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Danfoss Educational Program
Top 10 - TXV Operation and Troubleshooting
R E F R I G E R A T I O N A N D A I R C O N D I T I O N I N G

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Overview
• This presentation will cover 10 aspects of TXV’s that we feel are
important in both understanding how they function, as well as
what common problems the technician will face when dealing
with TXV’s
• Obviously there are many other things to know about TXV’s as
well as other things that can cause problems not only with
TXV’s but also in the rest of the system so learn all you can as
there will always be something new to face
• A key point to remember is that very often the problem does
not lie with the TXV but rather in another part of the system so
make sure you have eliminated all other possible problems
before making any adjustments

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Calculating Super Heat1a
Superheat = leaving temp – saturation temp = 28F° - 20F° = 8F°
28F°
For R-134a ,
Saturation
Temperature
@ 18 psig = 20F°
(From a P-T chart)
18 psig

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Calculating Subcooling1b
Condenser pressure
= 365 psig = 110F° from P-T Chart
Liquid temp @ outlet = 98F
Subcooling
= Saturation Temp – Liquid Temp
= 110F – 98F = 12F°
The liquid is sub cooled 12 degrees
Example for R-410A

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TXV Operating Principles & Components2
Bulb Pressure
• Valve Driver
• Responds to
refrigerant
temperature changes
exiting the evaporator
Spring Pressure
• Determines at which
superheat the valve
opens
• Must be physically
changed
Evaporator
Pressure
• Represents the
evaporator
pressure on the
system
• It’s load determines
the opening degree
of the valve
Bulb Pressure
Spring
Pressure
Evaporator
Pressure
The valve opening varies depending on the load and resulting superheat
Thus, TXV capacity is driven by superheat

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TXV Operating Principles & Components2
Here you can see what a TXV looks like inside
Orifice
Diaphragm (Red)
Superheat Spring
Valve Stem
To sensing bulb

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Internal and External Equalization3
Internally
equalized TXV
• In small, single circuit evaporators, an internal equalized TXV will work well
• This is due to the low pressure drop that occurs from the TXV outlet to where
the sensing bulb is located

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Internal and External Equalization3
• On larger multi circuit evaporators, external equalization allows for the
actual pressure where the sensing bulb is located to be supplied under
the diaphragm
Ext.
equalizer
line.
Refrigerant
distributor

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Internal and External Equalization3
• Whenever a refrigerant distributor is utilized, an external equalized TXV must
be used or substantial under feeding of the evaporator will occur!
Ext.
equalizer
line.
Refrigerant
distributor

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Operating Superheat4
Nominal Load
Pull-down Load
Low Load Load
Superheat
Valve
Capacity
The TXV does not maintain a constant
superheat value regardless of load!

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Operating Superheat4
Nominal Load
Pull-down Load
Low Load Load
Superheat
Valve
Capacity
Level of superheat
reflects evaporator load

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Troubleshooting
• In order to fully understand the importance of the symptoms that
we will be covering, we must first understand how a refrigeration
or AC system with a TXV responds to changes in load, airflow and
refrigerant charge
• A TXV equipped system will respond in a very specific manner
when changes in the above parameters occur if the all other
parameters are correct
• This is important to know so that you can discern the difference
between a normal reaction to a load change for example, or if
there are problems that may cause similar changes
• There will be one or more key differences between the 2 and
knowing the difference/s is key to trouble shooting

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Airflow5
Below are 2 identical evaporators being fed by a TXV with the evaporator on
the left getting proper airflow while the one on the right has inadequate
airflow. Note the superheat values and coil temps!!
Higher airflow
Evap temp = 47F°
Evap Superheat = 16F°
Lower airflow
Evap temp = 43F°
Evap Superheat = 14F°

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Low Ref. Charge6
If a low refrigerant charge is the issue, the evaporator charge will be lower,
the superheat will be high and the subcooling and condenser pressure
will be lower
Proper Charge
Evap temp = 20F°
Evap Superheat = 12F°
Condenser
subcooling = 12F
Low Charge
Evap temp = 5F°
Evap Superheat = 28F°
Condenser
subcooling = 2F

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Low Ref. Charge6
The biggest difference in low airflow and low charge is the level of superheat
and subcooling. In a low charge scenario, the load is still there, there is just
not enough refrigerant to absorb it!
Proper Charge
Evap temp = 20F°
Evap Superheat = 15F°
Condenser
subcooling = 12F
Undercharged
Evap temp = 5F°
Evap Superheat = 28F°
Condenser
subcooling = 2F

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Valve Blockage7
• A plugged valve will underfeed the evaporator and produce symptoms that
include the evaporator operating under a vacuum or very low pressures,
often less than ½ or even a third of the expected value
• Rarer, though still possible is a valve that will fail to close under low load.
In this case, expect low to zero superheat and liquid returning to the
compressor. In the case of an AC system performance may be great until
the compressor completely falls apart

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Valve Blockage7
• A plugged valve will underfeed the evaporator and produce symptoms that
include the evaporator operating under a vacuum or very low pressures,
often less than ½ or even a third of the expected value
• A plugged valve will not respond to a superheat decrease or will suddenly
open up if superheat when superheat is adjusted downwards.
• To test for loss of bulb charge, warm the bulb in your hand. If valve drives
open, bulb charge is likely ok

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Sensing Bulb Mounting8
If the bulb is not insulated
or is loose, it will cause the
TXV to overfeed, possibly
flooding the compressor
Since 50% of the heat
absorbed by the bulb can
come from the bulb strap
always use a metal strap
and insulate the bulb!!!
Header
Suction Line
TXV
Bulb
Bulb Strap

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Sensing Bulb Mounting
• Securely mount the
bulb on the evaporator
outlet downstream
from the refrigerant
header if present
• Mount the bulb in a
position on the pipe
least affected by liquid
refrigerant and oil
• Insulate the bulb!!
8
Header
Suction Line
TXV
Bulb
Bulb Strap

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Valve Capacity9
• An undersize TXV or orifice will impact the system most during
periods of high load in refrigeration systems, and pretty much all
of the time in AC systems
• In the case of an undersize TXV expect
• Long pull down times or a failure to get the conditioned space to
the correct temperature
• Lower evaporator temperatures and pressures opposite to what
you would expect. i.e. Low evaporator saturation temperature
and pressure even when under high load
• For an AC system, you may see frost on the inlet of the
evaporator circuits but abnormally high superheat at the outlet
• You can also expect poor dehumidification as a disproportionate
amount of the evaporator surface will be at a high temperature
even though the saturation temperature will be lower

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Valve Capacity9
Superheat
Valve
Capacity
Nominal Load
Pull-down Load
Excessive superheat and
failure to pull down to
temperature
Proper sized TXV
Undersized TXV

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Valve Capacity9
• Always ensure that the TXV or it’s orifice is of the correct size and
never assume that they are!
• Further, make sure that you match the TXV capacity with the
evaporator capacity at the same conditions
• Do not use nominal capacity unless you are sure they match the
system requirements!!!
Note: The capacity on the valve is the
nominal capacity and will be
substantially lower at lower evaporator
temperatures
At a -20F° evaporator temperature, this
valve has a capacity of only 1 ton!

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Upstream Obstructions10
R404A @ 235psi
8F° of subcooling
60psi pressure drop due
to plugged drier
Formation of flash gas and
liquid temperature drops to
~ 80F °
Low evap.
pressure, starved
evaporator
• An obstruction in the liquid line can produce flash gas even though there is
a proper refrigerant charge
• Make sure you check upstream for blockages if there is flash gas at TXV
inlet

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Upstream Obstructions10
R404A @ 235psi
8F° of subcooling
60psi pressure drop due
to plugged drier
Formation of flash gas and
liquid temperature drops to
~ 80F °
Low evap.
pressure, starved
evaporator
Never add refrigerant until you have checked the condenser subcooling at it’s
outlet to ensure that additional refrigerant is actually required

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Bonus: TXV Superheat Adjustment
• If you have eliminated all other causes of incorrect feeding of the
evaporator, then it is time to focus on the valve superheat setting
• Remember that it can take 10-15 minutes for the changes to work and for
the system to stabilize
• The best time to determine superheat is when the system is under a
relatively stable load (Not pulldown)
• However, in the case where adjustment is needed in order to allow for pull
down, make the initial adjustments to increase feeding and then wait for
the system to pull down and complete the adjustments to fine tune then
• When adjusting superheat, be sure to find the rate of change per turn for
the particular valve you are working on as the rate of change can be
significantly different for different models of valves
• Slowly change the superheat setting a half to full degree at a time and
then wait for system to stabilize
Turning CW = + SH
Turning CCW = - SH

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Danfoss E-Learning

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Essential Danfoss Apps and Web Sites
Home to the Information You Need
Danfoss tools to make your work easier and more efficient

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Essential Danfoss Apps and Web Sites

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Essential Danfoss Apps and Web Sites

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