pressure drop.

·         Liquid drain service stop valves must be installed in the vertical position above the drain trap into header back to the vessel at a height of at least 1.2mtr above the P trap.

·          Thermosyphon circulation is no different from a mechanically pumped system except that the motive force arises from the conversion of kinetic energy to pressure energy by virtue of the difference between the available static liquid head minus the circuit pressure drop & by the difference in density between the single phase liquid feed & the 2 phase liquid/vapour return. However unlike a mechanically pumped system or pumper drum system, thermosyphon systems are fixed head systems & cannot pump to a height above the available static head except for small differences between the flow and return due to differencies in refrigerant density plus Einstein bubble lift effects where gas bubbles lift the liquid as they rise through the column.

             If the pressure losses through the TSOC + return line are greater than the available liquid feed static             head, then liquid overfeed cannot take place The TSOC will then act as a flooded cooler on a 1:1 basis & a danger of gas binding arises.

             The TSOC/s return line/s must be larger than the liquid feed line by one pipe size to accommodate the vapour return & ensure friction losses are maintained at or below 0.2psig to ensure the design 4:1 recirculation rate is achieved. The vapour is carried back with the liquid by the force exerted by virtue of the static head & refrigerant liquid density difference + Einstein bubble lift flow. The vapour velocity may be greater than the liquid velocity but it is not necessarily the case that the vapour velocity is sufficiently high enough to assist in the liquid return up vertical risers and back to the priority vessel.

             The return flow in a vertical riser on a TSOC is not the same as a wet suction return on a freezer cooler,  where the circulating pump is designed to circulate the design refrigerant mass against the pressure differential between pump suction & surge drum return connection for bottom fed coolers below the surge drum, or to the cooler outlet connection for top fed coolers above the surge drum where gravitational forces

             assist in returning the liquid & vapour.

             The flow regime in a thermosyphon vertical riser or pumped cooler system will be 2 phase Annular, bubble or slug flow depending on the heat flux, liquid/gas velocities, pipe size.

             In case of pumped liquid overfeed systems or gravity flooded systems, the vertical riser pipe size may be smaller than for a TSOC application & the systems must not be confused. In the case of long vertical risers if the pipe size is larger than necessary the static head will increase requiring higher head pumps or in the case of Thermosyphon flow, stalling of the thermosyphon cycle.

• Where for some reason the TSOC return/s cannot be headered back to the pilot or priority vessel & are taken up to the condenser hot gas inlet pipe, it may be impossible to obtain liquid overfeed or a 4:1 rate of recirculation as there is insufficient static head or kinetic energy to drive the liquid above & beyond the liquid level existing in the priority vessel.
• The thermosyphon circuit is simply a closed loop U tube, where, when no heat is rejected in the oil cooler, no circulation will take place & the liquid level in the liquid feed & return line will be in equilibrium at the same height, as the pressure above both the feed & return in the priority vessel will be at the same saturated pressure as exists in the condenser. The liquid temperature will be the same as the condenser saturated temperature by virtue of the vent lines from the HP receiver + the priority vessel, back to the condenser inlet.
• The phase change from liquid to vapour in the TSOC will be at a higher temperature than the condensing temperature due to the submergence effect in the oil cooler due the static liquid head pressure exerted by the liquid feed line + vessel diameter. This provides the difference in refrigerant liquid density due to the difference in temperature. The refrigerant flow rate can be calculated by the TSOC THR divided by the difference in enthalpy between the liquid & vapour at the condensing temperature e.g. 40degC x by the recirculation rate.
• A common misconception with thermosyhpon oil cooling systems exists in relation to taking the TSOC return back to the priority vessel, where Sabroe recommend against it due a loss of liquid subcooling.
• They recommend the returns are taken up to the condenser inlet line. The notion of loss of subcooling is incorrect (In Saudi Arabia) as the liquid exiting the condenser & the liquid in the priority vessel + HP receiver  will all be at the same saturated condensing temperature e.g. 40degC by virtue of the gas balance or vent lines.

            Too, as explained above there will be insufficient motive force in the TSOC liquid feed line or liquid density difference to force the liquid refrigerant up the portion of return line above the liquid level in the priority vessel. In most evaporative condenser designs, the hot gas inlet connection will be approximately 1.5mtr above the outlet connection & upto 6mtrs above the priority vessel liquid level. 

            If condenser sub cooling is required then the condenser has to be ordered with a separate sub cooling

            section with its own inlet & outlet connections separate from the condenser main inlet & outlet

            connection. The liquid to the system is then fed from the HP liquid receiver through the condenser sub

            cooling coil and from the coil to the system. 

• Priority or pilot receivers are additional expense in terms of the vessel + installation costs. Wherever possible the main liquid receiver should be used to feed the TSOC/s. Generally there is little maintenance required on a receiver & if external liquid level gauge glasses are used there is no reason on any installation why the liquid receiver should not be installed in the plant room, out of the sun, installed at a height of 2 mtrs above the TSOC/s. The receiver should have a priority pot on the outlet sufficient to provide 2 minutes of liquid storage which is the amount of time for the motor/compressor to coast down to zero RPM. As noted above it is very easy to calculate the liquid overfeed flow rate in Kg/s or lbs/min and from that to

            calculate the priority pot capacity in M3 or ft3 by multiplying the 2 minute supply by the density of the

            refrigerant at the condensing temperature.

            If liquid receivers are installed on the roof along with the condenser then insufficient trapping height will be

            available, unless  a bottom inlet surge type receiver is used, plus the receiver will be subject to excessive

            heat ingress due to solar radiation whereby surface temperatures have been logged at upto 80degC.

3.3)  Pumped liquid recirculation oil coolers

           Pumped recirculation oil coolers may use Shell & Tube or Plate heat exchangers. Liquid is circulated using a  mechanical hermetic pump at a rate of  1.5 to 4:1. This system would generally only be used where insufficient static height exists to install a priority or pilot receiver or the main HP receiver cannot for some reason be elevated at least 2mtr above the oil cooler/s & refrigerant cooling is the only option available. Obviously the system is expensive in terms of initial capital costs of the pump, starter, controls, orifices, valves, strainer, relief devices etc, in comparison to a straight Thermosyphon system.

• The refrigerant mass flow rate is calculated in the same way as detailed above e.g. Oil cooler THR/ enthalpy difference between liquid & vapour @ condensing temperature x the rate of recirculation e.g. x 4.
• The oil cooler should be selected on whatever rate of flow provides the smallest heat exchanger, commensurate with the lowest pressure drop. If it is the same size cooler or same # of plate cassettes there is no point in selecting a recirculation rate of 4:1 if 1.5: 1 provides an identical cooler.
• The pump should be connected to an outlet connection on the main system HP liquid receiver ensuring the receiver is mounted at a height sufficient to meet the pump NPSH requirements. The return from the oil coolers can be taken back to a connection at the top of the HP liquid receiver.

A suitably sized vent line based on the mass vapour flow must be installed between the receiver & the condenser inlet. The vent or gas balance line would be selected based on the mass flow rate and pressure drop of less than 0.2pfsi100'.