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3 6 A S HR A E J o u r n a l a s h r a e . o r g A u g u s t 2 0 1 1
Early practitioners of industrial refrigeration systems found they
could increase the capacity of evaporators by supplying excess
liquid refrigerant to the unit (overfeeding). In evaporators congured
to operate with overfeed, the quantity of liquid refrigerant supplied is
greater than the minimum amount required to meet the cooling loads
as it undergoes the phase change from liquid to vapor. In this case, a
mixture of low temperature liquid and vapor leaves the evaporator
and returns to a vessel designed to separate the liquid from the vapor
prior to the vapor being recompressed.
Within limits, the cooling capacity o
an overed evaporator increases due to
the tendency or more o the evapora-
tors interior suraces being wetted with
saturated liquid rerigerant.1,2 Figure 1
shows a simple liquid overed system
typical o those designed and used today
or large built-up industrial rerigeration
systems.
In a mechanically pumped overeed
system, a centriugal pump draws low
temperature saturated liquid rerig-
erant rom a vessel reerred to as a
pumped recirculator, pumped ac-
cumulator, recirculator, or low-
pressure receiver and raises the pres-
sure o the liquid or delivery to one
or more evaporators having a common
rerigerant temperature requirement.
Once pressurized by the pump, the
saturated liquid becomes subcooled
as it leaves the pump discharge and
enters the recirculated liquid supply
line. As individual evaporators call
or cooling, local controls simply open
a liquid eed solenoid valve, which al-lows low temperature pressurized liq-
uid rom the liquid supply line to ow
into the evaporator. Manually adjust-
able hand-expansion (i.e., metering)
valves are used at each evaporator as
a means o balancing the supply o
About the Authors
Todd B. Jekel, Ph.D., P.E., is assistant director and
Douglas T. Reindl, Ph.D., P.E., is director of the
Industrial Refrigeration Consortium and professor
at the University of Wisconsin, Madison, Wis.
By Todd B. Jekel, Ph.D., P.E., Member ASHRAE, and Douglas T. Reindl, Ph.D., P.E., Fellow ASHRAE
Liquid Refrigerant PumpingIn Industrial Refrigeration Systems
TECHNICAL FEATUREThis article was published in ASHRAE Journal, August 2011. Copyright 2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more informationabout ASHRAE Journal, visit www.ashrae.org.
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A u g us t 2 0 1 1 A SHR A E J our n a l 3 7
liquid ow to individual evapo-
rators throughout the system.
The hand-expansion valves are
adjusted to achieve appropriate
liquid overeed rates as-required
or given evaporator designs.Leaving each unit will be a mix-
ture o saturated vapor produced
by absorbing heat rom the rerig-
eration loads and saturated liquid
that was overed. The two-phase
mixture is carried back to the same
pumped recirculator vessel through
the recirculated liquid return or
wet suction return. The recircula-
tor vessel separates overed liquid
rom vapor and liquid alls to the
bottom o the vessel to be pumped
back out to the evaporators while
the saturated vapor is directed to
the compressors through the dry
suction line. Liquid is made up to
the recirculator vessel rom a high
mass ow rate o vapor-phase rerigerant leaving the evapora-
tor in lb/min [kg/s].
The total liquid rerigerant mass ow rate the pump needs
to deliver is then:
m m OR refrigerant pumped refrigerant min, ,= +( )1 (3)
where mrefrigerant,pumped represents the mass ow rate o liquid
rerigerant the pump must supply to the connected overeed
evaporators expressed in lb/min (kg/s). The term (OR+1) is
commonly reerred to as the circulating rate, Nr. The circu-
lating rate represents the mass ratio o liquid pumped to the
evaporators to the amount o vaporized liquid in the evapora-
tors.1Table 1 shows the required recirculating liquid ow rate
expressed in gallons per minute o liquid ow or each ton o
rerigeration load over a range o saturation temperatures and
circulating rates (OR+1).
What overfeed rate is required for an individual evapo-
rator? Evaporator manuacturers typically publish recom-mended overeed rates based on the units specifc design.
Deviating signifcantly rom the manuacturers recommend-
ed overeed rate can lead to lower operating capacity o the
evaporator. Too low o a rerigerant ow rate will starve the
unit but excessive liquid supply will cause the evaporator
to brine. Excessive supply o liquid rerigerant ow also in-
creases the likelihood o pump cavitation. Finally, excessive
overeeding o liquid to evaporators increases the difculty
associated with returning the unused liquid to the recircula-
tor when the return path involves a vertical riser. In this case,
there is an increased tendency or liquid to accumulate or log
up in evaporators.
Figure 1: Mechanically pumped liquid overfeed system arrangement.
High Pressure Gas
EvaporativeCondenser
EvaporativeCondenser
HighPressureReceiver
Dry Suction
Compressor(s)Centrifugal Liquid
Refrigerant Pump
PumpedRecirculator
High PressureLiquid
KingValve
E
qualizerLine
OverfedEvaporator(s)
Recir
culat
edLiquid
Retur
n(We
tSucti
on)
Recir
culat
edLiquid
Supply
(Pum
pedL
iquid
Line)
T
pressure part o the system to replace the liquid that is evaporated
to meet the rerigeration loads; the recirculator vessel separates
the ash gas ormed as a result o this throttling process just as it
separates the two-phase mixture returning rom the evaporators.
How Much Liquid Refrigerant Needs to Circulate?
For industrial rerigeration systems using ammonia as the
rerigerant, the ow rate o liquid rerigerant delivered by thepump to connected evaporators is relatively low due to the re-
rigerants high heat o vaporization. The total pump ow rate
will depend on the minimum mass ow rate required to meet
the aggregate capacity o connected evaporators and the recom-
mended overeed rate or the evaporators. The minimum mass
ow rate o liquid required to meet the aggregate rerigeration
load is given by:
mQ
hrefrigerant, min
load, total
fg
= (1)
where mrefrigerant,min is the minimum liquid rerigerant that
must be supplied to all connected evaporators to meet theiraggregate load in lb/min (kg/s), Qload,total is the aggregate re-
rigeration load in Btu/min (kW), and hfg is the enthalpy o
vaporization or the rerigerant at the operating pressure o
that suction level in Btu/lb (kJ/kg). The overeed rate (OR) is
the ratio o liquid mass ow rate to vapor mass ow rate leav-
ing the evaporator:1
ORm
m
liquid
vaporwet return
=
(2)
where mliquid is the mass ow rate o liquid-phase rerig-
erant leaving the evaporator in lb/min [kg/s] andmvapor is the
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3 8 A S HR A E J o u r n a l a s h r a e . o r g A u g u s t 2 0 1 1
Recirculated Liquid Flow Rate (gpm/ton)
Circulating
Rate
Liquid Saturation Temperature (F)
60 40 20 0 20 40
2:1 0.1117 0.1164 0.1216 0.1274 0.1340 0.1415
3:1 0.1676 0.1746 0.1824 0.1911 0.2009 0.21224:1 0.2235 0.2328 0.2431 0.2548 0.2679 0.2829
5:1 0.2793 0.2910 0.3039 0.3185 0.3349 0.3537
Multiply by 0.0646 to convert table values to m3/h per kWT.
Table 1: Recirculated gallons per minute per ton of refrigeration.
Figure 2: Single-stage centrifugal pump.3
Discharge
Suction
Volute
Impeller
Impeller Eye
Anatomy of a Centrifugal Pump
At a undamental level, the centriu-
gal pump used or circulating reriger-
ants is similar to the centriugal pump
used to move water or other secondary
uids. Figure 2 shows a single-stagecentriugal pump common or moving
liquids. The main eature o a centriu-
gal pump is the impeller, which rotates
within the pump casing creating a low
pressure zone near its center (the eye).
This area o low pressure draws liquid
into the pump where the rotating impeller increases the kinetic
energy o the uid by accelerating the liquid outward radially
to the impeller tips. As the liquid leaves the impeller tips, its
kinetic energy is at a maximum. The pump housing or volute
surrounding the impeller then takes over to orderly collect the
liquid leaving the impeller. The process o gathering liquid
in the volute converts the kinetic energy o the uid to pres-
sure (potential) energy. The higher pressure uid then leaves
the pump through the discharge line.
Because a rerigerant pump is moving a volatile uid, it is
highly susceptible to cavitation during operation (see What is
Cavitation? sidebar). To reduce the likelihood o cavitation,
liquid rerigerant pumps include design details that dier
rom ordinary water or secondary uid pumps to decrease the
pressure loss through the pump suction.
Liquid Refrigerant Pumps
Beore discussing the operating details o centriugal re-
rigerant pumps, it is important to consider a ew conceptsundamental to their successul operation. One o the most
important concepts is net positive suction head (NPSH). Quite
simply, suction head represents the pressure at the pumps
suction. The term net positive is intended to account or the
balance o positive pressures (static, i.e., height, and absolute
pressure above the vapor pressure o the uid) and negative
pressures (losses) attributable to uid ow. Eectively, NPSH
is the dierence in pressure o rerigerant at the pump suc-
tion and the rerigerants saturation pressure. An NPSH o 0
or a pump attempting to move a volatile liquid rerigerant
indicates that the liquid will ash to a vapor state as it moves
into the pump.Two types o NPSH that need to be considered to ensure
proper pump operation: net positive suction head required
or NPSHr and net positive suction head available or NPSHa.
NPSHr is the minimum net positive suction head required to
prevent the liquid rerigerant rom ashing to a vapor. It is a
characteristic o each given pump and varies with the pumps
operating point (head and ow) as provided by the pump man-
uacturer.
NPSHa is the available net head at the pump suction ac-
counting or those actors that eectively increase head (static
head to the liquid elevation ahead o the pump suction, sub-
cooling o liquid rerigerant within the vessel) and decrease
head (rictional losses, heat gains, and orm losses). To pre-
vent pump cavitation, the NPSH available to the pump must
be greaterthan the minimum requiredby the pump.
NPSH NPSHa r> (4)
I a rerigerant pump is cavitating, several options can rem-
edy the situation. First, determine the pump impeller diameter
rom original equipment installation documentation or rom
the data tag on the pump. Next, read the pressure on the dis-charge side o the pump during operation and compare with
the vessels pressure to determine that the pump is developing
a pressure rise and note the magnitude (estimated by dier-
ence between the gauge reading and the vessel pressure). Then
obtain the pump curve or that specifc model pump and im-
peller diameter (see the next section).
With this inormation, look at the manuacturers pump
curve and determine the pumps ow rate and the required
net positive suction head (NPSHr) corresponding to that op-
erating point on the pump curve. I the pump is cavitating
due to operating out on the curve (toward the right side o
the pump curve), one approach to cure this cavitation is to
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Photo 1: Liquid recirculator package showing excessive ice ac-
cumulation on insulation indicating the failure of the insulation sys-
tem with corresponding increases in liquid refrigerant heat gain.
reduce down (close) the hand-expansion valves on the liq-
uid overed evaporators being served by the pump. Adjusting
down on, or more, hand-expansion valves will increase the
discharge head on the pump; thereby, decreasing its ow rateand the corresponding NPSHr. This process should ocus
on the largest evaporators frst since they have the greatest
impact on liquid demand. I cavitation-ree pump operation
cannot be achieved by this approach and the recirculator is
equipped with a capacitance probe or liquid level sensing/
control, it may be possible to raise the vessels liquid operat-
ing level to increase the available net positive suction head
(NPSHa). However, raising the operating level will reduce
the capacity o the recirculator vessel or surge (i.e., unex-
pected return o liquid) rom the system. Another possible
reason or pump cavitation is excess parasitic heat gain be-
tween the vessel and the pump suction. Visible rost on thesuction piping, as shown in Photo 1 is an indication o a
loss o insulation integrity, which increases the heat gain to
the rerigerant. To remedy this it will require reinsulating the
pump suction piping.
The Pump Curve
A pump curve is a compact, graphical representation o
a pumps perormance (Figure 3). Lets review the basics
o reading a pump curve or a liquid rerigerant pump. In
this case, we will consider an open-drive pump operating
with ammonia (specifc gravity o 0.7). The horizontal axis
o the pump curve shows the pumps developed ow rate
Cavitation is the formation of vapor bubbles within a
pump followed by their rapid collapse. The formation and
collapse of vapor bubbles produces a distinct audible sig-
nature that sounds as if the pump is circulating gravel.Although cavitation can occur in the process of pumping
any liquid, refrigerant pumps are more susceptible to cavi-
tation because uid being pumped is at or near saturation
conditions (i.e., its boiling point). When cavitation occurs,
the pump loses its ability to consistently move liquid and
to develop pressure lift. The loss of pump capacity (ow)
can starve evaporators leading to a loss of refrigeration
capacity. The action of vapor bubbles collapsing in the
pump causes erosion of the pump impeller with subsequent
degradation of pump capacity and efciency. For semi-
hermetic pumps, the reduction in refrigerant ow can result
in a reduction of electric motor cooling leading to prema-ture motor failure.
Preventing cavitation requires keeping the pressure of
the liquid refrigerant above the saturation pressure cor-
responding to the refrigerants temperature as it enters
the pump. This is accomplished by ensuring the refriger-
ant being pumped has adequate net positive suction head
(NPSH).
What Is Cavitation?
expressed in gal/min. The vertical axis o the pump curve
shows the generated pump pressure dierential (head) ex-
pressed in psi.
The lines that project horizontally rom the vertical axis to
the right sloping downward represents this models pump per-
ormance with varying, but discrete, impeller diameters rang-
ing rom 8 in. (200 mm), the lowermost curve, to 10.375 in.
(264 mm), the uppermost curve. The dashed lines that run on
a diagonal rom upper let to lower right represent the required
pump power ranging rom 1.5 to 5 hp (1.12 to 3.73 kW). The
semicircular lines represent the pump efciency range rom50% to 65%.
Below the upper portion o the plot (i.e., the pump curve)
is an additional plot o the pumps NPSH requirement with
a 10.375 in. (264 mm) impeller diameter operating with
a discharge pressure o 23.7 psig (265 kPa) connected to
an ammonia recirculator operating at a pressure o 8.8 in.
Hg (40F [40C]). Since the system is operating in a
vacuum, we must frst determine the pressure developed by
the pump. The saturation pressure in the recirculator ves-
sel is 8.8 in. Hg (40F [40C]) or 4.3 psig; thereore,
the total head or pressure developed by the pump is 28 psi
(294 kPa). The ow delivered at that pressure dierential(i.e., head) can be determined by drawing a horizontal line
rom the 28 psi (294 kPa) hash mark to the 10.375 in. (264
mm) diameter impeller pump curve (blue line). The inter-
section o this pressure with the pump curve represents the
pumps operating point. Projecting a vertical line rom the
pump operating point down to the horizontal axis gives the
ow delivered by the pump. In this case, the ow is 140
gpm (31.8 m3/h). From this operating point, other operat-
ing characteristics o the pump can be obtained such as the
pump efciency (~64%) and operating power required (~4
hp [~3 kW]). The last piece o inormation is the NPSHr,
which is 2 t (0.61 m) in this case.
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4 2 A S HR A E J o u r n a l a s h r a e . o r g A u g u s t 2 0 1 1
Other Considerations
Minimum Flow Protection.While operating, liquid reriger-
ant pumps always need to move some amount o liquid to avoid
orming vapor within the pump due to heat addition rom the in-
efciency o the pump or motor (i its a semi-hermetic confgu-
ration). The added heat at low ow conditions can also cause the
liquid rerigerant in the pump to boil, leading to cavitation. To
prevent this orm o cavitation, a bypass (or minimum ow)
line rom the pump discharge back to the pumped recircula-
tor or recirculated liquid return is installed with an orifce (or
metering valve) set at a manuacturers required minimum ow.
During normal operation, the heat gain rom the pump powerinput is small, or the example we used earlier the heat gain
would result in a temperature rise o 0.18F (0.1C).
Presence of EPRs on Pump Discharge Pressure Require-
ment. Pumping liquid to evaporators equipped with evapora-
tor pressure regulators (EPRs) requires special attention. First,
the liquid eed pressure at the evaporator, and thus the pump
discharge pressure, must be higher than the EPR set pressure
to get liquid into the evaporator. Second, it is important to re-
alize that the pumped liquid rerigerant being supplied to an
EPR-ftted evaporator is subcooledas it enters the evaporator.
The liquid rerigerant entering the evaporator is essentially at
the temperature o the liquid in the pumped recirculator (plus a
slight temperature rise due to heat gain in the pump and in the
liquid piping). Since the liquid rerigerant supply temperature
is below the saturation temperature corresponding to the EPR
pressure setting, the liquid rerigerant entering the evaporator
has to absorb heat to sensibly raise the rerigerant temperature
until it reaches its saturation temperature corresponding to the
operating pressure prior to its boiling. Evaporators operating
with liquid rerigerant supply temperatures more than 10F
(5.6C) below saturation temperature will likely result in poor
evaporator perormance.
Static Head Requirements in Open Loops (Why VFDs
Are Not Great For Refrigerant Pumps).Another issue withopen loop pumping circuits is that the static head requirement
sets the minimum discharge pressure to get ow to the evapo-
rators. This means that beore there is any ow to the evapora-
tors, the pressure required to lit the liquid to the roo (or to the
elevation o evaporators) must be overcome. I the pressure
generated by the pump is not sufcient, the pump will operate
at its minimum ow with a column o liquid standing in the
riser supplying liquid rerigerant to the roo. In other words,
the pump is not capable o delivering ow to the evaporators.
This is the main reason that variable requency drives (VFDs)
are generally not suitable applications or rerigerant pumping
in liquid overeed applications. Note that a 50 t (15.24 m) rise
Figure 3: Pump curve for a liquid refrigerant pump.
gpm
10.375 in. 50
50
55
55
60
62 64 65
1.5 hp55
50
2 hp
64
6260
55
50
3 hp
8 in.
35
30
25
20
15
10
5
0
NPSHr
(ft)
Head
(psi)
40
30
20
10
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210
5 hp
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210
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A u g us t 2 0 1 1 A SHR A E J our n a l 4 3
in an ammonia liquid line requires a pressure dierence o
approximately 15 psi (103 kPa) just to lit liquid to the roo.
Hydrostatic Lock-Up. Hydrostatic lock-up is the trapping
o subcooled (or pressurized) liquid rerigerant in a fxed vol-
ume and exposing that trapped volume to a heat source. As the
trapped liquid absorbs heat, it causes an increase in temperature,which causes the trapped liquid to volumetrically expand leading
to a substantial increase in rerigerant pressure and an increased
likelihood o component or equipment ailure. It is important
to identiy locations within a rerigeration system that can trap
liquid and provide suitable means o overpressure protection.
ASHRAE Standard 15-2010, Safety Standard for Refrigerating
Systems, provides requirements or protection rom hydrostatic
lock-up. The liquid eed valve train on an overed evaporator is a
location where hydrostatic lock-up is only a danger during main-
tenance procedures; thereore, can be eectively managed with
proper procedures and employee training. However, the liquid
supply piping between the pump discharge check valve and the
liquid eed solenoids on the evaporators must be protected rom
hydrostatic lock-up in the event o a plant-wide power ailure.
No amount o procedures or training can mitigate this hydraulic
lock-up scenario; thereore, the installation o a hydrostatic re-
lie device downstream o the pump discharge check valve piped
back to the recirculator vessel is required.
Summary
This article introduced concepts that are distinctive to
pumping rerigerants with centriugal pumps. In industrial
rerigeration systems, the use o centriugal pumps or sup-
plying low temperature liquid rerigerant to loads has be-
come quite common. Although a simple concept, the suc-cessul design, installation, and operation o centriugal
liquid rerigerant pumps does require care and attention to
a number o details. One o the most common operational
problems ought in the feld is rerigerant pump cavitation.
In a number o installations, we have ound the contributing
or root cause to cavitation is the excess ow o liquid through
the pump due to hand-expansion valves on individual evapo-
rators being set too ar open. When cavitation occurs, cool-
ing capacity is lost and the likelihood o premature pump
(or motor) ailure increases. We hope this article helps you
understand and troubleshoot potential problems that may ex-
ist in rerigerant pumping systems.
References
1. 2010 ASHRAE HandbookRefrigeration.
2. Stoecker, W.F. 1998.Industrial Refrigeration Handbook. McGraw
Hill Publishers.
3. Chaurette, J. 2011. Personal communication.