Danfoss Educational Program
Interpreting System Symptoms
Competitors Comparison
REFRIGERATION AND AIR CONDITIONING
DS, TSO
Nov-08 / 1
Index
• Welcome & Intro to Danfoss
• Laying the foundation:
• Basic System Operation and components
• SH and SC
• Equilibrium and system balance
• Load effects on:
• Superheat
• Subcooling
• Impacts of valve capacity
• Ambient Effects on:
• Superheat
• Subcooling
• Dealing with Low Ambient Conditions
• Pressure Controls and Regulators
• Causes of Abnormal Superheat
REFRIGERATION AND AIR CONDITIONING
Nov-08 / 2
Danfoss – Yesterday and Today
• Founded by Mads Clausen in 1933
– First product was a TXV
– Rapidly expanded into other products and markets
• Today, still owned by founder’s family
• Net sales in 2012, $6.0 Billion US
• 26,000 employees worldwide
• 93 Manufacturing plants
• 139 Sales Companies
• Largest controls manufacturer in the world
– One of the largest manufacturers of compressors
• What we Hope to Accomplish
• The relationship balance between key components such as expansion valves,
compressors and heat exchangers
• How to relate ambient and load conditions to the measurements you are
seeing to fully understand what subcooling and superheat values are
indicating
• Don’t touch that superheat dial! (How to quickly troubleshoot common TXV
feeding problems.)
• Recognizing and understanding the key information to solve the real issues
and reduce call backs
Calculating Super Heat
Superheat = leaving temp – saturation temp
= 28F° - 20F° = 8F°
For R-134a , Saturation
Temperature @ 18 psig =
20F° (From a P-T chart)
Danfoss Refrigeration
18 psig
28F°
Refrig TXV TUA/TUAE
|5
Calculating Subcooling
Example for R-410A
Subcooling =
Saturation Temp – Liquid Temp
= 110F – 98F = 12F°
The liquid is sub cooled 12 degrees.
Liquid temp @ outlet = 98F
Danfoss Refrigeration
Condenser pressure = 365 psig
= 110F° from P-T Chart
Refrig TXV TUA/TUAE
|6
Equilibrium
In this basic system, there is really only one path for the refrigerant to flow
through
In this respect, it follows the same rules as a series circuit in electricity
What flows in one area also must flow in the rest of the system
Danfoss Refrigeration
Refrig TXV TUA/TUAE
|7
Equilibrium
• The main concept here is that the flow through the TXV must be
balanced by the pumping capacity of the compressor
• This state can be referred to as ‘equilibrium’
• In equilibrium, the pressures and temperatures do not change
• We see this when the load
and ambient conditions are
constant
Danfoss Refrigeration
Refrig TXV TUA/TUAE
|8
Load Changes and System Pressures
• If one of the parameters changes, say the air flowing over the
evaporator becomes warmer, the equilibrium will be broken and
the system conditions will begin to change
• In this case, if a TXV is used, the valve will begin to open and
inject more refrigerant into the evaporator
Danfoss Refrigeration
Refrig TXV TUA/TUAE
|9
Load Changes and System Pressures
• This extra refrigerant flowing into the evaporator has to go
somewhere and in this case it must be pumped thru the
compressor
• However, in order for the compressor to move more mass of
refrigerant, the density of the refrigerant must increase
• As a result, the pressure and temperatures in the evaporator
will increase
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 10
Load Changes and System Pressures
• For a certain volume, the higher the density of the vapor, the
more mass it will contain
This cylinder contains 50% more
refrigerant
1 ft3
1 ft3
R404A @ 20 psig
1 ft3 = 0.79 lbs.
Danfoss Refrigeration
R404A @ 40 psig
1 ft3 = 1.21 lbs.
Refrig TXV TUA/TUAE
| 11
Load Changes and System Pressures
• As a result of this phenomena, if a system is coming out of
defrost, what would we expect the evaporator pressure to be?
• Higher than normal?
• Lower than normal?
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Superheat and Load - TXV
• One of the symptoms that will be observed during periods of
high load will be an increase in evaporator superheat
• This is because of the increase in bulb pressure required to
compress the superheat spring to further drive the valve open.
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Superheat and Load - TXV
Static Superheat (SS)
Valve Capacity
Full Open Capacity
Reserve
Capacity
Superheat necessary to
overcome spring force
Rated Capactiy
Opening Superheat (OS)
Superheat required to move
valve pin from seat
SS
OS
Superheat
OPS
Danfoss Refrigeration
Operating Superheat
(OPS)
Superheat at which the valve
operates (SS + OS)
Refrig TXV TUA/TUAE
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Superheat and Load – TXV Sizing
Valve Capacity
Full Open Capacity
Reserve
Capacity
Rated Capactiy
SS
OS
The impact of
valve size on
superheat will be
greatest during
periods of heavy
load.
Superheat
OPS
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Superheat and Load - TXV
Proper sized TXV
Valve Capacity
Undersized TXV
Pull-down Load
Nominal Load
Excessive
superheat
Superheat
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Load Effects on SC (TXV Equipped)
• By it’s nature, subcooling requires a temperature difference between the
refrigerant and the ambient surroundings
• The greater the temperature difference, the greater the amount of
subcooling that is possible
• This is true regardless of what is causing the temperature difference
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Load Effects on SC TXV Equipped
• When system load is high, the TXV’s will inject more refrigerant into the
evaporators resulting in greater compressor hp to pump the vapor
• Because of this, the condenser will have a higher Total Heat of Rejection
(THR) requiring a greater TD between the air and the refrigerant resulting
in a greater level of sub-cooling in the liquid
High load Condensing temp =
110F°, Liquid = 96F°
Danfoss Refrigeration
Low load Condensing temp =
104F°, Liquid = 93F°
Refrig TXV TUA/TUAE
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Condenser Size- TXV Equipped
• A smaller condenser for a given capacity will generate more subcooling than a larger condenser
• Similarly, higher ambient conditions will do the same
9F°
SC
12F° SC
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Condenser Size- TXV Equipped
• This effect is the result of the condensing temperature needing
to be higher to maintain the TD between the ambient air and the
refrigerant temperatures
• This results in a higher compression ratio, greater compressor
work and more THR for a given load
12F° SC
9F° SC
Ambient is 97F° Condensing temp
= 120F°, Liquid = 108F°
Ambient is 76F° Condensing temp
= 95F°, Liquid = 86F°
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Ambient Effects on SC - TXV Equipped
A condenser that is recycling discharge air will have an
abnormally high condensing pressure and a high subcooling
level
92F°
Ambient
112F°
112F°
102F°
Danfoss Refrigeration
92F°
Ambient
102F°
Refrig TXV TUA/TUAE
| 21
Amb. Effects on SC – Piston & Cap
Tube
• Below is a graph plotting subcooling vs. outdoor temperatures
• Notice how the subcooling can drop to low levels on very hot days due
to large amounts of refrigerant situated in the evaporator “starving” the
condenser
Subcooling
14F°
This is a result of flow rate into
evaporator increasing as
condensing pressure increases.
10F°
6F°
2F°
70F°
Danfoss Refrigeration
80F°
90F°
Refrig TXV TUA/TUAE
100F°
Ambient T (F°)
| 22
Subcooling
• A cooler with a TXV has just been loaded with warm product.
Superheat and subcooling will be:
• Higher than normal?
• Lower than normal?
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 23
Subcooling
• On a coldish day, you notice that on a 6 fan condenser, the
condensing fans are all running despite 2 out of 3 compressors
being idle due to low evaporator loads.
• What would you expect the level of subcooling leaving the
condenser to be if TXVs is used?
• Higher than normal?
• Lower than normal?
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 24
Ambient Effects on Superheat
• Capillary tube flow rates depend on length, diameter and pressure
differential
• All else being the same, the greater the pressure difference across
them, the more flow
110 psi differential
210 psi differential
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Ambient Effects on Superheat
• Pressure differential is the difference between condensing and
evaporator pressures
• An increase in condensing pressure will result in greater refrigerant feed
into the evaporator
• On high ambient days, the condensing pressure will increase in outdoor
condensers
Cool Day
Hot day
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Amb. Effects on SH – Piston & Cap
Tube
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Amb. Effects on SH – Piston & Cap
Tube
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Ambient Effects on Superheat
• Below is a graph plotting superheat vs. outdoor temperatures
• Notice how the superheat can drop to low levels on very hot days
• If the cap tube is slightly oversized for the condenser, liquid flood back
to the compressor may occur
SH (F°)
14F°
This is a result of cap tube flow
rate increasing more than the
load on the evaporator
10F°
6F°
2F°
70F°
Danfoss Refrigeration
80F°
90F°
Refrig TXV TUA/TUAE
100F°
Ambient T (F°)
| 29
Ambient Effects on Superheat (TXVs)
For TXV’s it is the opposite concern of too little pressure difference
across the valve
This can occur during low ambient conditions in refrigeration systems
that are located where the temperature can vary widely such as in the
northern US and Canada
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Ambient Effects on Superheat
• This problem is especially acute during system start or during periods
of low load and in units with no fan control
• The issue stems from a lack of pressure difference across the valve
because of abnormally low condensing pressures
• This results in the valves sticking or failing to feed properly
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Ambient Effects on Superheat
• Capacity can be reduced by as much as 50%
• High superheat levels , long run times, and the inability to maintain
temperature set points
Valve Capacity
Nominal Pressure
Differential
Insufficient Pressure
Differential
Superheat
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Scenario Investigation
• A cooler with a 1 hp out door condenser is equipped with a TXV
It cannot meet the pull down times required and superheat is
22F° at evaporator outlet. The outdoor ambient is 38F°
• What would you expect the pressure and temperature readings
to be?
Hint: The cardboard test is great for this
scenario!
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 33
Methods of Condenser Pressure Control
• The most common method of controlling pressure in
the condenser is through the use of fan staging.
• This incorporates either 2 or more fans, a variable
speed fan or combination of both
• The volume of air flow through the condenser is
increased or decreases in response to a rise or fall in
condensing pressure and ambient conditions
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Condenser Pressure Control
Temperature = 90F
When the outdoor temperature
is lower, the condenser acts like
it is a lot larger
Temperature = 50F
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Condenser Pressure Control
Condensing
Temperature =
110 Fahrenheit
90 degree Fahrenheit
Air Inlet Temperature
•Condensers are designed for a certain difference between the
outside ambient temperature and the condensing temperature
•Fan cycling allows the use of large condensers that can reject
heat with the smallest difference between the ambient and
condensing temperatures feasible while allowing for lower load
and ambient conditions
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Condenser Pressure Control
Condensing
Temperature =
90 Fahrenheit
Less airflow needed when air inlet is only 50
degree °F
•Because the condenser must be able to reject heat even at
high loads, the condenser must be large enough to reject this
heat even when it is hot outside i.e. 90F
•However, during periods of low load and or low ambient
conditions, less airflow is required to remove heat while still
maintaining sufficient high side pressure
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Condenser Pressure Control
Condensing
Temperature =
90 Fahrenheit
Less airflow needed when air inlet is only 50
degree °F
•To accomplish this balance, having multiple fans that can be
turned off and on allows substantial flexibility in controlling
airflow
•It is not uncommon for all fans to be off during cool weather
when the system is first started and it may take several
minutes to build pressure before the first stage is started
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Condenser Pressure Control
Setting of Controls
Setting of pressure controls requires
the adjusting of the set point and the
differential value if it is not fixed.
Example:
Cut out = Cut in – Diff.
= 50psig – 20 psig = 30 psig
Cut out = 30 psig
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 39
Condenser Pressure Control
Fan Control Operation
P
I. When the pressure exceeds
the upper set point,
contacts 1 and 4 make and
bring on fan/s
II.
I.
USP
II. When the pressure falls to
the lower set point, the contacts
change back to the initial
position and turn fan/s off.
LSP
USP = Upper Set Point
LSP = Lower Set Point
Danfoss Refrigeration
Refrig TXV TUA/TUAE
Differential
| 40
Condenser Pressure Control
Minimum required condensing temperature = 80°F
When the ambient is 30°F or lower, it can be very difficult to
maintain a minimum level of condensing temperature even if
all fans are off.
This is especially acute in regions with large seasonal
temperature swings
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 41
Condenser Pressure Control
Pressure regulators
• Pressure regulators are used to maintain pressures
within an acceptable or desired level
• They are used in applications where it is possible for
operating pressures to develop that are outside of the
operating limits for components or for the “product”
that is being maintained
• Pressure regulators are also used for capacity control
to make up for a shortfall in evaporator load compared
to compressor pumping capacity ( Air driers)
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 42
Condenser Pressure Regulators
Condenser
Used in the
liquid line
before the
receiver
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Condenser Pressure Regulator
•When the condenser effectively becomes larger, the
condensing pressure can drop substantially ,causing the TEV
to operate erratically
•The CPR acts by reducing the area available for heat
rejection, effectively making the condenser smaller
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 44
Condenser Pressure Regulator
This the way the condenser
behaves when liquid is backed up
in it
Only a portion of it can reject heat
•The CPR accomplishes this by backing up the liquid in the
condenser, using up free volume
•The condenser then has a smaller area available to reject
heat from the refrigerant
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 45
Heat Energy - Enthalpy
Temperature
Enthalpy is the heat in BTUs per pound added to
or removed from a substance, in this case water.
212 °F
heating
water (liquid)
superheating
vapor
boiling water
(liquid + vapor)
970 BTU/lb
180
BTU/lb
Danfoss Refrigeration
Heat (Energy)
Enthalpy
Refrig TXV TUA/TUAE
| 46
Receiver Capacity!!
• In the summer months, the condenser will hold
substantially less refrigerant and this refrigerant will need
to be stored in the receiver
• It is important to ensure that there is enough receiver
capacity to hold the extra refrigerant charge that is
necessary to properly accomplish this
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 47
Basic Troubleshooting
Causes of Abnormal Superheat
• There are several scenarios that can cause an undesirable
level of superheat at the evaporator outlet
• The causes can differ depending on the metering device
utilized
• This section will focus on both simple restriction and TXV
type metering devices
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 48
Causes of Abnormal Superheat
Metering Device Sizing
• We have already seen how the ambient conditions can effect the
feeding of simple restrictions resulting in abnormally high or low
SH readings
• Similar symptoms will also be observed for metering devices that
are not sized correctly
• Cap tube flow rate is determined by their length and diameter
Longer length/smaller diameter
Shorter length/larger diameter
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 49
Causes of Abnormal Superheat
Metering Device Sizing
• Piston fed systems are also affected by
orifice size and one that is larger or
smaller than specified will not feed
properly
• Always check the size and/or length of
simple restriction fed systems against
the manufacturers specifications and
always make sure you correctly
measure the ambient conditions as
you will need this information
Danfoss Refrigeration
Refrig TXV TUA/TUAE
The piston in this
distributor
should be sized
correctly
| 50
Causes of Abnormal Superheat
Metering Device Sizing
• TXV’s will have either a fixed or
interchangeable orifice that determines
the capacity range of the valve
• An undersized orifice will result in
excessively high superheat levels ,
longer pull down times and the inability
to meet temperature set points
• An oversized orifice may amplify any
problems that normally would not be
severe enough to adversely affect the
system
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 51
Causes of Abnormal Superheat
Sensing Bulb Heat Transfer
Proper Bulb Strap Placement
• 60% of the heat transferred to the sensing bulb comes
via the bulb mounting strap
• It must always make good physical contact with the bulb
and the refrigerant tubing
• Do not use zip ties or other non conductive materials!
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Causes of Abnormal Superheat
Sensing Bulb Heat Transfer
Guidelines:
• Securely mount the bulb on the
evaporator outlet downstream
from the refrigerant header if
present
Header
• Mount the bulb in a position on
the pipe least affected by liquid
refrigerant and oil
TXV Bulb
Suction Line
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Causes of Abnormal Superheat
Sensing Bulb Heat Transfer
Guidelines:
• Bulb should always be completely
insulated with a foam type
insulation
• Exposure to the air may cause the
valve to overfeed, resulting in low
superheat and possible flooding of
the compressor
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Causes of Abnormal Superheat
Metering Device Blockages
Of those with faults, 90% of returned valves are inoperable because of
outside influence, primarily contamination or overheating
Generally debris will cause the valve to underfeed but it can also cause the
valve to stick, resulting in superheat levels that are too high or low for the
corresponding load.
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Causes of Abnormal Superheat
Metering Device Blockages
Screen traps dirt
in system
• If the filter screen in the TXV becomes excessively plugged, it can
cause liquid flashing as well as reduced flow into the TEV
• If possible, when you suspect the screen is plugged, remove orifice to
inspect and clean
• If system is already open, take the time to check!
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 56
Causes of Abnormal Superheat
Line component Restrictions
R404A @ 235psi
60psi pressure drop
due to plugged drier
8F° of subcooling
Formation of flash gas and
liquid temperature drops to ~
80F °
Low evap.
pressure, starved
evaporator etc.
An obstruction in the liquid line can produce flash gas and can
be detected by a pressure drop as well as by a detectable
temperature difference.
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 57
Causes of Abnormal Superheat
Internal Equalization with High PD Evaporators
An evaporator with a high equivalent pressure drop requires the use
of an externally equalized TXV
Internally Equalized
TXV
Evaporator pressure
Externally Equalized
TXV
Evaporator
pressure
acting on diaphragm
is from outlet, not inlet
acting on diaphragm
is from inlet
To Evaporator
Outlet
Equalizer pressure reaches diaphragm
from valve outlet – along pushpin or a
bored hole
Danfoss Refrigeration
External
Equalizer Inlet
Refrig TXV TUA/TUAE
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Causes of Abnormal Superheat
Internal Equalization with High PD Evaporators
 A distributor introduces a large pressure drop right after the TEV
 This results in a higher pressure being applied under the diaphragm
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Causes of Abnormal Superheat
Internal Equalization with High PD Evaporators
• Example #1 System Conditions
–
–
–
–
Refrigerant is R404A
Evaporator pressure is 57 psig.
TXV is set for 9°F static superheat
Pressure drop across distributor and evaporator is 11psi
• Example #2 System Conditions
–
–
–
–
Refrigerant is R404A
Evaporator pressure is 25 psig.
TXV is set for 9°F static superheat
Pressure drop across distributor and evaporator is 11psi
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Causes of Abnormal Superheat
Internal Equalization with High PD Evaporators
 External equalization allows for the actual evaporator pressure to be supplied
under the diaphragm so the valve does not under feed the evaporator resulting
in excessive superheat
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Causes of Abnormal Superheat
Incorrect Superheat Adjustment
Turning CW
= + SH
Turning
CCW
= - SH
• Very rarely will a TXV superheat setting need adjustment in the
field
• Unless the issue is a failure to feed or liquid flood back, make only
small inputs and then wait for the system to stabilize.
• It can take as long as 10 minutes for the changes to fully take effect
• Always check with the manufacturer's literature to determine the
rate of change per turn!
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Summary
Troubleshooting - Superheat
• System Conditions
– Before making any adjustments, ensure that the system is
stable and not under high load
– Under conditions such as pull down or coming off of defrost,
excessive superheat and evaporator pressure and
temperatures are normal and are the result of the TEV
opening in reaction to these high loads
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Basic Troubleshooting - Failure to feed
•properly
Check to ensure the sensing bulb is properly mounted (and
•
•
•
•
insulated) to the suction line
If there are shut off or solenoid valves up stream from the TEV,
ensure they are open.
Check the valve for symptoms of over heating such as
discoloration
Remove the filter screen and check for debris. If present, clean
and replace and be sure to replace the gasket with a new one.
While the valve is open, check the valve internally for excess
brazing material which may have plugged the valve seat
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Troubleshooting - Suction line too warm
(High Superheat)
• Check for proper system subcooling. Flash gas may be present at TEV
inlet.
– Additional refrigerant may be needed but check SC at condenser first
– Check for dirty condenser
– Check for clogged Liquid Line filter drier
• Check the orifice size against the system capacity. If it is incorrect, then
replace using the sizing chart to make the proper selection. Be sure to
use the correct evaporator temperature when making the selection, and
ensure that you have accounted for additional capacity needed during
pull-down!
• Check for obstructions in the liquid line such as plugged line
components or valves that may be only partially open.
• Try adjusting the superheat 1 full turn counter-clockwise. If no response
after 5 minutes, then remove and check the filter screen for dirt and
debris. While the valve is open, check for excess brazing material
plugging the valve seat. Replace gasket with a new one.
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Troubleshooting - Cannot adjust superheat
• Note that superheat spindle is located on the side (not bottom) of all TU
TXVs
• Check to make sure that you have removed the protective cap screw
which will reveal the superheat adjustment screw inside the stem
• Make sure the sensing bulb is in good contact with the suction tubing
and that it is well insulated.
• When adjusting superheat, be sure to make ~ 1 full turn one time and to
wait 5 minutes or so for the inputs to take effect and for the system to
stabilize
• If the valve is unstable, slowly increase super heat until the valve stops
hunting by inputting ¼ turn at a time
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Troubleshooting - Flooding the evaporator
• Check for proper airflow across evaporator and that the evaporator is
clear of ice and dirt
• Make sure that the sensing bulb is mounted properly, is in good contact
with the tube and is well insulated
• This could be caused by poor superheat adjustment or bulb
placement/insulation
• Slowly increase the superheat by 1/8 of a turn clockwise until there is
sufficient superheat at the evaporator outlet
Danfoss Refrigeration
Refrig TXV TUA/TUAE
| 67
Helpful Website Link
http://www.danfoss.com/North_America/BusinessAreas/Refrigeration+and+Air+Conditioning/Aftermarket+Contractor+Corner.htm
Danfoss Refrigeration
Refrig TXV TUA/TUAE
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Danfoss Refrigeration
Refrig TXV TUA/TUAE
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September 2009 | | 69