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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

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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.

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3
Q

(24.4) What are the components of the TXV?

A

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

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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.

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5
Q

(24.7) What is the spring also known as?

A

superheat spring

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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.

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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.

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8
Q

(24.10) What systems are liquid charge bulbs used?

A

ones with very narrow operating ranges.

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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.

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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.

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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)

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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.

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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.

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14
Q

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

A

the TXV will starve the evaporator.

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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.

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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)

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