Metering Devices

Front 1

Describe the steps required to assist superheat on a TX v

Back 1

- 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

Front 2

What provides the opening force for a TX V

Back 2

The sensing bulb

Front 3

What provides the closing force on a TXV?

Back 3

Evaporator pressure and adjustable spring

Front 4

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)

Back 4

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

Front 5

What does decreasing spring pressure on a TX valve achieve?

Back 5

Less closing force = more refrigerant flow into evap coil

Front 6

What effect does decreasing spring pressure on a TX valve have on superheat?

Back 6

Decrease pressure = less closing force = more refrigerant into coil = lower superheat

Front 7

Why are externally equalized valves necessary at times?

Back 7

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

Front 8

What theoretically might occur if an externallly equalized valve is installled without utilizing the external sensing port?

Back 8

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

Front 9

What is the primary difference between e external and internal equalized TX valves?

Back 9

Internal is equalized at the inlet of the evaporator coil

External is equalized at the outlet to the condenser

Front 10

What is the proper placement position for tx valve sensing bulbs?

Back 10

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)

Front 11

What letter is designated for medium temperature applications?

Back 11

C

Front 12

What letter is designated for medium temperature applications?

Back 12

C

Front 13

What letter is designated for low temperature applications?

Back 13

Z

Hint 13

FREEZER = low temperature

Front 14

What does ZP40 refer to?

Back 14

Low temperature with an evaporator outlet pressure limited to 40 PSIG

Front 15

What are a few examples of units that have relatively low loads And use automatic expansion valves?

Back 15

Slush machines

Soft serve ice cream

Flake ice machines

Water coolers

Front 16

What are the two main components of an EEV?

Back 16

Electric stepper motor

Nellie and orfice

Front 17

What are two common sensor types that are used on an EEV?

Back 17

Temperature Thermister

Pressure transducer

Front 18

What does a fixed metering device maintain?

Back 18

Pressure difference between evaporator and condenser

Front 19

High _________ _____________ rates of refrigerant will overload the compressor leading to high _____________ __________ draws, which can overheat compressor motors and ___________ motor overload protection devices.

Back 19

Mass flow

Compressor-motor amperage

Open

Front 20

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:

Back 20

Vapor charge bulb / gas-charged bulb / MOP maximum operating pressure valves

Front 21

Which type of tx valve charge closely follows the refrigerant’s T/P relation?

Back 21

Liquid charged bulb

Front 22

True or false?

Liquid charged tx bulbs always have some refrigerant in liquid state regardless of temperatures

Back 22

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

Front 23

Which type of sensing bulb charge can lead to over feeding of valves in high pressure situations?

Back 23

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

Front 24

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?

Back 24

Cross charge valves

(Cross liquid?)???

Front 25

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?

Back 25

Cross liquid charge valves

Front 26

What application are cross liquid charge valves useful for?

Back 26

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

Front 27

Which type of bulb charge tends to be reserved for custom installations ?

Back 27

Cross vapour charged bulbs (similar to vapour charged but has different type of refrigerant from the system)

Front 28

Which type of bulb charge tends to be reserved for custom installations ?

Back 28

Cross vapour charged bulbs (similar to vapour charged but has different type of refrigerant from the system)

Front 29

What degree of pressure drop across an evap coil necessitates an external equalizer ?

Back 29

3 PSIG for AC

2 PSIG medium temp

1 psig low temp

Front 30

True or false?

An evaporator is most efficient when it has the maximum amount of refrigerant flow

Back 30

True ONLY as long as the liquid does not flood back to the compressor (I.e state change is priority over superheating)

Front 31

True or false?

The sensing bulb should be placed downstream of the external equalizer connection

Back 31

False

Or should be placed before OR upstream

Front 32

In low ambient conditions, what force tends to have the biggest impact when a standard tx valve is used?

Back 32

- 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

Front 33

In low ambient conditions, what force tends to have the biggest impact when a standard tx valve is used?

Back 33

- 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

Front 34

What is the main difference between a balance port and standard tx valve?

Back 34

- 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

Front 35

How is SEER rating calculated?

Back 35

The total amount of BTU (cooling) the system will provide in one year divided by the total number of watt hours it will consume

Front 36

List some benefits to parallel flow plate and fin heat exchangers

Back 36

- 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

Front 37

What should a tx valve do when a conditioned space is under heavy load?

Back 37

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)

Front 38

The needle and seat of a TXV are normally made of _____________

Back 38

?

Front 39

The needle and seat of a TXV are normally made of _____________

Back 39

Stainless steel

Front 40

The different types of charge that the tx v uses to control refrigerant flow are:

Back 40

- Vapor (gas charged bulb)

- cross vapour

- liquid

- cross liquid

Front 41

What 3 factors determine the capacity of aTX valve?

Back 41

- liquid temp

- evaporator temp

- pressure drop drop across the valve

Front 42

Why does the entering liquid have no effect on the opening force of a balance port TX?

Back 42

The flat Pin pushes on the diaphragm ???

Front 43

How does a PID controller work?

Back 43

?

Front 44

True or False?

Capillary tube metering devices tend to run low superheats when under heavy load conditions.

Back 44

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

Front 45

What is one downside to using capillary tube metering devices?

Back 45

Small particles may block the tubes

I.e. contaminants, desiccant from broken down filter drier

Front 46

Capillary tube metering devices are __________ _________devices so they does not stop refrigerant flow to the _________ __________during off cycle. This has the benefit of ______________

Back 46

Fixed bore

Low side

Reducing motor starting torque

Front 47

What does critical charge refer to?

Back 47

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.

Front 48

Capillary tube metering devices are typically used in what type of system?

Back 48

Hermetically sealed, small factory assembled units

Beer fridges, restaurants.

Not good in low ambient conditions!

Front 49

What is the unique characteristic of a pressure limiting tx valve and what situation is it useful for?

Back 49

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

Front 50

When are balanced per TXVs used?

Back 50

Wide varying load applications

Heavy load application

Wide varying head pressure apps

Very low liquid line temps

Front 51

Review section of unit 24 about stereo motors aevs and PID’s

Back 51

…

Front 52

How many steps a does a lightbulb have?

How does this compare to a step-motor expansion valve?

Back 52

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.

Front 53

What components make up a step motor expansion valve?

Back 53

> electric stepper motor (contains digital linear actuator)

> needle and orifice

Front 54

What type of PID controller calculates the area under the curve of the temperature/time graph?

Back 54

> 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

Front 55

What do the first three letters on a TEV stamp indicate?

I.e FSE 1ZP

Back 55

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)

Front 56

What are AEV’s specifically foods at maintaining?

What applications are they typically used in?

Back 56

Preventing low suction pressures (good in low ambient conditions)

Slush/flake ice machines, water coolers, ice cream machines

Front 57

What is an AEV useful for?

Back 57

Maintaining low load/consistent loads. Ie soft serve machines. Quirks like a pressure regulator.

Front 58

Do not pressure test when this component is in place:

Back 58

Transducer

(Remove from EEV during pressure test)

Front 59

What dies GA refer to?

Back 59

Air conditioning