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Thursday 16 April 2015
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 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'.
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