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Competitors Comparison

Danfoss Educational Program. Interpreting System Symptoms . Competitors Comparison. 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

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Competitors Comparison

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  1. Danfoss Educational Program Interpreting System Symptoms Competitors Comparison DS, TSO

  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

  3. 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

  4. 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

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

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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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. R404A @ 40 psig 1 ft3 = 1.21 lbs.

  12. 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?

  13. 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.

  14. Superheat and Load - TXV • 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) Valve Capacity Full Open Capacity Reserve Capacity Rated Capactiy OS SS Superheat OPS

  15. Superheat and Load – TXV Sizing Valve Capacity The impact of valve size on superheat will be greatest during periods of heavy load. Full Open Capacity Reserve Capacity Rated Capactiy OS SS Superheat OPS

  16. Superheat and Load - TXV Proper sized TXV Undersized TXV Valve Capacity Pull-down Load Nominal Load Excessive superheat Superheat

  17. 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

  18. 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°

  19. 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° SC 12F° SC

  20. 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°

  21. Ambient Effects on SC - TXV Equipped A condenser that is recycling discharge air will have an abnormally high condensing pressure anda high subcooling level 92F° Ambient 92F° Ambient 112F° 112F° 102F° 102F°

  22. 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° Ambient T (F°) 100F° 70F° 80F° 90F°

  23. Subcooling • A cooler with a TXV has just been loaded with warm product. Superheat and subcooling will be: • Higher than normal? • Lower than normal?

  24. 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?

  25. 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

  26. 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

  27. Amb. Effects on SH – Piston & Cap Tube

  28. Amb. Effects on SH – Piston & Cap Tube

  29. 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° Ambient T (F°) 100F° 70F° 80F° 90F°

  30. 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

  31. 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

  32. 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

  33. 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!

  34. 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

  35. Condenser Pressure Control When the outdoor temperature is lower, the condenser acts like it is a lot larger Temperature = 90F Temperature = 50F

  36. 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

  37. 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

  38. 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

  39. 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

  40. I. II. I. When the pressure exceeds the upper set point, contacts 1 and 4 make and bring on fan/s II. When the pressure falls to the lower set point, the contacts change back to the initial position and turn fan/s off. Condenser Pressure Control Fan Control Operation P USP LSP USP = Upper Set Point LSP = Lower Set Point Differential

  41. 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

  42. 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)

  43. Condenser Pressure Regulators Condenser Used in the liquid line before the receiver

  44. 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

  45. 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

  46. superheating vapor 212 °F boiling water (liquid + vapor) heating water (liquid) Heat Energy - Enthalpy Enthalpy is the heat in BTUs per pound added to or removed from a substance, in this case water. Temperature 970 BTU/lb Heat (Energy) Enthalpy 180 BTU/lb

  47. 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

  48. 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

  49. 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

  50. Causes of Abnormal SuperheatMetering 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 The piston in this distributor should be sized correctly

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