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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 DS, TSO Nov-08 / 1

Competitors Comparison

Danfoss Educational Program

Interpreting System Symptoms

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 Nov-08 / 2

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

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

Refrig TXV TUA/TUAEDanfoss Refrigeration | 5

Calculating Super Heat

28F°

For R-134a , Saturation Temperature @ 18 psig = 20F° (From a P-T chart)

Superheat = leaving temp – saturation temp= 28F° - 20F° = 8F°

18 psig


Calculating Subcooling

Liquid temp @ outlet = 98F

Subcooling = Saturation Temp – Liquid Temp= 110F – 98F = 12F°

The liquid is sub cooled 12 degrees.

Condenser pressure = 365 psig = 110F° from P-T Chart

Example for R-410A


EquilibriumIn this basic system, there is really only one path for the refrigerant to flow throughIn this respect, it follows the same rules as a series circuit in electricityWhat flows in one area also must flow in the rest of the system


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


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


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


Load Changes and System Pressures• For a certain volume, the higher the density of the vapor, the

more mass it will contain

R404A @ 20 psig1 ft3 = 0.79 lbs.

R404A @ 40 psig1 ft3 = 1.21 lbs.

1 ft3 1 ft3

This cylinder contains 50% more refrigerant


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?


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.


Superheat and Load - TXV

Superheat

Valve Capacity

ReserveCapacity

Rated Capactiy

Full Open Capacity

SS OS

OPS

Static Superheat (SS)

Superheat necessary to overcome spring force

Opening Superheat (OS)

Superheat required to move valve pin from seat

Operating Superheat (OPS) Superheat at which the valve operates (SS + OS)


Superheat and Load – TXV Sizing

Superheat

Valve Capacity

ReserveCapacity

Rated Capactiy

Full Open Capacity

SS OS

OPS

The impact of valve size on superheat will be greatest during periods of heavy load.


Superheat

Valve Capacity

Superheat and Load - TXV

Nominal Load

Pull-down Load

Excessive superheat

Proper sized TXV

Undersized TXV


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


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°

Low load Condensing temp = 104F°, Liquid = 93F°


Condenser Size- TXV Equipped

• A smaller condenser for a given capacity will generate more sub-cooling than a larger condenser

• Similarly, higher ambient conditions will do the same

9F° SC12F°

SC


• 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

9F° SC12F° SC

Ambient is 97F° Condensing temp = 120F°, Liquid = 108F°

Ambient is 76F° Condensing temp = 95F°, Liquid = 86F°

Condenser Size- TXV Equipped


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° 102F°

92F° Ambient


• 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

Ambient T (F°)

14F°

10F°

6F°

2F°

70F° 80F° 90F° 100F°

This is a result of flow rate into evaporator increasing as condensing pressure increases.

Amb. Effects on SC – Piston & Cap Tube


Subcooling

• A cooler with a TXV has just been loaded with warm product. Superheat and subcooling will be:




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?




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


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


Amb. Effects on SH – Piston & Cap Tube


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°)

Ambient T (F°)

14F°

10F°

6F°

2F°

70F° 80F° 90F° 100F°

This is a result of cap tube flow rate increasing more than the load on the evaporator


For TXV’s it is the opposite concern of too little pressure difference across the valveThis 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

Ambient Effects on Superheat (TXVs)


• 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

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

Ambient Effects on Superheat


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!


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


Condenser Pressure Control

When the outdoor temperature is lower, the condenser acts like it is a lot larger

Temperature = 50F

Temperature = 90F



• 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

90 degree Fahrenheit Air Inlet Temperature

Condensing Temperature = 110 Fahrenheit



• 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

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

Less airflow needed when air inlet is only 50 degree °F



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

Setting of Controls



P

DifferentialUSP = Upper Set PointLSP = Lower Set Point

USP

LSP

I. When the pressure exceeds the upper set point, contacts 1 and 4 make and bring on fan/s

I.

II. When the pressure falls to the lower set point, the contacts change back to the initial position and turn fan/s off.

II.

Fan Control Operation




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

Minimum required condensing temperature = 80°F


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)



Condenser Pressure Regulators

Used in the liquid line before the receiver

Condenser


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


Condenser Pressure Regulator

• 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

This the way the condenser behaves when liquid is backed up in it

Only a portion of it can reject heat


Heat (Energy)Enthalpy

Tem

pera

ture

Enthalpy is the heat in BTUs per pound added to or removed from a substance, in this case water.

superheatingvapor

heatingwater (liquid)

212 °F

boiling water(liquid + vapor)

970 BTU/lb

180 BTU/lb

Heat Energy - Enthalpy


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


Basic TroubleshootingCauses 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


Causes of Abnormal SuperheatMetering 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



• 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

The piston in this distributor should be sized correctly


• 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



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!

Causes of Abnormal SuperheatSensing Bulb Heat Transfer


Header

Suction Line

TXV Bulb

Guidelines:• 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




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


Of those with faults, 90% of returned valves are inoperable because of outside influence, primarily contamination or overheating

Causes of Abnormal SuperheatMetering Device Blockages

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.


• 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!

Screen traps dirt in system

Causes of Abnormal SuperheatMetering Device Blockages


R404A @ 235psi8F° of subcooling

60psi pressure drop due to plugged drier

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.

Causes of Abnormal SuperheatLine component Restrictions


External Equalizer Inlet

To Evaporator

Outlet

Evaporator pressure acting on diaphragm is from

outlet, not inlet

Evaporator pressure acting on diaphragm is from

inlet

Internally Equalized TXV

Externally Equalized TXV

Equalizer pressure reaches diaphragm from valve outlet – along

pushpin or a bored hole

Causes of Abnormal SuperheatInternal Equalization with High PD EvaporatorsAn evaporator with a high equivalent pressure drop requires the use of an externally equalized TXV


Causes of Abnormal SuperheatInternal 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


• 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



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


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!

Causes of Abnormal SuperheatIncorrect Superheat Adjustment


Summary

• 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

Troubleshooting - Superheat


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


• 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.

Troubleshooting - Suction line too warm (High 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

Troubleshooting - Cannot adjust superheat


• 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

Troubleshooting - Flooding the evaporator


Helpful Website Linkhttp://www.danfoss.com/North_America/BusinessAreas/Refrigeration+and+Air+Conditioning/Aftermarket+Contractor+Corner.htm

Refrig TXV TUA/TUAEDanfoss Refrigeration | 69 September 2009 | 69

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