Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

racm > REF UNIT 24 > Flashcards

REF UNIT 24 Flashcards

1
Q

(24.2) What are the 3 types of expansion devices?

A

thermostatic, automatic, fixed-bore

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

(24.2) What determines the opening and closing of the TXV?

A

the temperature sensing element. An increase in superheat will open the valve, a decrease in superheat will close the valve.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

(24.4) What are the components of the TXV?

A

valve body, diaphragm, needle and seat, spring, sensing bulb and transmission tube.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

(24.7) What are the 3 and sometimes 4 forces that act on the diaphragm?

A

spring, sensing bulb, evaporator pressure, balanced port.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

(24.7) What is the spring also known as?

A

superheat spring

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

(24.10) What is a liquid charge bulb?

A

a sensing bulb containing a fluid with the same characteristics as the refrigerant in the system. There will always be some liquid left in the bulb.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

(24.10) Why are liquid charge bulbs flawed?

A

When the system goes into defrost, the sensing bulb will heat up, causing the TXV to open, causing flooding the evaporator.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

(24.10) What systems are liquid charge bulbs used?

A

ones with very narrow operating ranges.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

(24.11) What is the cross liquid charge bulb?

A

a sensing bulb containing fluid that has different characteristics from system fluid. It has a flatter PT curve than the system refrigerant, so it will close the valve faster and open not as fast.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

(24.12) What is the vapor charge bulb?

A

a sensing bulb with a small amount of liquid refrigerant. Once it all boils off, the valve can not open anymore. This is used to limit the feeding to the evaporator.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

(24.12) What are the other 2 names the vapor charge bulb goes by?

A

critical charge bulb and maximum operating pressure valve (MOP)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

(24.13) What is the cross vapor charge bulb?

A

similar to vapor charge bulb but with a fluid with different characteristics from the system refrigerant. Only applied to very specialized systems.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

(24.15) What are external equalizers for?

A

compensating for pressure drop across the valve. They must always be used when a distributor is used. They provide a line from the suction line straight to the TXV.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

(24.15) What is the result of too much pressure drop across the TXV?

A

the TXV will starve the evaporator.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

(24.18) How do balanced-port TXVs work?

A

They let liquid refrigerant on both sides of the diaphragm of the valve, so they cancel each other out. This is used for systems where the ambient temperature changes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

(24.19) What is the pressure-limiting TXV?

A

a TXV with a bellows that only allows the evaporator pressure to rise to a pre-determined level. Used so that the suction pressure is low during hot pulldown so that the compressor isn’t overloaded

17
Q

(24.22) What is a stepper motor expansion valve (EEV)?

A

uses a small motor to control valve’s port. Controlled by a solid-state controller that has a temp sensor or thermistor (usually a thermistor).

18
Q

(24.22) Where is the algorithm for the stepper motor found?

A

in the microprocessor in the controller.

19
Q

(24.23) What is the P in PID controllers?

A

proportional controllers. Change valve directly based on superheat measurement.

20
Q

(24.23) What is the I in PID controllers?

A

integral controller. measures offset to make the integral controller more accurate.

21
Q

(24.23) What is the D in PID controllers?

A

derivative (or differential) controller. Takes the data of the proportional and integral controllers to make more accurate.

22
Q

(24.24) How do AXV open and close?

A

based on the diaphragm, which senses pressure. It maintains constant evaporator pressure. These valves are adjustable.

23
Q

(24.24) In what systems are AXVs most effective?

A

when the load is constant.

24
Q

(24.24) What is it called when liquid refrigerant enters the compressor’s crankcase and turns to flash gas?

A

flooding the crankcase (or migration when it occurs during offcycle)

25
Q

(24.2) how long should system run before making adjustment to TXV?

A

15 minutes

26
Q

(24.4) What are the 3 methods a TXV can be fastened to a system?

A

flare, flange, or solder.

racm (23 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions