RACT Unit 24 Flashcards
Example of a TXV functioning with an internal equalizer: Load changes
- Load changes with _____ added to the cooler
1A) addition of warm food ______ evap load - Load changes with food ______ from the cooler
2A) Removal of food ______ load on the evap
- food
1A) increases - removal
2A) reduces
TXV Functioning with External Equalizers:
- Designed and applied with pressure drop from the _____ to the ______
- Used to compensate for, not get rid of, pressure drops from the inlet to the outlet of the ________
- 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
- inlet and outlet
- evaporator
- distributor
3A) starve
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
- faster
1A) opens - longer
2A) closes
TXV Selection:
- 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 ___________
- capacity of the system is very _________
- particular
- alternative refrigerants
2A) more than one refrigerant - important
Balanced-Port TXV:
- 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
- Have larger than normal ________
1. low ambient conditions A) varying head pressures B) pressure drops C) evap loads D) liquid line temps
- orifices
Pressure Limiting TXV:
- 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
- predetermined
A) throttle the flow of liquid
B) low-temperature applications
Servicing the TXV:
Things to be considered
- Type of Fastener (______, solder, or ______)
- Location of ______ for service
- Expansion valve bulb _______
- Valve has moving parts that are subject to wear; exact __________ is best
- ________ a TXV may cause problems
- flare, solder or flange
- valve
- Location
- replacement
- Adjustment
Installing the Sensing Element:
- Follow __________ ____________
- Bulb should be mounted at the end of the evap on the _________ line
- For small suction lines, bulb is usually secured to the _____ of the line
- Recommended location for large suction lines is near the bottom of the line in the __________ run
- manufacturers instructions
- suction line
- top of the line
- bottom of the line in the horizontal run
Stepper Motor Expansion Valves:
- Uses a small motor to control the valve ____
A) valve port controls evap ________ - Temp sensor sends a signal to the ________
- Controller sends a signal to the _______
- Motor turns a fraction of a rotation for each _______
- Motor is powered by signals that change in _________
- port
A) superheat - the controller
- the motor
- signal
- polarity
Stepper Motor Expansion Valves: Controller Functioning:
- The controller consists of many ________ for each switching function
- 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 _________ - ________ loops let the controller know the process needs to be changed or modified
- transistors
- microcomputer
A) microprocessor - Feedback
Algorithms and PID Controllers:
Proportional Controllers:
- Generate an _______ output signal
- difference between actual superheat and superheat set point is the ______ or “error”
Integral controller modes:
- help _____ the offset or error
- calculates error size and length of ______
Derivative (differential) controller modes:
5. Estimate ______ or rate of change of temp/time curve
- analog
- offset or error
- reduce
- time
- slope
Automatic Expansion Valve:
- Meters the refrigeration to the evaporator by using a _______ sensing device
- Maintains constant pressure in the _________
A) When the pressure _____ the valve ______
B) Turning the adjustment screw into the valve ________ the suction pressure
- pressure sensing device
- maintains constant press in the evap
A) pressure drops valve opens
B) increases
The automatic expansion valve uses the ________ as the sensing element and maintains constant pressure in the _________ but does NOT control _________
diaphragm, evaporator, superheat
Automatic Expansion Valve Response to Load Changes:
Responds differently than the TXV to load changes:
- It actually acts in ________
- When a load is added to the coil the suction pressure starts to _______
- When the load is decreases the suction pressure _____ the AVX will start to open and feed more ref to the coil
- reverse
- rise
- falls
Special Considerations for the TXV and AXV:
- Both are _______ devices that allow ____ or ____ refrigerant flow
- Both need a storage device (______) for refrigerant when it is not needed
- The ______ serves as both a storage tank as a tank into which refrigerant can be pumped when ________ the system
- expansion devices, more or less
- Receiver
- Receiver, servicing
