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Throttling valves
The main purpose of the throttling device in a refrigeration system is to control the refrigerant flow rate based on the difference between evaporating and condensing pressure. Depending on the type of system this flow is feeding the evaporator as to keep the refrigerant flow tuned with the required cooling capacity (DX systems) or to keep a constant refrigerant level (pump circulated systems)
The refrigerant flows thanks to the pressure
difference between condenser and evaporator
and is controlled mainly to the degree of
opening of the throttling valve. Depending on
the type of valve the opening is controlled in
different ways, maintaining the correct flow
under the influence of the above mentioned
pressure difference that changes (normally by a
small extent) during the system operation,
according to the working conditions and the
loads of the evaporator (heat source) and the
condener (heat sink) ( mainly temperatures and
flow rates).
An example can help to clarify the throttling
device duty: Let us consider a direct expansion
(DX) type refrigeration system equipped with a
compressor, running at a fixed speed under
stable conditions. Let’s assume that, at a given
moment, the cold store requires more cooling
capacity, because of changed cooling load. For
the sake of simplicity, let’s also consider a
compressor without any automatic part loading
device or system.
Given the previous hypotheses, the refrigeration
system spontaneously adapts itself to the
changed working conditions. In fact, the increased
heat load for the evaporator heat exchanger brings
about an increased amount of vapour that
generates in the evaporator itself. Accordingly, also
the evaporation pressure increases inside the heat
exchanger. In this way, also the density of the
refrigerant at the compressor suction port tends to
increase (if constant vapour superheat at the
evaporator outlet is considered).
Since we are considering a volumetric compressor
at constant rotating speed, also the refrigerant mass
flow rate increases due to the higher density of the
sucked of refrigerant gas. Just what we are looking
for to obtain a higher cooling capacity.
Now the achievement of a new stable working condition fulfilling the cold store cooling load requirements is possible only if the throttling device is able to increase the mass flow rate exactly to the value that is elaborated by the compressor: Otherwise, the compressor would progressively
empty the evaporator, causing the system to work
not properly.
Furthermore, as pointed out also in the “Heat
exchangers” Info Pack, the throttling device strongly
affects the heat transfer effectiveness of the
evaporator in DX type systems, because the
expansion valve has to control the super heat of the
refrigerant gas leaving the evaporator in order to
avoid liquid refrigerant to leave the evaporator (and
be sucked in by the compressor), still must evenly
distribute the refrigerant inside different parallel
circuits of the heat exchanger: An excessive vapour superheat caused by a not proper setting of the throttling valve will cause a part of the circuit to be not optimally used.
The throttling device strongly affects the heat transfer effectiveness of the evaporator.
ICE-E
INFORMATION
PACK
In the present Information Pack, only constant level
valves and thermostatic valves (mechanical and
electronic) are considered. Other type of throttling
devices that are not able to change the own settings to
follow the changed cooling load (and so the throttling
device itself affects the system working conditions) are
the constant pressure valves and the capillary tubes.
The use of these last two types of expansion devices is
rather limited in cold stores.
Constant level valves These types of valves are used with flooded
evaporators, i.e. heat exchangers with the refrigerant
side heat transfer surface completely flooded by liquid or
pump separator vessel (“surge drum”) in case of pump
circulated system. In both cases only saturated vapour
conditions are are leaving the evaporator. The
compressor is sucking from an adequate void separator
volume ensuring no liquid is led to the compressor
suction.
Constant level valves are found in two configurations:
• Low pressure valves
• High pressure valves
The working principle of the low pressure valve is rather
simple: It is a float valve that changes the opening percentage to keep the level of refrigerant inside the evaporator. Since the
liquid/vapour interface inside the evaporator shell is
rather agitated, normally the valve is installed in a
chamber in parallel to the evaporator to avoid excessive
wear of the valve mechanisms (see figure 1). It is a
common practice to install a solenoid valve before the
float valve, to ensure tight closing of the refrigerant, when
needed.
High pressure float valves work attempting to keep constant the liquid level inside a small cell fed by the saturated liquid condensate drained from the condenser. In this case, contrary to the
low pressure float valve, the valve closes if the liquid
level decreases. The high pressure float valve can be
installed also above the condenser if a suitable “out-
gassing” tube is installed, according to figure 2.
This out-gassing tube avoids also the entrapment of non-
condensable gases that could block the evaporator
feeding. The valve working principle is the following: In
case that the liquid level inside the evaporator decreases
because of increased cooling load, the refrigerant charge
removed by the compressor tends to accumulated inside
the condenser (in fact the compressor elaborates, for a
while, more refrigerant mass flow rate, in comparison to
the flow rate that is flowing through the valve).
The increased liquid level inside the condenser forces the
valve to open, thus annulling the evaporator liquid level
change.
ICE-E INFO PACK
At the contrary to the low pressure float valves, high pressure float valves close if the liquid level decreases.
Figure 1. Low pressure float valve feeding: a) a shell and tube evaporator; b) a separator for ammonia plant. 1) expansion valve; 2) feeding pipe; 3) auxiliary hand settled valve; 4) filter.
Figure 2. High pressure float valve. 1) condenser; 2 and 2’) float valve; 3) out-gassing pipe; 4) separator; 5) evaporator; 6) stem; 7) orifice; 8) float.
.
ICE-E INFO PACK
High pressure float valve present some advantages in
comparison to low pressure float valves and so the use of
high pressure float valves is advisable when possible. The
main advantages are:
• There is no need to install a liquid receiver at the
bottom of the condenser. This high pressure reservoir
is needed with low pressure float valves to
compensate the changes in the refrigerant mass
collected inside the evaporator (depending on the load)
and the condenser (depending on the condensing
temperature). These changes are caused by the
variation of the average density of the refrigerant inside
the evaporator. The average density changes mainly
because of shifts in average refrigerant vapour quality
induced by heat flux changes because of varied
cooling load. The mass variation in the condenser (and
high pressure piping) is due to the changes in gas
density when the condensing temperature and
pressure changes
• Reduced overall refrigerant charge.
• The high pressure valve operation is “smoother” than
the low pressure one, so there is no need of installing
(and tuning) in series other calibration valves
(manually operated and set) as sometimes happens
with low pressure float valve.
An option is to replace the float valve with a float switch that, according to the measured liquid level, creates a feedback signal that operates a suitable two-ways solenoid valve. This latter strategy allows remote
automatic operation and monitoring of the refrigeration unit.
As the main limitation to the use of high pressure float valve, one should consider that it is not possible to feed more than one (low pressure) liquid receiver from each condenser. One liquid receiver can obviously
feed several evaporators, but in case of remote installation
of the evaporators, the investment costs for piping will
increase markedly, depending also on the type of circulation
(natural or pumped) of the refrigerant, because of the lower
density of two-phase refrigerant in comparison with
saturated liquid. A fair comparison of investment and
operational costs may advise the installation of several liquid
separators, each installed close to a single evaporator fed by
a dedicated low pressure float valve.
The installation of several liquid separators is mandatory for systems with one compressor and different working evaporation pressures (i.e. frozen and chilled
stores). A constant pressure valve (suction pressure
regulator) must then be installed at the outlet of the higher
pressure evaporators to equalize the pressure level along
the suction line.
Thermostatic valves
This kind of valves are used in DX systems and
regulate the refrigerant flow rate by controlling the
refrigerant superheat at the evaporator outlet. The
refrigerant superheat is defined as the difference
between the vapour temperature at the evaporator
outlet and the saturation temperature at the actual
refrigerant pressure. Obviously, this type of valve is
used in dry-expansion evaporators, i.e. the final part
of the evaporator is not wetted by liquid, and only
vapour flows inside the tube.
Two type of thermostatic valves exists
• Mechanical
• Electronic
The mechanical thermostatic expansion valve consists of a body, a stem connected to a spring and a metallic diaphragm and a sensing system composed by a bulb and a capillary tube connecting the bulb with the metallic bellow.
The working principle of this kind of valves is
described in the e-learning section of the ICE-E
website.
The thermostatic expansion valve can be selected
when the following are known:
• Refrigerant
• Evaporator capacity
• Evaporating pressure
• Condensing pressure
• Subcooling
• Pressure drop across valve
• Internal or external pressure equalization.
Given the number of variables, a proper selection can
be done only by a well trained technician or designer.
A not proper choice will strongly affect the system operation, with risk of damage of the compressor or of remarkable increase of energy consumption. The problem is rather complex
and cannot be treated exhaustively in short
document, like an info pack. Here just a simple
example is provided, using the Minimum Stable
Signal (MSS) concept.
A detailed analysis can be found for example in
Langer et al. (1998). The valve needs a minimum
stable signal from the evaporator to work properly.
High pressure float valve present some advantages in comparison to low pressure
float valves and so the use of high pressure float valves is advisable when possible.
ICE-E INFO PACK
It is worth noticing that the position and the slope of
the curve indicating the boundary
layer between stable and unstable working
conditions for the thermostatic valve (MSS line) in
the diagram plotting the rated cooling capacity
against the vapour superheat, is not linked to the
valve but only to the evaporator design, working
conditions and boundaries (parasitic effects due to
the evaporator installation).
In figure 3, the cooling capacity vs. superheat plane
is divided in two regions, stable and unstable
operation by the MSS line. Furthermore, the working
lines of three different valves are reported (1 is the
largest valve, 3 is the smallest). Valve 2 is optimized
for the chosen evaporator and will ensure the
minimum possible superheat in all the working
conditions. Valve 3 line falls entirely in the stable
region and so it will work stable, but with a too high
superheat, causing an excessive portion of the
evaporator to work dry and thus reducing the
evaporation temperature as a smaller portion (area)
of the evaporator shall do the whole work. The
consequence will be increased energy consumption
because of increased compression work. Valve 1 is
too large and will cause instable operation of the
evaporator (“starving” followed by “overfeeding”)
with consequent liquid flowing to the compressor,
causing possible failure of the compressor (Granryd
et al. 2002).
Also for the installation of the thermostatic valve
particular care should be given. The bulb should be in good thermal contact with the exit of the evaporator, avoiding parasitic effects of external inputs (heat) different from the superheated vapour temperature. The
suppliers mounting recommendations shall be
followed as to avoid possible effects of the
refrigerant/oil liquid mixture flowing at the bottom of
the pipe.
In case of air draughts, it is advisable to insulate the
bulb. Another common practice is to avoid installing
the bulb close to large mass devices,
since this may affect the pipe surface
temperature (that is the one measured by the
bulb) because of heat conduction through the
pipe wall.
Electronic thermostatic valves
Getting back to previous considerations about
MSS, in principle a suitably controlled and sized
electronically controlled expansion valve is able
to work better than valve 2 in figure CC, at any
working conditions.
It comes out that the dry expansion evaporator always should work with the minimum necessary superheating and with the highest (relatively to its heat transfer area) evaporation pressure. The consequent energy consumption optimization is obvious.
Normally the valve is a pulsing or modulating
solenoid valve. For a proper control of the
valve, at least two signals are needed. The
most obvious one is the measurement of the
temperature of vapour at the evaporator outlet.
Other possible signals are the evaporating
temperature or pressure, or the cold store
temperature, or the discharge temperature or
the condensing pressure.
The optimal working of the valve, in terms of
safe compressor operation and minimum
energy consumption, can only be achieved if a
suitable electronic controller is used. Often, the
same electronic controller used for the
expansion valve can be used also for
controlling condenser and evaporator fan
speed, compressor speed, defrosting cycle etc.
The optimal working of an electronic valve, in terms of safe compressor operation
and minimum energy consumption, can only be achieved if a suitable controller is used.
Reference
Lenger M.J., Jacobi A.M., Hrnjak P.S.
1998, Superheat stability of an evaporator
and thermostatic expansion valve, ACRC
TR-138, retrived on line
https://www.ideals.illinois.edu/bitstream/ha
ndle/2142/11847/TR138.pdf
Granryd E. et al, 2002, Refrigerating
Engineering, Royal Institute of
Technology, KTH, Sweden
For more information, please contact: Claudio Zilio ([email protected])
Figure 3. Matching between evaporator and thermostatic expansion valve (from Granryd et al. 2002)