RTP Technical Bulletin Category: REFRIGERATION REGISTERED TECHNICIANS PROGRAM Volume 1 Bulletin 2 the information contained in this bulletin contains some important c o n c e p t s regarding pressure/ temperature conditions throughout the circuit. if you have a good q u a l i t y thermocouple or temperature probe we would encourage you to reinforce the concepts presented here with some practical testing. this testing will not be required on all systems (although many professionals do) but is vital for conceptual understanding you can apply in p r o b l e m systems or w h e n performance is below standard PRINCIPLES OF REFRIGERATION (Stage 2) In Bulletin 1 we discussed the basics of refrigeration. In this bulletin we will continue with that theme and introduce the more complex aspects of system operation. In particular we will focus on specific pressure and temperature conditions that may be exhibited throughout the system. It must be stated at this point that some of the concepts and readings presented here may be foreign to you. The modern automotive air conditioning system is significantly different in many areas from those of bygone eras. The second point is that not all systems are the same when in operation. We are not talking about system designs here (TX Valve/Orifice Tube etc.), we are talking about system dynamics - the actual behaviour of refrigerants through the system. If the contents of this Bulletin are understood the identification of system limitations and system faults becomes much clearer, particularly when reinforced by workplace learning/experiences. Pressure/Temperature Relationships Bulletin 1 presented the pressure/temperature (P/T) relationship of refrigerants and the concepts of change of state and change of temperature. The relationship was shown on graph form. Please review the graph (Bulletin 1) at this point if required. The more common way to present the P/T relationship of refrigerants is not with a graph but with a table or chart. There are 2 P/T charts included in this bulletin as Technical Data Sheets 4 & 5. Please refer to them when reading this Bulletin. With reference to R12 and R134a charts it can be seen that the relationship is as Bulletin 1 P/T graph indicated. 1
ie: R134a - at 191 kpa (28 PSIG) pressure the temperature of the refrigerant is 0 C - at 1580 kpa (229 PSIG) the temperature of the refrigerant is 60 C. Further discussion in Bulletin 1 highlighted that this is not only the pressure/ temperature relationship, but it is also the change of state temperature or liquid saturation temperature. Therefore when we read the gauges what can we deduce from the readings? Low Side Gauge = 191 kpa = 0 C refrigerant inside the evaporator that is changing state at this temperature (as the name evaporator suggests - LIQUID to a VAPOUR). High Side Gauge = 1580 kpa = 60 C Refrigerant inside the condenser that is changing state at this temperature (as the name condenser suggests - VAPOUR to a LIQUID). The change of state is facilitated by absorbing heat into the cold refrigerant in the evaporator and releasing heat from the hot refrigerant in the condenser. The gauges tell you the conditions that exist inside the heat exchangers. Now let s go one step further: The reality is the pressure temperature graph/chart only relates to the refrigerant undergoing a change of state inside the two heat exchangers (the evaporator and the condenser). The pipes entering and exiting the heat exchangers may be (and in some cases must be) at significantly different temperatures than the gauges indicate. It is the concepts of Superheating and Subcooling that we now need to identify. These concepts are best demonstrated with the use of a basic form ENTHALPY DIAGRAM. Enthalpy The pipe temperatures may not be in accordance with what the gauges indicate - let s find out why! Enthalpy can be defined as the total heat content of a substance -otherwise defined as the sum of the energy applied to a medium. (Heat is a form of energy). Let s clarify what actually happens as we heat up and cool down a liquid, We will use water as an example since it has a familiar boiling point (or change of state temperature), or as we identified Liquid Saturation Temperature. 2
Between 0 C and 100 C adding heat to water causes a rise in temperature (the water gets hotter), THIS IS KNOWN AS SENSIBLE HEAT (add heat to water, it gets hotter - it makes sense - hence the name sensible heat). 100 C is the boiling point of water - in refrigeration terms this would be referred to as its Saturation Temperature. At 100 C we have reached a SATURATED LIQUID CONDITION. Water has absorbed as much heat as it can in liquid form and any further heat added will cause a CHANGE OF STATE. This extra heat to cause a change of state from Saturated Liquid to a Saturated Vapour is known as LATENT HEAT. Whist this change of state occurs there is no change in temperature. The extra head added is the energy to cause a change of state - not a change in temperature. If a complete change of state occurs and extra heat is added past this point the vapour becomes SUPERHEATED. Let s show this in diagrammatic form. Added heat will do one of two things - it will either raise temperature OR cause a change of state. Period A to B = Heating a liquid up to its boiling point = Adding Sensible Heat Period B to C = Saturated Liquid to Saturated Vapour = Change of State = Adding Latent Heat (no temperature rise during this phase) Period C to D = Adding heat to Saturated Vapour (Change of state has finished). = SUPERHEATING OF VAPOURS. Let s define the phases. 3
A liquid that is at below its boiling point is known as a SUBCOOLED LIQUID. A liquid that is boiling off is called a SATURATED LIQUID undergoing a change of state to a SATURATED VAPOUR (LATENT HEAT PHASE). A vapour that has extra heat added to it after the change of state phase is known as SUPERHEATED. QUANTITIES OF HEAT Strictly we can t simply measure heat with a thermometer. The only readily measurable heat is Sensible Heat which we see as a change in temperature. The head added during a change of state (Latent Heat) is otherwise referred to as Hidden Heat. Large quantities of heat are present in vapours in the form of Molecular Activity which we cannot readily measure or feel. Latent heat present in vapours we cannot measure because it is in the form of increased Molecular Activity. The true measurement of heat is the previously mentioned ENTHALPY. You have probably heard of the Imperial Measurement of heat - The British Thermal Unit (BTU). 1 BTU is the heat required to raise 1 pound of water by 1 F. The metric unit of heat is the kilojoule or kilocalorie. We will use kilocalories (KCal) because of their ease of use in these examples. 1 kcal is the heat required to raise 1 kg of water by 1 C. Technical data will normally refer to kilojoules of heat energy since joules/kilojoules are the true units of energy. Since we cannot simply measure heat content with a thermometer we need to go one step further and incorporate enthalpy into the equation. 1 kcal = 4.17 kj. 4
Let s now review and add to our previous example and view the heat quantities during the Sensible Heat and Latent Heat ranges. 100 kcal of heat energy is required to raise 1kg of water from 0 C to 100 C (from its freezing point to its boiling point)... Whereas... 539 kcal of heat energy is required to get that 100 C water to boil off to a vapour. In the period of Sensible Heat - raising the temperature of the liquid up to its boiling point (100 C) from 0 C we add 100 kcal of heat energy: 1 kcal = 1 C rise for the 1 kg water 100 kcal = 100 C rise for the 1 kg water. We now have a liquid at its boiling point - a saturated liquid. If we look at the next phase (evaporation) a further 539 kcal of heat is required to get that 1 kg of liquid to all change to a vapour. THIS IS A VITAL ASPECT OF AIR CONDITIONING. TO CAUSE A CHANGE OF STATE FROM A LIQUID TO A VAPOUR LARGE AMOUNTS OF HEAT MUST BE ADDED WITH NO CHANGE IN TEMPERATURE. THE HEAT ENERGY IS HIDDEN IN THE FORM OF MOLECULAR ENERGY (INCREASED MOLECULAR ACTIVITY) CONTAINED IN THE VAPOURS. THIS IS WHAT OCCURS IN THE EVAPORATOR. 5
If extra heat is added past the point of a Saturated Vapour (ie all the liquid has boiled off) we will cause the temperature of the vapour to rise - that is we will now have a Superheated Vapour. Notice the heat quantity - only 9kCal of added heat energy will drive the temperature of the vapour up by 20 C. THE SECOND VITAL POINT - PARTICULARLY APPLICABLE TO MODERN SYSTEMS: IF HEAT IS ADDED PAST THE SATURATED VAPOUR POINT THE VAPOUR WILL GET HOTTER. ie IT WILL BECOME SUPERHEATED. SMALL HEAT INPUT WILL GENERATE LARGE SUPERHEAT LEVELS. If heat is applied to a vapour it will rapidly increase in temperature. This is known as Superheat. Now let s look at the reverse of evaporation - the condensation phase. If we have a vapour it contains large quantities of heat locked in it in the form of Latent Heat Energy. This vapour therefore has high levels of molecular activity. The way to get a vapour to condense back into a liquid is to remove heat from it. This will reduce molecular activity (cause the molecular to slow down) to a level where the vapours can reform into a liquid. With reference to the diagram below, if we take heat our of a Saturated Vapour it will return to a Saturated Liquid. But remember the vapours have large quantities of heat that must be removed in order for condensation to occur. (539 kcal/kg). The place where this change of state occurs is the condenser. Condensing efficiencies must be at an optimum to ensure enough heat is dumped off to allow complete condensation to occur. The latent heat present in vapours must ALL be removed if the vapour is to condense back into a liquid. 6
ENTHALPY IN THE AIR CONDITIONING SYSTEM Now it s time to relate enthalpy to an Air Conditioning System. The refrigerant exiting the TX valve is just like water at 100 C - if we add extra heat it will begin to boil off. We know that refrigerants are a naturally cold substance. In fact we know that R134a is approximately - 26 C at atmospheric pressure and that is also its boiling point (liquid saturation temperature), (Refer to graph. chart). We also know that if we raise the pressure of the refrigerant not only do we raise its temperature but also its boiling point (liquid saturation point). That is the three way relationship for pure refrigerants. Let s now analyse the thermodynamic properties of the refrigerant as it flows through the system. This will be done in two stages - a basic explanation then a detailed analysis including superheating and subcooling. IN THE EVAPORATOR Exiting the TX Valve we have a low pressure, low temperature/low boiling point liquid refrigerant. (Refer to P/T chart.) Example Pressure = 190 kpa Temperature = 0 C Boiling Point = 0 C The conditions that allow the refrigerant to boil off so readily in the evaporator is that it is cold and has a low boiling point. Both as a result of its low This is just like water at 100 C. If we add any more heat we enter into the latent heat phase. Where does the heat come from? The cabin. How much heat does it absorb? Massive amounts. What does this heat do? Causes the refrigerant to boil off or evaporate inside the evaporator. What happens to the temperature of the refrigerant as it boiled off? No Change. the whole evaporator will be at 0 C (refrigerant temperature) whilst the change of state is occurring. (Disregard pressure drop across the evaporator coil - we will discuss this later.) All that has happened in the evaporator is we have absorbed massive amounts of heat from the cabin to boil off the refrigerant. The conditions that allowed this to happen are: the refrigerant is at a low pressure hence it is cold hence it already absorbs heat from the cabin. 7
In addition: At low pressure it has a low boiling point otherwise known as saturated liquid temperature, therefore: as the cold refrigerant absorbs cabin heat it boils off or more correctly turns to a saturated vapour whilst no change in refrigerant temperature occurs. You can test this simply on a system where in inlet and outlet pipes from the evaporator are accessible. Feel (or better still, measure with a good quality thermometer) the pipes entering and exiting the evaporator. You will find on a correctly operating system they are very close to the same temperature. Under test you will find the entry and exit pipes of the evaporator will be at near the same temperature if the system is operating correctly. IN THE COMPRESSOR The low pressure, low temperature Vapour now enters the compressor via the suction pipe. The important concept to grasp here is that the vapour contains heat energy - even though it is still cold - it contains LATENT HEAT absorbed from the cabin. The compressor, acting against the restriction of the TX Valve raises the temperature of the vapour to say 1580 kpa (229 PSI) with a corresponding temperature of 60 C (134a) (Refer to P/T chart) by pressurizing the vapours. IN THE CONDENSER The heat laden, high temperature vapour now enters the condenser. With a refrigerant temperature much higher than the surrounding air ie: refrigerant at 60 C, air at say 35 C the condenser will readily remove heat from the refrigerant to the cooler air providing airflow is adequate. If sufficient quantities of heat are removed at the condenser then the vapours will condense back into a liquid. Therefore the refrigerant has been cleaned of latent heat that was absorbed in the evaporator, and is ready to be reloaded with heat as it next passes through the evaporator coil. THROUGH THE TX VALVE/ORIFICE TUBE The high pressure, high temperature liquid is next fed to the TX Valve/ Orifice Tube where its pressure and temperature are dramatically reduced back to 190 kpa - 0 C (example only). The cold liquid refrigerant is sprayed into the evaporator where it can once again absorb heat. Even though the suction pipe is cold to touch it contains heat energy - Latent Heat hidden in the vapours. The refrigerant MUST be cleaned of latent heat to allow it to condense back into a liquid. 8
SUMMARY OF THE BASIC OPERATION Cold liquid refrigerant is sprayed into the evaporator via the TX Valve/ Orifice Tube. For a system to be balanced the condenser must be capable of dumping off the heat absorbed in the evaporator. Large quantities of latent heat are absorbed to cause a change of state to a vapour. The heat laden vapours are then fed to the compressor via the suction pipe. Note: The Suction pipe will be cold to touch - remember this is latent or hidden heat, we cannot feel or measure with a thermometer. The compressor raises vapour temperatures by pressurizing the refrigerant to ensure the refrigerant inside the condenser is much hotter than the surrounding air. The condenser will then dump off the latent heat that was absorbed in the cabin to the correspondingly cooler air, allowing condensation to occur. The heat cleaned liquid is then returned to the TX Valve/Orifice Tube (via the receiver drier for TX Valve systems) where its pressure and temperature are reduced. The low pressure cold refrigerant is the re-fed into the evaporator and the cycle starts over again. Superheating And Subcooling Air conditioning systems work in the change of state phase that occurs in both heat exchangers. The next phase in understanding air conditioning operation is to identify SUPERHEATING and SUBCOOLING characteristics. As has been explained in Bulletin 1 and in the preceding part of this Bulletin, air conditioning systems principally operate in the change of state phase in both heat exchangers. Up until recently with R12 and relatively simple systems this is all we have needed to be concerned with, but it is becoming increasingly obvious we need a deeper understanding of what actually occurs in a system including the subcooling of liquids and the superheating of vapours. 9
If the dynamics of Superheating are fully understood we can identify (together with other indicators): correct TX Valve operation the need for suction line insulation (lagging) the causes of thermal limiter (superheat) switch activation the possible causes of premature compressor failure or wear identify recompression (internal bypass) in compressors identify possible system contamination If we understand the principles of liquid subcooling it can identify (together with other indicators): The concepts of superheating must be understood for the technician to fully appreciate the possible causes of premature compressor failure. inadequate condensing efficiencies causes of inadequate condensing (ie airflow) incorrect charge rates causes of sight glass foaming/bubbling possible system contamination excessive pressure drops (in conjunction with gauge analysis) Subcooling directly relates to the condensing efficiency of the system. SUPERHEATING We will split superheating into 2 categories: 1. Superheating in the evaporator coil/suction line 2. Superheating in the discharge line (exiting compressor) As previously explained when a liquid boils off from a saturated liquid to a saturated vapour it does not change temperature - but it any further heating occurs the saturated vapour will become superheated. In the evaporator a small amount of superheating must occur. This is the superheat setting of the TX Valve, and is the safety margin that is built in to ensure liquid slugging of the compressor does not occur. How Does This All Work? The refrigerant is sprayed into the evaporator as a saturated liquid and immediately starts taking on heat from the cabin air. This heat (as previously explained) is used to facilitate a change of state to a saturated vapour. There are two categories of superheat we need to consider - superheat in the suction line and superheat in the discharge line. 10
The TX valve capillary tube/ internal sensor checks a predetermined amount of superheat is present in the vapours exiting the evaporator. If however this change of state is only just completed as the refrigerant exits the evaporator there is too much risk that a liquid may get back to the compressor with varying heat loads and TX valve openings. What actually happens is a complete change of state occurs a little bit back in the evaporator. Therefore in the last portion of the evaporator coil the vapours will heat up or become superheated. It is superheated vapours (normally 2ºC - 6ºC) that are sampled by the TX valve - sensing bulb, to calibrate flow into the evaporator coil. (This sensing is done internally in block valves.) If the vapours leaving the evaporator are superheated to above normal levels the sensing bulb will drive the TX open to provide for more refrigerant flow. If the vapours leaving the evaporator have no superheat present there is a risk of liquid slugging (we are still in the change of state phase). In this case the TX sensing bulb will shut down the TX valve to restrict flow to reintroduce some superheating. By sampling superheat content the TX valve will critically control flow in exact proportion to the heat load placed on the evaporator. A detailed explanation of TX valve operation and calibration will be presented in future bulletins. CHECKING EVAPORATOR SUPERHEAT LEVELS (BASIC GUIDELINES) The refrigerant inside the evaporator will normally be colder than the actual fins and tubes. Things are not as easy as they might appear in checking evaporator superheat levels. In theory if a TX valve is set at 4ºC superheat one could imagine that if we were to check the inlet and outlet pipes to the evaporator (where accessible) the pipe leaving the evaporator coil would be 4ºC warmer than when it entered. This is not the case due to the existence of a pressure drop across the evaporator coil. In reality many automotive evaporator coils have approximately 3ºC to 4ºC temperature drop as the refrigerant boils off due to a reduction in pressure cause by a restriction to flow. This pressure drop varies significantly with the coil design but lets work with 4ºC for this example. Refer to P/T chart - the temperatures given in this example (as with our discussions thus far) relates to refrigerant temperature. The external surfaces of the evaporator will be above 0ºC due to heat gains through the tubes/fins. Refrigerant to evaporator differentials will be discussed in detail in advanced analysis. 11
If the refrigerant entered the evaporator at say 170 kpa (25 PSIG) its temperature would be -2ºC. PRINCIPLES OF REFRIGERATION As the refrigerant flows through the evaporator coil loses some pressure - let s say it drops to 133 kpa (19 PSIG) therefore its vaporisation temperature drops to -6ºC. It actually gets 4ºC colder as it boils off. Once it has all vaporised (at -6ºC) it then superheats by 4ºC before exiting the evaporator coil. The result is the vapour temperature exiting the evaporator is -2ºC - exactly the same temperature as it was when entering the coil in liquid form. HOW IS THIS OF USE TO US? If we were to externally measure pipe temperatures on the entry and the exit of the evaporator (when accessible) what would we expect to get? Answer - nearly the same temperature if the TX valve is controlling flow correctly. Of course to do this accurately you need to know the exact pressure drop across the evaporator coil, at a certain flow rating, and also the superheat setting of the TX valve. But as a general field test guideline most automotive systems run + 2ºC inlet to outlet variation. The refrigerant will actually get colder as it travels through the evaporator due to a pressure drop. This is what causes the cold spot of the evaporator some technicians may have observed. We can now do some basic fault diagnosis using this principle. If the pipe leaving the evaporator is much warmer than when it enters what does it indicate? Answer - A partially filled coil with a longer than normal superheat run. This is normally a result of low charge, blocked liquid line, blocked TX valve/orifice tube, all of which will starve the coil. If the pipe leaving the evaporator is much colder than when it enters what does it indicate? Answer - A flooded coil with a significant risk of liquid floodback. This may be accompanied by frosting of the suction line but there are other causes of frosting which will be discussed in detail in future editions of RTP. If frosting exists basic tests would include a check of thermostat setting (set too low) or partial internal blockage of the evaporator (causing an excessive pressure drop across the coil), poor airflow or inoperative blower fan (evaporator). Pressure drops vary dramatically with the design and flow ratings. We are only presenting some general guidelines here - but the concepts are CRITICAL. 12
The capillary tube/ upper diaphragm force directly controls the opening and closing of the valve to control flow... What causes the valve to open? EXCESS Of course in many (most) cases you cannot readily access the inlet and outlet pipes of the evaporator to do this testing, so in future Bulletins we will be presenting guidelines for comparing gauge readings to pipe temperatures, analysing frost lines, and setting of TX valves when applicable. This edition has presented the concepts of evaporator superheat only. DISCHARGE LINE SUPERHEAT vs EVAPORATOR SUPERHEAT. When the valve opens we more effectively fill the coil. As air conditioning technicians many of you have probably touched the discharge line of the compressor to find it is extremely hot - yet the gauge may not indicate a high temperature/pressure. Remember though the gauge only indicates the conditions that exist within the heat exchangers. The reality is the superheating that occurred in the evaporator will not be shown on the low side gauge. That is the gauge may read 190 kpa = 0ºC but the refrigerant in the suction line may be 5ºC as it leaves the evaporator (5ºC superheating). Superheat cannot be read on a gauge - it can only be measured with a thermometer. Superheated vapours have increased molecular activity to cause them to rise in temperature - not to rise in pressure. 13
The superheated vapours travelling down the suction line may absorb further heat from the engine bay causing them to rise even further in temperature (increased superheat). The vapours may be 15ºC when they enter the compressor - even though the gauge reads 190 kpa = 0ºC. These vapours returning to the compressor are critical to provide cooling and for oil return. If the vapours returning to the compressor are excessively superheated (compounded evaporator superheat and suction line superheat) then discharge line temperatures may rise to unacceptably high levels, together with insufficient oil return. This is the role of the superheat switch/thermal limiting switch/thermal fuse. Discharge line superheat levels vary from system to system dependent on a number of factors but in simple terms if a thermal switch is activating we must ask ourselves the question. Is the compressor being fed with cold vapours to prevent excessive superheat generation? If the vapours returning to the compressor are cold then there is nothing we can do about the high superheat levels. At excessive highway speeds some compressors/ systems will experience thermal switch activation particularly under high ambient conditions. If a customers complaint suggests the activation of the thermal switch (air conditioner cuts out on highway cycle) then all we can do is ensure evaporator superheat levels are okay and that excessive heat loading of the suction line is not present (most manufacturers now insulate the suction line, and in some cases the accumulator on CCOT/FOT systems to limit suction line superheat). If the evaporator is full (with normal superheat levels) and the suction line is cold then high discharge line superheat levels are probably a normal condition for the system under question. Many modern systems run high discharge line temperatures as a normal operating condition - whereas in older systems superheating did not exist. It is becoming increasingly important to ensure excessive superheating does not exist as this is an extremely common cause of compressor failure - particularly in systems that have no superheat switch protection and work in high ambient conditions/highway cycles. The vapours returning to the compressor not only provide cooling, but also are responsible for carrying sufficient oil back to Excessive suction line superheat will be directly reflected as discharge line superheat on most modern systems. The most common causes of excessive superheat are partially blocked TX valves/orifice tubes, undercharged systems (common in retrofit) and partial restrictions of the liquid line/drier. (Check gauges for verification of these faults.) High discharge line superheat levels existing in modern systems are generated by high under bonnet temperatures, compressor placement, compressor design etc. 14
SUMMARY Remember: superheat cannot be read on the gauge - that is evaporator and condensing temperature - superheat is an overheating of the vapours to above the normal pressure/ If the discharge line appears extremely hot, or hotter than normal, then check the suction line temperature - it should be cold to touch to ensure the compressor is being adequately cooled. If it is cold at the evaporator outlet, and warm at the compressor then insulating (lagging) of the suction line may be necessary - this is often recommended on long hose run systems. IMPORTANT WARNING CONTAMINATED REFRIGERANTS (EITHER AIR AND OTHER NON CONDENSABLES) OR MIXTURES OF REFRIGERANTS ARE COMMON CAUSES OF EXCESSIVE SUPERHEAT AND/OR COMPRESSOR FAILURE. THE PRECEDING TEST HAS BEEN ADDRESSING PURE R12 and R134a ONLY. IF YOU SUSPECT CONTAMINATION EITHER TEST THE REFRIGERANT WITH AN APPROVED IDENTIFIER OR DUMP THE CONTAMINATED JUNK (OR RECLAIM AS REQUIRED INTO A DUMP CYLINDER WHEN REQUIRED BY LEGISLATION) AND DO A COMPLETE SERVICE WITH FILTER/DRIER CHANGE. NITROGEN PURGING AND/OR A 134a SCAVENGE CHARGE In addition to excess superheating causing premature compressor failure there is the aspect of oil return to also be considered. As refrigerant quantities and velocities reduce so do oil return rates to the compressor - thereby raising the possibility of oil starvation, particularly in low oil charge systems with upflow condenser/evaporator designs. Oil circulation rates will be discussed in detail in future Bulletins. 15