ICE-E Info Pack 4 Throttling Valves

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.

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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.

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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.

.

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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.

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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)