RACT Unit 24

Question 1

Example of a TXV functioning with an internal equalizer: Load changes

1. Load changes with _____ added to the cooler
   1A) addition of warm food ______ evap load
2. Load changes with food ______ from the cooler
   2A) Removal of food ______ load on the evap

Answer 1

1. food
   1A) increases
2. removal
   2A) reduces

Question 2

TXV Functioning with External Equalizers:

1. Designed and applied with pressure drop from the _____ to the ______
2. Used to compensate for, not get rid of, pressure drops from the inlet to the outlet of the ________
3. Must always be installed when a ref _________ is used because of the distributors large pressure drop characteristics

3A) Excess pressure drop will cause the valve to _______ the coil of refrigeration

Answer 2

1. inlet and outlet
2. evaporator
3. distributor

3A) starve

Question 3

TXV Response to Load Changes:

When load increases:
1. Refrigerant boils _______ and the suction line temp increases
1A) Valve ______ to feed more refrigerant to the evaporator

When load decreases:
2. Refrigerant takes _________ to boil
2A) Valve ______
to feed less ref to the evap

Answer 3

1. faster
   1A) opens
2. longer
   2A) closes

Question 4

TXV Selection:

1. Each TXV is designed for a __________ refrigerant or group of refrigerants

Many newer TXV are designed to;
2. Handle newer, alternative __________ and blends
2A) Be used with more than one ___________

3. capacity of the system is very _________

Answer 4

1. particular
2. alternative refrigerants
   2A) more than one refrigerant
3. important

Question 5

Balanced-Port TXV:

1. Designed to operate in ____ ambient conditions

Used if any of the following conditions exist;
1A) Large varying ______ pressures
1B) Large varying _______ drops across the TXV
1C) Widely varying ________ loads
1D) Very low _______ line temps

2. Have larger than normal ________

Answer 5

1. low ambient conditions 
A) varying head pressures 
B) pressure drops
C) evap loads 
D) liquid line temps 

2. orifices

Question 6

Pressure Limiting TXV:

1. Allows evaporator pressure to only reach a ___________ pressure

A) If the evaporator pressure exceeds this pressure, the valve will _______ the flow of liquid

B) Desirable on low -_________ applications

Answer 6

1. predetermined

A) throttle the flow of liquid

B) low-temperature applications

Question 7

Servicing the TXV:

Things to be considered

1. Type of Fastener (______, solder, or ______)
2. Location of ______ for service
3. Expansion valve bulb _______
4. Valve has moving parts that are subject to wear; exact __________ is best
5. ________ a TXV may cause problems

Answer 7

1. flare, solder or flange
2. valve
3. Location
4. replacement
5. Adjustment

Question 8

Installing the Sensing Element:

1. Follow __________ ____________
2. Bulb should be mounted at the end of the evap on the _________ line
3. For small suction lines, bulb is usually secured to the _____ of the line
4. Recommended location for large suction lines is near the bottom of the line in the __________ run

Answer 8

1. manufacturers instructions
2. suction line
3. top of the line
4. bottom of the line in the horizontal run

Question 9

Stepper Motor Expansion Valves:

1. Uses a small motor to control the valve ____
   A) valve port controls evap ________
2. Temp sensor sends a signal to the ________
3. Controller sends a signal to the _______
4. Motor turns a fraction of a rotation for each _______
5. Motor is powered by signals that change in _________

Answer 9

1. port
   A) superheat
2. the controller
3. the motor
4. signal
5. polarity

Question 10

Stepper Motor Expansion Valves: Controller Functioning:

1. The controller consists of many ________ for each switching function
2. A small __________ in the controller has a programmed algorithm which can control or sequence the electrical signal
   A) Algorithms, or sets of instructions,
   often referred to as software, reside in the _________
3. ________ loops let the controller know the process needs to be changed or modified

Answer 10

1. transistors
2. microcomputer
   A) microprocessor
3. Feedback

Question 11

Algorithms and PID Controllers:

Proportional Controllers:

1. Generate an _______ output signal
2. difference between actual superheat and superheat set point is the ______ or “error”

Integral controller modes:

3. help _____ the offset or error
4. calculates error size and length of ______

Derivative (differential) controller modes:
5. Estimate ______ or rate of change of temp/time curve

Answer 11

1. analog
2. offset or error
3. reduce
4. time
5. slope

Question 12

Automatic Expansion Valve:

1. Meters the refrigeration to the evaporator by using a _______ sensing device
2. Maintains constant pressure in the _________
   A) When the pressure _____ the valve ______
   B) Turning the adjustment screw into the valve ________ the suction pressure

Answer 12

1. pressure sensing device
2. maintains constant press in the evap
   A) pressure drops valve opens
   B) increases

Question 13

The automatic expansion valve uses the ________ as the sensing element and maintains constant pressure in the _________ but does NOT control _________

Answer 13

diaphragm, evaporator, superheat

Question 14

Automatic Expansion Valve Response to Load Changes:

Responds differently than the TXV to load changes:

1. It actually acts in ________
2. When a load is added to the coil the suction pressure starts to _______
3. When the load is decreases the suction pressure _____ the AVX will start to open and feed more ref to the coil

Answer 14

1. reverse
2. rise
3. falls

Question 15

Special Considerations for the TXV and AXV:

1. Both are _______ devices that allow ____ or ____ refrigerant flow
2. Both need a storage device (______) for refrigerant when it is not needed
3. The ______ serves as both a storage tank as a tank into which refrigerant can be pumped when ________ the system

Answer 15

1. expansion devices, more or less
2. Receiver
3. Receiver, servicing

Question 16

The Capillary Tube Metering Device:

1. Controls ref flow by ______ drop
2. ______ and _______ determine flow at any given pressure drop
3. does not control ______ or _______
4. Used when the load is relatively _______
5. No _______ parts to wear out
6. Oil logging may occur with; Too much _____, wrong oil ______, f______ or, mi_______

Answer 16

1. Pressure drop
2. diameter and length,
3. superheat or pressure
4. constant
5. no moving parts
6. too much oil, wrong oil viscosity, flooding, migration

Question 17

Piston Metering Device:

1. Functions similar to the _________ tube metering device
2. Has a precisely sized hole _____ through it
   A) Controls flow of ref to _________
3. Available in various ______
   A) Can easily be changed to different _____
4. _______ housing can be opened for cleaning
5. Typically used in residential a/c and _____ ______ systems

Answer 17

1. capillary tube
2. bore
   A) evaporator
3. sizes
   A) size
4. Piston housing
5. Heat pump systems

Question 18

Operating Charge for the Capillary Tube System:

1. Cap tube systems are ________ charged
2. Cap tubes sometimes fastened to the _______ line for _______ exchange
3. Responds very slowly to _______ _____ changes or charge modifications
4. Often causes high evap superheated of ___F or 50F under high evap loads

Answer 18

1. critically
2. suction line for heat exchange
3. system load changes
4. 40F