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59 Cards in this Set
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- Back
- 3rd side (hint)
Describe the steps required to assist superheat on a TX v |
- remove seal cap - loosen packing gland - adjust stem 1/4-1/2 turn (clockwise increases superheat) - wait 15-20 min for system to settle - tighten packing nut and seal cap |
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What provides the opening force for a TX V |
The sensing bulb |
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What provides the closing force on a TXV? |
Evaporator pressure and adjustable spring |
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IF the suction line pressure of an R22 system is 84 PSIG (50 deg f ) and the evaporator temperature is 40 deg f what (theoretically) should the spring tension of the TXV be set to (assuming it is internally equalized and the sensing bulb also has R22) |
15 PSIG - 84 is opening force measured by sensing bulb post evap coil - 69 (r22 at 40 deg f is 69 psig) is closing force (internal back pressure) - 84-69 = 15 PSIG (required closing force required by spring to active equilibrium |
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What does decreasing spring pressure on a TX valve achieve? |
Less closing force = more refrigerant flow into evap coil |
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What effect does decreasing spring pressure on a TX valve have on superheat? |
Decrease pressure = less closing force = more refrigerant into coil = lower superheat |
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Why are externally equalized valves necessary at times? |
When distributors are used there is more pressure drop entering the coil which does not allow for internal closing pressure at the inlet. The external equalizer measures pressure at the evaporator outlet |
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What theoretically might occur if an externallly equalized valve is installled without utilizing the external sensing port? |
TXV back Pressure will be read at the inlet which may appear normal (ie 69 psig 40 deg f evap) but the pressure drops at the distributors so the actual evap p/t could be half of that (35 psig / 12 deg f). When the coil empties at the outlet, the split pressure situation reverts and p/t May read normal again (50 deg f/84 psig) Therefore the sensing bulb may have 84 psi opening force, the valve may have 70 psi closing force leading to too |
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What is the primary difference between e external and internal equalized TX valves? |
Internal is equalized at the inlet of the evaporator coil External is equalized at the outlet to the condenser |
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What is the proper placement position for tx valve sensing bulbs? |
8 OR 4 o’clock position if it is 7/8 OR LARGER Anywhere between 8-4 o’clock if it is smaller than 7/8 (but never 6:00) |
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What letter is designated for medium temperature applications? |
C |
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What letter is designated for medium temperature applications? |
C |
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What letter is designated for low temperature applications? |
Z |
FREEZER = low temperature |
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What does ZP40 refer to? |
Low temperature with an evaporator outlet pressure limited to 40 PSIG |
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What are a few examples of units that have relatively low loads And use automatic expansion valves? |
Slush machines Soft serve ice cream Flake ice machines Water coolers |
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What are the two main components of an EEV? |
Electric stepper motor Nellie and orfice |
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What are two common sensor types that are used on an EEV? |
Temperature Thermister Pressure transducer |
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What does a fixed metering device maintain? |
Pressure difference between evaporator and condenser |
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High _________ _____________ rates of refrigerant will overload the compressor leading to high _____________ __________ draws, which can overheat compressor motors and ___________ motor overload protection devices. |
Mass flow Compressor-motor amperage Open |
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This type of tx valve charge tends to follow a pressure curve that flattens at a max value which acts to limit max operating pressures at the evaporator: |
Vapor charge bulb / gas-charged bulb / MOP maximum operating pressure valves |
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Which type of tx valve charge closely follows the refrigerant’s T/P relation? |
Liquid charged bulb |
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True or false? Liquid charged tx bulbs always have some refrigerant in liquid state regardless of temperatures |
True. They are not full of liquid but are charged so the liquid will never be 100% vapourized. This allows for a straight/predictable plotted line on the T/P graph |
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Which type of sensing bulb charge can lead to over feeding of valves in high pressure situations? |
Liquid charged bulbs can over feed evaporators as they follow a very direct line of pressure^=valve opens. These charged bulbs don’t have a built in limit by nature of their design |
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Which type of bulb charge has a flatter curve than the system refrigerant’s parameters? Leading to faster closing times when pressures rise and slower opening times when the bulb gets warm? |
Cross charge valves (Cross liquid?)??? |
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Which type of bulb charge has a flatter curve than the system refrigerant’s parameters? Leading to faster closing times when pressures rise and slower opening times when the bulb gets warm? |
Cross liquid charge valves |
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What application are cross liquid charge valves useful for? |
Cross charge valves are useful in low temp uses. They give high superheats under high load. Normal superheat under design conditions, and they close off during off cycle (bc pressures rise when the compressor shuts off) which prevents refrigerant flooding the compressor on startup |
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Which type of bulb charge tends to be reserved for custom installations ? |
Cross vapour charged bulbs (similar to vapour charged but has different type of refrigerant from the system) |
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Which type of bulb charge tends to be reserved for custom installations ? |
Cross vapour charged bulbs (similar to vapour charged but has different type of refrigerant from the system) |
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What degree of pressure drop across an evap coil necessitates an external equalizer ? |
3 PSIG for AC 2 PSIG medium temp 1 psig low temp |
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True or false? An evaporator is most efficient when it has the maximum amount of refrigerant flow |
True ONLY as long as the liquid does not flood back to the compressor (I.e state change is priority over superheating) |
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True or false? The sensing bulb should be placed downstream of the external equalizer connection |
False Or should be placed before OR upstream |
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In low ambient conditions, what force tends to have the biggest impact when a standard tx valve is used? |
- low inlet pressure leads to low evaporator pressure. - typically you would think this would lead to higher superheats? But lower pressure means lower temps. So the sensing bulb would have lower opening force. - the back pressure from the evaporator would lower the closing force - so overall there seems to be no overwhelming force on open or close - acc to handouts, the needle tends to shut down refrigerant flow in this situation |
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In low ambient conditions, what force tends to have the biggest impact when a standard tx valve is used? |
- low inlet pressure leads to low evaporator pressure. - typically you would think this would lead to higher superheats? But lower pressure means lower temps. So the sensing bulb would have lower opening force. - the back pressure from the evaporator would lower the closing force - so overall there seems to be no overwhelming force on open or close - acc to handouts, the needle tends to shut down refrigerant flow in this situation |
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What is the main difference between a balance port and standard tx valve? |
- Balanced port has a flat needle so the liquid at the inlet acts as an added opening force - this will lead to higher refrigerant flow to the evaporator in low ambient conditions |
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How is SEER rating calculated? |
The total amount of BTU (cooling) the system will provide in one year divided by the total number of watt hours it will consume |
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List some benefits to parallel flow plate and fin heat exchangers |
- reduced static pressure (more energy savings) - reduced size (cheaper shipping, easier cleaning, smaller internal volume reduces refrigerant charge, reduced weight) - All aluminum means less corrosion -quieter operation |
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What should a tx valve do when a conditioned space is under heavy load? |
The high pressure/high superheat should lead to the sensing bulb to warm and throttle the valve open, driving more refrigerant into the coil… leading to lower superheats as the main goal is to soak up as much heat as possible with dense amounts of saturated liquid. Therefore superheats may be low during high load conditions. (I think) |
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The needle and seat of a TXV are normally made of _____________ |
? |
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The needle and seat of a TXV are normally made of _____________ |
Stainless steel |
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The different types of charge that the tx v uses to control refrigerant flow are: |
- Vapor (gas charged bulb) - cross vapour - liquid - cross liquid
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What 3 factors determine the capacity of aTX valve? |
- liquid temp - evaporator temp - pressure drop drop across the valve |
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Why does the entering liquid have no effect on the opening force of a balance port TX? |
The flat Pin pushes on the diaphragm ??? |
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How does a PID controller work? |
? |
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True or False? Capillary tube metering devices tend to run low superheats when under heavy load conditions. |
False, when charged properly, capillary tube metering devices run high superheats under heavy load and returns to the 8-12 deg f superheat range over normal conditions resume |
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What is one downside to using capillary tube metering devices? |
Small particles may block the tubes I.e. contaminants, desiccant from broken down filter drier |
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Capillary tube metering devices are __________ _________devices so they does not stop refrigerant flow to the _________ __________during off cycle. This has the benefit of ______________ |
Fixed bore Low side Reducing motor starting torque |
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What does critical charge refer to? |
Proper charge in a capillary tube metered system is critical. Because there is no valve it relies on pressure drop via restriction so is easy to overcharge. |
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Capillary tube metering devices are typically used in what type of system? |
Hermetically sealed, small factory assembled units Beer fridges, restaurants. Not good in low ambient conditions! |
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What is the unique characteristic of a pressure limiting tx valve and what situation is it useful for? |
It has a (second?) bellows that only throttles once a predetermined pressure has been reached. This is useful in low temp app because it keeps load suction pressure during heavy Load which prevents compressor overload |
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When are balanced per TXVs used? |
Wide varying load applications Heavy load application Wide varying head pressure apps Very low liquid line temps |
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Review section of unit 24 about stereo motors aevs and PID’s |
… |
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How many steps a does a lightbulb have? How does this compare to a step-motor expansion valve? |
A light bulb had two steps Expansion valve stepper motors can have up to 100 steps (electromagnets each with a degree of rotation equal to as little as 3.6 deg) for a total of 360 degrees in 100 increments. |
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What components make up a step motor expansion valve? |
> electric stepper motor (contains digital linear actuator) > needle and orifice |
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What type of PID controller calculates the area under the curve of the temperature/time graph? |
> in other words, this controller is calculating the offset OR error value between actual evaporator SH and SH set point. > this controller is known as an integral controller > integration is a mathematical term used to refer to finding the area under a curve on a graph |
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What do the first three letters on a TEV stamp indicate? I.e FSE 1ZP |
F - body style (b would be balanced port) S - Refrigerant type E - type of equalizer (external) 1 - refers to tonnage Z - refers to low temperature (FreeZer) P - refers to pressure limiting (GA would indicate air conditioning) |
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What are AEV’s specifically foods at maintaining? What applications are they typically used in? |
Preventing low suction pressures (good in low ambient conditions) Slush/flake ice machines, water coolers, ice cream machines |
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What is an AEV useful for? |
Maintaining low load/consistent loads. Ie soft serve machines. Quirks like a pressure regulator. |
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Do not pressure test when this component is in place: |
Transducer (Remove from EEV during pressure test) |
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What dies GA refer to? |
Air conditioning |
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