Unit 24 Expansion Devices

Explain what is meant by a maximum operating pressure (MOP) TXV.

Maximum operating pressure = limit the evaporating pressure to prevent the compressor from overloading

EEV's use ..... controllers for their operation

PID = proportional integral derivative

The four types of charges that the TXV uses to control refrigerant flow are ......

The fluid in the expansion valve bulb is known as the charge for the valve - Liquid charge - Cross liquid charge - Vapor charge - Cross vapor charge

The needle and seat of a TXV are normally made of ....

Types of very hard metal such as stainless steel = prevents passing refrigerant from eroding the seat

The three factors that determine the capacity of a TXV?

(1) liquid temp entering TXV (2) evaporator temp (3) pressure drop across TXV

vapor charge bulb

(gas charge bulb) has only a small amount of liquid refrigerant and is called a critical charge bulb. - when bulb temp rises, more and more liquid will boil until there is none left - when this point is reached, an increase in temp will not bring an increase in pressure - the pressure curve will be flat - this acts to limit the Maximum Operating Pressure (MOP)

the colder the liquid temp entering the TXV, the more ......... in tons the TXV will have

capacity

liquid charge bulb

charged with a fluid characteristics of the refrigerant in the system - high temps = high pressure - during defrost, the expansion valve bulb may reach high temps = cause excessive pressure on diaphragm and valve will open wide when bulb gets warm - overfeed evaporator and cause liquid to flood to compressor when defrost is terminated - used for desuperheating in the high side of system

the external equalizer on a TXV is used to ........ for, not get rid of, ........... drops from the inlet to the outlet of the evaporator

compensate pressure

capillary tube systems are ......... charged

critically

MOP (maximum operating pressure) TXV's limit the ......... pressure to prevent the .......... from overloading

evaporator compressor

cross liquid bulb

filled with a fluid that is different from the system fluid so that it can react faster on a rise of evaporator pressure - closes valve fast on rise in evaporator pressure - good for low temp applications - valve closes during off cycle defrost when compressor shuts off and the evaporator pressure rises - this helps prevent liquid floodback

the capillary tube expansion device is a ....... ......... metering device. It is usually made of ....... , with a very small inside diameter and no moving parts

fixed bore copper

as the liquid temp gets colder, less ....... (loss effect) there will be at the entrance to the evaporator

flashing

What is meant by a dual port TXV?

has two independent capacities. Large port = pull downs Small port = holding loads

electronic expansion valves (EEV) use either ...... motors or ...... motors for valve movement

heat step

the expansion device is one of the dividing points between the ..... and ..... sides of the system

high low

continuous overfeeding/underfeeding is called .....

hunting

increase net refrigeration effect = ......... capacity

increase

What is a king valve?

it is a service valve that stops refrigerant from leaving the receiver when the low side of the system is being serviced

dual port TXV have a ....... and ...... port to eliminate overfeeding/underfeeding in high/low loads

large small

Explain why the entering liquid pressure has no effect on the opening force of a balanced port TXV.

liquid pressure is cancelled out because it acts on equal areas but in opposite directions.

the more liquid subcooling there is, the lower the ....... ....... at the evaporator entrance

loss effect

balanced port TXV will not reduce refrigerant flow when ambient temps are ..........

low

balanced port TXV's are used where .....

low ambient temps exist head pressures vary widely evaporator loads vary widely large pressure drops exist across TXV low liquid line temps exist

What condition does the automatic expansion valve maintain the evaporator?

maintains a constant pressure in the evaporator ..... not superheat!!!!!

The TXV responds to an increased heat load by ....

metering more refrigerant into the coil. This causes an increase in suction pressure for the refrigerant boiling at a faster rate

When do some TXV's require an external equalizer?

must always be installed when a refrigerant distributer is used because of the large pressure drop characteristics.

the force inside the expansion valve all work together to hold the ........ and ...... in the correct position so that the evaporator will have the correct amount of refrigerant

needle seat

The bulb of the TXV is mounted on the ....

outlet of evaporator

pressure limiting TXV has another bellows that will only allow the evaporator to build a predetermined ........ , and then will throttle the flow of liquid

pressure

when a TXV or AXV are used, a storage device is required. The ....... stores refrigerant when it is not needed

receiver

the capillary tube system uses a very limited amount of ......... compared to other metering devices and is popular in ....... systems

refrigerant small

the automatic expansion valve responds in ........... to a load change. When the load increases, the AEV ....... the refrigerant. When the load decreases, the AEV ........ the refrigerant

reverse throttles feeds

balanced port TXV is the only valve immune to capacity variations due to head pressure swings as ......... change

seasons

cross vapor charge bulb

similar to vapor charge bulb - charged with different fluid than the refrigerant system - different pressure/temp relationship under different conditions - special valves = special systems

In the evaporator, TXV tries to maintain a constant .....

superheat

the TXV maintains a constant .......... in the evaporator

superheat

three things that determine the TXV capacity?

temp of liquid refrigerant evaporator temp pressure drop across the TXV

What determines the amount of refrigerant that flows through a capillary tube metering device?

the diameter and length of tube

solid state controlled expansion valve uses a ......... as a sensing element

thermistor

solid state controlled expansion valve uses a ....... to monitor the evaporator ........... temperature to control ........... flow to the evaporator

thermistor outlet refrigerant

How does the automatic expansion valve respond to a load increase?

when suction pressure rise .... the AXV will start to throttle the refrigerant, maintaining suction pressure set point

the ......... pressure is the only force that acts to open the TXV

bulb

Which is not a pressure or force acting on a TXV diaphragm? Head, Evaporator, Spring or Bulb

Head

an externally equalized TXV must always be installed when a refrigerant ........ is used because of the large pressure drop

distributor