Characteristics, principle of operation and installation of thermostatic expansion valve


In heat­ing and air con­di­tion­ing sys­tems oper­at­ing under vari­able envi­ron­men­tal con­di­tions, it is absolute­ly nec­es­sary to adjust the pow­er of the exist­ing instal­la­tion. This allows you to main­tain the required tem­per­a­ture and save ener­gy con­sump­tion dur­ing its oper­a­tion. In auto­mat­ic mode, this task is han­dled by a ther­mo­sta­t­ic expan­sion valve. It con­trols the flow of the work­ing medi­um, react­ing to exter­nal tem­per­a­ture changes.

The appear­ance of the ther­mo­sta­t­ic device in the cool­ing sys­tem

Design and principle of operation

In refrig­er­a­tion units and air con­di­tion­ers, a closed cir­cuit is used, through which the refrig­er­ant cir­cu­lates, chang­ing its state of aggre­ga­tion in the evap­o­ra­tor. In heat­ing sys­tems, heat­ing is car­ried out by pump­ing hot liq­uid to ther­mo­cou­ples. Despite the devel­op­ment of var­i­ous alter­na­tive meth­ods of cool­ing and heat­ing, such a scheme of oper­a­tion is the main one.

With a low pow­er of the device, con­stant adjust­ment to exter­nal changes is not required. In low-pow­er cool­ing sys­tems, the role of the reg­u­la­tor is per­formed by a throt­tle from a cap­il­lary tube. Its oper­a­tion does not depend on the per­for­mance of the evap­o­ra­tors and is not able to change the refrig­er­ant lev­el in the cir­cuit.

Heat­ing cir­cuits are equipped with man­u­al con­trollers. In them, the change in the flow of hot liq­uid is car­ried out by turn­ing the han­dle, low­er­ing or rais­ing the restric­tive rod.

Heat­ing man­u­al valve device

In sys­tems where con­stant adjust­ment to chang­ing exter­nal con­di­tions is required, the cool­ing or heat­ing pow­er is adjust­ed by chang­ing the flow of the work­ing medi­um.

The main flow reg­u­la­tor is an expan­sion valve, which stands for ther­mo­sta­t­ic expan­sion valve. This is a direct action device. It does not require any exter­nal ener­gy to oper­ate. The valve reacts to the over­heat­ing of the vapors leav­ing the evap­o­ra­tor. And it, in turn, depends on the load on the cool­ing sys­tem.

An addi­tion­al advan­tage of using expan­sion valves is that the sys­tem is not crit­i­cal to the exact amount of fill­ing refrig­er­ant.

The inter­nal struc­ture of the reg­u­la­tor is shown in the fig­ure.

Clas­sic expan­sion valve for refrig­er­a­tion sys­tems


The main ele­ments of the TRV are:

  • a mem­brane or diaphragm that con­trols the move­ment of the shut­off rod;
  • a cap­il­lary tube with a ther­mal bulb that trans­mits to the device changes in the tem­per­a­ture of the vapors at the out­let of the evap­o­ra­tor,
  • adjust­ing spring for adjust­ing the instal­la­tion lev­el,
  • inlet and out­let fit­tings.

The com­bi­na­tion of a diaphragm, a ther­mal bulb and a cap­il­lary tube is called a ther­moele­ment. It is he who per­ceives the ambi­ent tem­per­a­ture and reg­u­lates the sup­ply of refrig­er­ant.

The prin­ci­ple of oper­a­tion of the valve is to move the mem­brane under the action of three forces:

  • medi­um pres­sure from the bulb,
  • evap­o­ra­tor equal­iz­ing pres­sure,
  • spring action.

After reach­ing a bal­ance between these three force com­po­nents, the diaphragm sets the required amount of refrig­er­ant flow.

Bulb pres­sure = equal­iz­ing pres­sure + spring pres­sure on diaphragm.

With a change in tem­per­a­ture and an increase in the ther­mal load in the evap­o­ra­tor, the heat­ing of the bulb and the pres­sure of the liq­uid fill­ing it increase. Through the cap­il­lary tube, it is trans­ferred to the diaphragm, as a result of which the valve opens and the refrig­er­ant sup­ply to the evap­o­ra­tor increas­es.

The ther­mo­sta­t­ic valve of the heat­ing radi­a­tor is arranged accord­ing to a sim­i­lar prin­ci­ple.

Ther­mo­stat for heat­ing sys­tems


In it, the role of a ther­mal bulb is per­formed by a sen­si­tive ele­ment (float) locat­ed in a cav­i­ty filled with liq­uid or gas. When the tem­per­a­ture changes, the vol­ume of the medi­um decreas­es or increas­es. As a result, the float changes its posi­tion, shift­ing the stem, which changes the valve flow area.

Gas-filled ther­mo­cou­ples are con­sid­ered the most sen­si­tive. They react to tem­per­a­ture changes faster than liq­uid ones. But they also cost more.

Characteristics and types of expansion valves

When choos­ing a device, you need to pay atten­tion to the fol­low­ing para­me­ters:

  • The max­i­mum tem­per­a­ture at which the valve can oper­ate. It can reach 200 °C.
  • The pres­sure of the work­ing envi­ron­ment. Usu­al­ly in the range of 16 — 40 bar.
  • Man­u­fac­tur­ing mate­r­i­al. The body is made of bronze or brass. But stain­less steel valves have the best anti-cor­ro­sion prop­er­ties.
  • TRV per­for­mance. This is the max­i­mum flow allowed by a ful­ly open valve. It must cor­re­spond to the capac­i­ty of the refrig­er­a­tion unit.
  • The diam­e­ter of the inlet and out­let fit­tings must cor­re­spond to the pipelines of the entire reg­u­lat­ed sys­tem.

Expan­sion valves for refrig­er­a­tion and air-con­di­tion­ing are dis­tin­guished by the type of sup­ply of equal­iz­ing pres­sure from the evap­o­ra­tor.

Internal adjustment

The pres­sure is trans­ferred under the low­er edge of the diaphragm through the machined gaps around the stem. This type of valve is used only for sin­gle pass evap­o­ra­tors with low hydraulic resis­tance.

The pres­sure of the refrig­er­ant on the mem­brane is car­ried out before it enters the evap­o­ra­tor.

External adjustment

In a more advanced con­trol sys­tem, equal­iz­ing pres­sure enters the valve direct­ly from the out­let of the evap­o­ra­tor. To sup­ply this pres­sure, an addi­tion­al inlet pipe is pro­vid­ed in the hous­ing, which ensures the flow of refrig­er­ant from the evap­o­ra­tor under the ther­moele­ment mem­brane. In this case, the sub­di­aphrag­mat­ic cav­i­ty is iso­lat­ed by a sep­a­rate seal from the out­let pres­sure of the valve.

Scheme of sup­ply­ing pres­sure to the ther­moele­ment with exter­nal equal­iza­tion


Such reg­u­la­tors are applic­a­ble to work with any meth­ods of cool­ing and with dif­fer­ent types of refrig­er­ant. But they can­not be used accord­ing to the scheme with inter­nal adjust­ment. The equal­iza­tion tube must be con­nect­ed to the evap­o­ra­tor out­let. It can­not be silenced.

Meth­ods for con­nect­ing valves to sys­tem pipes:

  • using a thread­ed con­nec­tion;
  • through the flange
  • per­ma­nent weld­ed joint.

The ther­mo­sta­t­ic expan­sion valves of heat­ing sys­tems dif­fer in shape depend­ing on their loca­tion on the pipe. Straight or axi­al cut into a flat sec­tion of the pipeline. Cor­ner options are installed in places where the pipe bends and change the direc­tion of flu­id move­ment.

Angle ther­mo­sta­t­ic valve with air vent

Mounting Features

The instal­la­tion of ther­mo­sta­t­ic expan­sion valves for heat­ing and air con­di­tion­ing should be con­sid­ered sep­a­rate­ly, since the require­ments and rec­om­men­da­tions in these cas­es are dif­fer­ent.

Installation in the air conditioning system

A gen­er­al view of the inclu­sion of a tem­per­a­ture con­trol device in the pip­ing scheme for refrig­er­a­tion units is shown in the fig­ure.

A typ­i­cal scheme for installing a ther­mo­sta­t­ic expan­sion valve in a cool­ing sys­tem


Dur­ing instal­la­tion, the fol­low­ing rules must be observed:

  • The valve is installed on the line in the imme­di­ate vicin­i­ty of the evap­o­ra­tor. The part of the hous­ing with the diaphragm must be ver­ti­cal.
  • The instal­la­tion loca­tion of the bulb is as close as pos­si­ble to the out­let of the evap­o­ra­tor. But it should be installed only on a hor­i­zon­tal sec­tion of the pipeline. The loca­tion of the cylin­der on a ver­ti­cal pipe will lead to mal­func­tions of the ther­mo­stat, espe­cial­ly at the time of start­ing the air con­di­tion­er.
  • The bulb must fit snug­ly against the out­let pipe of the evap­o­ra­tor. Loca­tion — only on top of the pipe, it is unac­cept­able to install a bulb under the pipe or on the side.
  • Fas­ten­ing to the pipe must be car­ried out with a spe­cial clamp includ­ed in the ther­mo­sta­t­ic valve kit. Oth­er meth­ods do not pro­vide reli­able con­tact, which ulti­mate­ly leads to a dis­tor­tion of the pres­sure trans­mit­ted to the valve ther­mo­cou­ple.
  • For devices with exter­nal pres­sure equal­iza­tion, it is manda­to­ry to con­nect the equal­iza­tion pipe to the evap­o­ra­tor out­let. The out­let must be car­ried out from the top of the out­let pipe at a dis­tance of at least 100 mm from the bulb and at the same dis­tance from the oil lift­ing loop.

If it is not pos­si­ble to install a ther­mal bulb on a hor­i­zon­tal sec­tion of the pipeline, then it is allowed to mount it on a ver­ti­cal pipe. But the direc­tion of the refrig­er­ant must be from top to bot­tom, and the cylin­der is fixed with a cap­il­lary tube upwards.

Installing a thermostatic valve in heating pipes

The main ele­ment of the cen­tral­ized sys­tem is a heat radi­a­tor or con­vec­tor. It is most con­ve­nient to adjust the amount of hot liq­uid flow in each device sep­a­rate­ly.

Con­nec­tion dia­gram for ther­mo­sta­t­ic valves in the heat­ing sys­tem


For reli­able reg­u­la­tion of heat flow, two devices are installed on each radi­a­tor — at the inlet and out­let. In sin­gle-pipe sys­tems, where the move­ment of the work­ing medi­um through the ele­ments is sequen­tial, it is nec­es­sary to install bypass­es. These are bypass tubes that ensure the func­tion­ing of the line in case of block­age or clog­ging of one of the radi­a­tors.

Possible installation errors and malfunctions

The main prob­lems in the oper­a­tion of the expan­sion valve arise due to the incor­rect instal­la­tion loca­tion of the valve itself or the bulb. The accu­ra­cy of adjust­ment can also be affect­ed by minor fac­tors when fix­ing the ele­ments of the device.

Pos­si­ble errors when installing an expan­sion valve for a refrig­er­a­tion unit

One of the com­mon prob­lems is the inac­cu­rate trans­fer of the required pres­sure by the bulb to the ther­mo­cou­ple. The rea­son for this may be its poor con­tact with the out­let pipe of the evap­o­ra­tor. The instal­la­tion site must be care­ful­ly cleaned and cov­ered with heat-con­duct­ing paste. Do not place the bulb on the welds con­nect­ing the pipes.

The sen­sor itself must be insu­lat­ed so that ambi­ent air does not affect its tem­per­a­ture.

The com­plete expan­sion of the expan­sion valve is often due to the use of mod­els with plas­tic inter­nals.


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