Valv_Termo

A thermostatic expansion valve is a passive device that is used to control the flow

of refrigerant to an evaporator (a heat exchanger that allows the refrigerant to accept

a heat transfer from a refrigerated space)

Saturated (or nearly saturated) refrigerant enters the thermostatic expansion

valve through the liquid line. The flow rate of refrigerant is controlled by the position

of the valve seat. The refrigerant subsequently flows through the evaporator

at nearly constant pressure. The heat transfer causes the refrigerant to completely

evaporate and so it leaves the evaporator as superheated vapor at temperature

Tevap,out . The job of the thermostatic expansion valve is to regulate the refrigerant

flow so that it leaves at a precisely controlled temperature even if the load on

the evaporator (i.e., the heat transfer to the refrigerant) changes. If the load on the

evaporator decreases then Tevap,out will tend to decrease and the thermostatic valve

should act to reduce the refrigerant flow rate. Alternatively, if the load increases

then Tevap,out will tend to increase and this should result in the thermostatic valve

increasing the flow rate.

A bulb filled with refrigerant is attached to the evaporator outlet. The bulb

is connected, through a tube, to the head space of the valve. The pressure in the

bulb and the head space are the same; therefore, an increase in Tevap,out tends to

increase the pressure in the head space of the valve which causes the diaphragm

in the thermostatic expansion valve to move downward and open the valve. If

Tevap,out decreases then the opposite happens; the head space pressure decreases

which, along with the force exerted by the spring, causes the diaphragm to move

up thereby closing the valve.

Figure 2 provides a simplified schematic of the thermostatic expansion valve

and bulb. The bulb volume is Vbulb = 10 cm3 and the head space volume is Vhead

= 2 cm3 , the volume of the connecting tube can be neglected. The temperature of

the refrigerant contained in the head space is equal to the evaporator inlet temperature,

Tevap = –5 ºC. Under nominal operating conditions, the temperature of

the refrigerant leaving the evaporator (and therefore the temperature of the refrigerant

contained in the bulb) is Tevap,out,nom = –2 ºC and the bulb and head space

pressure are both Pbulb,nom = 700 kPa. The refrigerant is R22.

What is the total mass of refrigerant used to charge the bulb and the head

space?

The evaporator outlet temperature rises from Tevap,out,nom = –2 ºC to Tevap,out =

–1 ºC. Determine the increase in bulb pressure that results from this change.

The spring constant is K = 72 kN/m and the area of the diaphragm that

the bulb pressure acts on is A = 1 cm2 . Determine the change in the valve

opening that is caused by the increase in evaporator outlet temperature (from

its nominal value of Tevap,out,nom = –2 ºC to its new value Tevap,out = –1ºC).