RTP Technical Bulletin

Transcription

1 RTP Technical Bulletin Category: RETROFITTING REGISTERED TECHNICIANS PROGRAM Volume 1 Bulletin 1 this information is presented to clarify some misconceptions that occur, in c h a r g i n g retrofitted systems. it does not contain s p e c i f i c recommendatio ns regarding component changes but gives guidelines for analysing s y s t e m s correctly to e n s u r e performance is maintained for all ambient conditions. a significant amount of retrofits are conducted with charge rates insufficient for high ambient conditions causing poor performance under high heat loads and can also lead to compressor failure. System Charging In Retrofit With the changeover to R134a most technicians have either attended formal training or have simply learnt by experience what is required in an average retrofit. The very early change almost everything except for the air conditioning switch mentality has largely been scaled down to a more realistic level. There are two key factors that have generated this scaling down of retrofit requirements: 1 Overstated initial guidelines 2 Price driven market/industry forces. The early recommendations were in many cases overstated due to a shorter than ideal time for testing. With the recognition of the depletion of the ozone layer, and the severity of the depletion, alternative refrigerants were introduced into the industry without a suitable trial/test period for accurately identifying the absolute requirements of a retrofit. This scenario was unavoidable in the interest of ozone protection but it did leave the manufacturers of systems and components in an awkward situation where they had no choice but to overstate the requirements of retrofit in order to protect their systems and components. An example of this is: given the 20% higher flow rate of R134a how did they know that under certain operating conditions there would not be a liquid flood back to the compressor if the existing R12 valve was left in circuit? The answer was to recommend a TX Valve replacement (R134a type) to ensure a perfect matching to the system which would give absolute protection against liquid flood back. Only after substantial field testing have we determined that in most cases the existing TX will shut down to compensate for the excessive flow rate, but if the system is identified as one that will/may be prone to liquid migration then TX replacement is still recommended. The key point from the manufacturers perspective is not that the system or component will get out of warranty but rather that it must maintain its original life expectancy. If the life expectancy of components, particularly compressors, is noticeably shortened then the supplier of that componentry will, in the long term, become non competitive. 1

2 Given this there was no option but to be overprotective until suitable field testing and reliability trials were completed. The second key point is that we are operating in a price competitive market. What has therefore happened is that the price driven market forces are controlling the service and repair sector to a point where in many cases the level of service and repair (ie the procedures and replacement of components in retrofit) is principally a dollar driven decision. This is in fact a key point in charge determination of any system. Ideally any retrofitted system that has not been fitted with alternative componentry (ie parallel flow condensers, smaller receiver driers, alternative hose runs etc) should accept 90% of original R12 weight (see next). In reality if the componentry does not have the capability to happily accept R134a (particularly the condenser) then we are in an area of compromise. An area where we are charging a system within the limitations of that system. This is where the sub 90% charging rules originate. Many VASA technicians will have heard of 60, 70 and 80% charge ratios, and possibly practice these charging procedures. In some systems and operating environments this is fine - in others it is highly undesirable. This article will address the issues to consider when practicing sub 90% charging. In a price driven market many retrofits are conducted with componentry that is inadequate and the charge rate tailored accordingly. The 90% Charge Rule When a system is charged with refrigerant we have characteristically used the weight charging method. This has led us to the belief that it is the weight of refrigerant that enters the system that is important. In fact it is the LIQUID VOLUME that is the critical factor. When comparing R134a and R12 there is approximately a 10% weight differential to liquid volume (density differential). In simple terms 1kg of R134a will occupy a 10% larger volume than did 1kg of R12. Now ask yourself the question - where is the 10% extra liquid going to fit in the system? The answer is nowhere - you will be in the overcharge band with excessive head pressures. liquid flood back etc a distinct possibility. The answer is to reduce the WEIGHT charge by 10%. This will ensure the liquid volume in the system is equal to what is was designed for. The system should operate correctly if component compatibility is acceptable. If however problems start to arise before the 90% charge ratio is achieved then there is either a limitation in a component design/size with respect to its ability to handle R134a or there is a component malfunction which is limiting its ability. These limitations are usually related to condensing capability. Low charge rates are undesirable in retrofit for 4 reasons 1 Inadequate performance under high heat loads, 2 Excessive superheat ing under high heat loads, 3 Reduced oil circulation, 4 No safety margin when minor leakage occurs. 2

3 R134a - Sight Glass Charging It is imperative the technician identify the cause of a foaming sight glass. It must be proved it is not an undercharge or lack of condensing in retrofitted systems. Lack of performance under high heat loads will result from both conditions. Most technicians by now realise the limitations of sight glass observation of R134a systems. It must be identified however that the sight glass (if fitted) may still, in many systems, by an indicator of charge rates. It is just that it cannot be used as an absolute indicator - because in many cases it will bubble or even foam severely. Our role is to identify charge rates by all of the indicators that the system under test shows. WHY DOES THE SIGHT GLASS FOAM/BUBBLE ON R134a WHERE IT DIDN T ON R12? Basically there are 4 reasons for a bubbling/foaming sight glass: 1 Oil foaming 2 Turbidity of R134a 3 Lack of Condensing 4 Undercharge 3

4 OIL FOAMING Basically a problem of retrofitting. If mineral oil is left in the system excessive foaming and agitation with the R134a compatible synthetic oils may/will occur. This renders the sight glass useless. NOTE: Genuine R134a systems with known oil types do not have oil foaming problems - there are of course no oil mixing problems in these systems. However the sight glass may still foam for one of the following reasons. A case for flushing - it removes the variable of oil foaming in the receiver drier due to oil types being mixed. TURBIDITY R134a exhibits turbid characteristics in some systems in the drier. This will cause unwanted bubbling or foaming of the sight glass even though the eductor tube (stand pipe) is covered with liquid. Foaming or bubbling due to turbidity can be ignored if it is the identified cause of the bubbling/foaming. With R12 there were basically 4 stages of sight glass indication whilst charging: 1 Clear sight glass - oil smearing - pure vapour in the system- very low charge rates. 2 Heavy foaming of sight glass - a small amount of liquid in the base of the drier mixed with a high vapour content - under flow conditions this caused an agitated foamy mixture. 3 Bubbling sight glass - the stand pipe/educator tube was covered but some bubbling, due to agitation under flow, was still picked up. 4 Clear sight glass - there was a sufficient quantity of R12 in the system to cover the bottom of the stand pipe to a degree that no bubbles were picked up. This was our correct charge where a solid liquid stream (liquid head) to the TX valve was guaranteed. This ensured maximum system performance under all operating conditions/heat loads. With the knowledge that the sight glass may foam due to one of the above conditions, it is important that we establish some critical guidelines for charging of systems and the analysis of condensing action. These main points are the subject of the rest of this Bulletin. A clear pool of liquid refrigerant must be present in the base of the receiver/drier - this ensures liquid feed for the TX and gives a reserve quantity of refrigerant for leakages/weeping. If a reserve is not present there is an increased possibility the customer will return within the season complaining of lack of performance - particularly if their system is serviced early in summer (seasonal locations) 4

5 Charging Options There are four options of charging: 1 Weight charging 2 Sight Glass charging 3 Pressure charging 4 Monitoring of Condenser Subcooling. The 90% charge rule in retrofit allows for the density differential between R12 and R134a. R134a is 10% lighter so charging to 90% ensures an equal liquid volume enters the system. CAUTION: None of the above methods of charging should be used in isolation. In all cases pressures must be analysed and in retrofitted systems subcooling checks are recommended to verify correct charge rates and/or satisfactory condenser performance. WEIGHT CHARGING The simplest of all methods of charging is to weigh in a specified quantity of refrigerant as specified by the manufacturer. As previously mentioned the recommendation is for 90% charge ratios to be used in retrofit to ensure the equal liquid volume is maintained. (Accounts for the R12 to R134a density differential.) Before weight charging a system ensure the following factors are taken into account. A thorough inspection must be carried out to ensure the system is unaltered from manufacture. Pulling hoses dry with the high side depressor backed off is a dangerous practice - unless some pretesting verifies there is no danger of runaway head pressures. To accurately weight charge a system the hoses must be pulled dry or an allowance made for the quantity of the refrigerant that is retained in the hoses at the completion of charging. Note: A 2 metre set of service hoses will contain approximately 100 grams of refrigerant unless they are pulled dry. CAUTION: When weight charging retrofitted systems DO NOT pull the hoses dry or balance off the gauges unless some pre-testing has been done on an identical system to ensure that the extra quantity pulled into the system will not drive head pressures up to an unacceptable level. When pulling hoses dry once the high side valve depressor is backed off the high side pressure can no longer be assessed. The extra quantity (up to 100 grams) can be a massive overcharge to a retrofitted or small capacity system. It is for this reason it is strongly recommended the hoses are left full on completion of charging UNLESS prior verification testing is performed. 5

6 SIGHT GLASS CHARGING Contrary to popular opinion the sight glass may still clear at correct charge rates in a correctly operating system. Whilst weight charging or pressure/ subcooling charging keep an eye on the sight glass. If it happens to clear then it still remains a valid indicator. The important thing to realise is it may not clear and this is where the other indicators are critical. PRESSURE CHARGING Clear sight glass indicates a liquid supply to the TX valve - If the sight glass does clear it is still a valid indicator of charge rates. In reality pressure charging should never be done in isolation. It should be married with weight charging, sight glass indication and liquid line subcooling as required. Pressure dynamics will vary significantly dependent on the heat loads placed on the system. However there are some rules that can be used for basic pressure analysis of both genuine and retrofitted R134a systems. In a correctly operating, correctly charged system the evaporator and the condenser will Balance Off. In simple terms this means the condenser will dissipate the heat that the evaporator absorbs - in fact it has to also dissipate suction line superheat and compressor superheat. For this reason the condensers heat radiation capacity is above that of an evaporator. Critically an evaporator will absorb approximately 25 to 30ºC out of the cabin air. On humid days this temperature absorption factor will drop because it has the dehumidification heat loads to contend with (to be discussed later). If the evaporator is absorbing 30ºC out of the cabin then it stands to reason the condenser will need to dump off heat at a differential of 30ºC. Due to the frontal surface area of a condenser being considerably larger than an evaporator its efficiency as a heat exchanger is higher - Given this, well designed systems will balance off when the refrigerant in the condenser is approximately 25ºC hotter than the air surrounding the condenser. In general terms the condenser heat radiation capacity needs to be 1.5 times that of the evaporator in a modern system. This allows for suction line superheating and compressor superheating. Using this +25ºC rule will arm the technician with a basic guideline of establishing what the condensing pressure should be for normal heat loads of 20 to 35ºC (moderate to low humidities). 6

7 Example: Most technicians are familiar with monitoring high side presures - But in both retrofit and genuine 134a systems there are significant advantages in using condensing temperatures as the benchmark for system analysis. Sample the air temperature the condenser is working with - ie the temperature of the air 50mm in front of the condenser with no air flow. Let s use the example of 28ºC. Condenser air on = 28ºC Air to Refrigerant Differential required for = 25ºC adequate condensing = Ideal condensing temperature 53º C = (Refer to P/T chart) Approximately 1320kPa (190PSI) Therefore this system operating and charged correctly will operate with a head pressure of 1320 kpa (190 PSI) to give a condensing temperature of 53ºC. Of course this is in an ideal world with everything working perfectly - but the reality is we must make an allowance to the condensing temperature of + 10% for low humidities and + 20% for high humidities (above 60% Relative Humidity) In the real world a condensing temperature of 60ºC (1580 kpa) (230PSI) would be acceptable for low humidities and up to 66º C (1800 kpa) (265PSI) for high humidity conditions. Using these guidelines establishes a basic pressure/temperature rule for ascertaining head pressures but there are limitations to this clinical approach. For low ambient conditions the doubling rule should be used due to low evaporator heat loads. Low Ambient Conditions Under low ambient conditions (below 25ºC) especially when humidities are also low the +25ºC rule is limited. Going back to the basic system operation why would the condenser establish a 25º differential when the evaporator is only absorbing 18ºof heat (18º day)? The answer is it wouldn t. It would only establish a differential to balance off the system which will only be 18ºC (possibly only 15º to 16º given the efficiency ratio condenser to evaporator). For low ambient conditions it is therefore recommended the doubling rule be used. The doubling rule is simply - sample the ambient temperature and double it. ie: 18ºC day + 18ºC differential = 36º condensing temperature = 810 kpa (118 PSI) 7

8 Once again an allowance of 10% must be made for low to moderate humidities and 20% for high humidities (above 60%). High Ambient Conditions At above 35ºC, especially with high humidities, the evaporator is working at peak capability, the TX valve will be open for a considerable percentage of time causing high flow rates and dense suction vapours to the compressor and the flow rate through the condenser to be higher. These factors load the condenser to a point where it will need to operate at approximately 30ºC higher above its sourcing air (air on) in order for it to dump heat effectively. DETERMINING CHARGE RATES BY PRESSURES A 30º differential will be established at high ambients due to high TX flow rates and dense suction vapours loading the condenser. In addition underbonnet heat leadings will be higher. Things are not as simple here as what they might seem - and those technicians who have been in the trade a while will know this. The relationship between charge rates and condensing pressures/ condensing temperature is not linear. That is we do not have 75% of ideal pressures at 75% charge rates and correct pressures at 100% charge rates. This is not to say we can t use pressures for system evaluation but rather that we need to identify the system dynamics when charging. Let s Go Back To Basics The condenser has to dissipate the heat that the evaporator absorbs. Whilst charging at approximately ½ charge on a 25º day there is a considerable amount of vapour being fed to the TX valve (the receiver drier stand pipe is not covered in liquid). With excess vapour feed to the TX excess superheating will occur across the evaporator (ie reduced volume of refrigerant entering the evaporator = excess superheat). In this case the TX valve will open up to compensate for the vapour feed. At 25º (nominal) the TX will fully open to fill the evaporator coil. In this scenario the evaporator can therefore absorb cabin heat relatively effectively. If we are absorbing cabin heat then the head pressures will already be at recommended levels (ambient +25º) = 50ºC. Past this point we are increasing liquid feed rates to the TX which will effectively cause the TX valve to shut down. The amount of heat absorbed by the evaporator will however remain relatively stable. At moderate heat loads (eg 25ºC) the evaporator can be filled at relatively low charge rates - due to the TX opening up. Once the evaporator is filled and absorbing heat the head pressures will be at or near recommended levels. 8

9 Therefore the head pressures will not rise significantly. This is the formation of the high side Plateau. The high side stabilises off at 50ºC (1220 kpa) (180 PSI) as charging continues up to a point of overcharge. When entering the overcharge band the head pressures kick or begin to wander off the prestabilised plateau. NORMAL PRESSURE CURVE CORRECTLY OPERATING SYSTEM The system should hold the plateau up to 90% charge rates (for retrofitted systems). The correct procedure when charging is to watch the high side gauge carefully for the establishment of the plateau. Past this point the high side should stabilize off or only increase marginally (due to a marginal efficiency rise in the evaporator due to liquid feed.) With continued charging the pressures should remain stable until the system has either: been charged with the correct weight/volume of refrigerant (90% of R12 weight) OR the sight glass clears. 9

10 If the high side kicks or wanders before a 90% charge rate is achieved or before the sight glass clears it clearly indicates a condensing deficiency. The high side kick indicates the condenser is no longer handling the evaporator heat loads at full charge rates when the evaporator is working at peak efficiency and absorbing maximum heat. CAUTION: Below 30ºC (dry air) or 25ºC (humid air) the evaporator is not operating at maximum heat load and therefore a pressure kick may not be indicated. Overcharging is common on cooler days because of the reduced load on the condenser. At these reduced heat loads the system can hold the plateau past the maximum recommended charge point. The problem is on hot days the head pressure runs away. This is where the additional subcooling check (see next) is recommended. A kick off the plateau clearly indicates the condenser is no longer dissipating the heat that the evaporator is absorbing. NORMAL PRESSURE CURVE MARGINAL CONDENSING LIMITATION 10

11 NORMAL PRESSURE CURVE POOR CONDENSING LIQUID LINE SUBCOOLING Liquid line subcooling is a check of condensing efficiency and charge rates. It is a vital check to systems that: Liquid Line subcooling is an additional check that can be used to verify condensing efficiencies or limitations. Do not hold the plateau ( or exhibit rising head pressures) at lower than recommended charge rates Are lacking performance on hot days Fail to clear a sight glass The concept of liquid line subcooling is simple. With reference to Refrigeration Bulletins 1, 2 and 3 we know that: NO LIQUID LINE SUBCOOLING CAN OCCUR UNLESS THE CHANGE OF STATE IS COMPLETED BACK IN THE CONDENSER. 11

12 Once the vapour has condensed it will cool in the last portion of the condenser. It is this subcooling that is the key indicator of adequate condensing. There are however various reasons for lack of condensing. Most of them are obvious and have been our basis for evaluation for years: blocked condenser (external) blocked radiator (external) restricted airflow (coolers, insect screens etc) undersized condensers inoperative electric fans loose drive belts (mechanical fans) faulty viscous drive hubs. Any one or more of the above factors may lead to lack of condensing. There are two additions to this list which are largely a product of the retrofit era: contaminated refrigerants (mixtures) higher latent heat capacity of R134a. Many technicians fail to realise that R134a from a purely technical stand point is approximately 20% harder on condensing. If the R12 condenser fitted to the vehicle was easily handling R12 then it will probably have adequate capacity to handle R134a. Limited condensing performance may not dictate a new high performance condenser be fitted. Fans, shrouding or radiator to condenser sealing are all viable options in certain cases. These methods are widely practiced in America. NOTE: Recent documentation from USA published by a large vehicle manufacturer gives guidelines of retrofitting for all their vehicles 1980 to Reviewing the list over 80% of their vehicles had a condenser change or modification stipulated. Most vehicles retrofitted in Australia do not have a condenser change because it is a price driven not a performance driven market. The common practice is to tailor charge rates to compensate for the lack of condensing. There are problems with this procedure in high heat load environments (see next section undercharging). Subcooling is the method used to identify condensing is in fact adequate. The liquid line should be 5 to 16º cooler than the condensing temperature. How do you do a subcooling test? Check the condensing temperature on the gauge (ie 1220 kpa = 50ºC condensing) Measure the liquid line with a good quality thermocouple/thermistor probe It should read 34 to 45ºC This verifies the liquid has subcooled in the bottom of the condenser. 12

13 LACK OF SUBCOOLING What if in the previous example the liquid line was at 50ºC (equal to condensing temperature). This means there is no subcooling with a distinct possibility of incomplete condensation. This could be the reason for bubbles or foaming in the sight glass (it probably is!!!) A liquid line at the same temperature as the condensing temperature indicates a lack of subcooling. The vehicle must not be sent out with no subcooling - The cause must be identified. When there is a lack of, or no subcooling, there are two possibilities: insufficient condenser capacity low charge rates To differentiate between them we need to go back to the head pressure analysis. If the head pressure is on the plateau or below the recommended level then it is likely a low charge rate. If however the head pressure is creeping then it is clearly lack of condensing. IF THE HIGH SIDE IS NOT STABLE AT RECOMMENDED LEVELS AND SUBCOOLING LEVELS ARE LOW THE PROBLEM WITH THE CONDENSER MUST BE RECTIFIED. SUBCOOLING MUST BE PRESENT WHILST WE ARE STILL ON THE PLATEAU. Do not compare inlet and outlet temperatures of the condenser. THIS IS AN INVALID TEST as it merely compares a superheated pipe temperature with a subcooled pipe temperature - a totally useless test. USING SUBCOOLING TO DETERMINE CHARGE RATES Subcooling levels are dependent on charge rates providing condensing is adequate. Subcooling is the overcooling of the liquid in the last portion of the condenser below its ideal condensing temperature. If the high side gauge is creeping or climbing to above normal condensing pressures and temperatures due to poor condensing action subcool testing is an invalid testing method. Pressures and temperatures must still be on the plateau. With ideal condensing pressures/temperatures present in the system the subcooling level can be checked to verify charge rates. At low charge rates subcooling levels will be low due to a low condensing efficiency (poor refrigerant to condenser wall contact). At correct charge rates subcooling levels will be optimised due to condensing efficiency being at a maximum (refrigerant to condenser wall contact is maximised. In an overcharge condition testing becomes invalid when condensing temperatures rise to above normal levels (kick off the plateau). 13

14 Using this knowledge together with the guidelines of 5ºC to 16ºC being an ideal subcooling range we can monitor subcooling until it is in the ideal range. If we can t promote any subcooling before the head pressure creeps or climbs condensing is inadequate. If we can achieve +5ºC subcooling whilst maintaining head pressure then we are in the normal charge band at moderate to high heat loads. On cooler days (low heat loads) 10ºC subcooling is normal due to the ease of promoting a subcooling run. Subcooling in excess of 16ºC indicates a possible overcharge. On cool days the high side may be stable with 18 to 20ºC of subcooling but on a hot day the head pressure is likely to run away. If a system is checked on a cool day with high subcooling levels it is suggested some refrigerant is removed until the normal subcooling level is reached unless the technician can guarantee against an overcharge on a hot day. (Pre-tested an equivalent system for charge rates.) The subcooling charts included here graph the correct charge and for a property condensed system. These are included as Technical Data Sheets 6 and 7 for future easy reference. PSI SUBCOOLING REFERENCE 14

15 kpa SUBCOOLING REFERENCE Use the subcooling charts to analyse charge rates (providing condensing is adequate). Comparing the high side pressure (condensing temperature) with liquid line temperature will indicate which band you are in (overcharge, undercharge or correct). As little as 20 grams overcharge may present problems in marginally condensed systems with low capacity. OVERCHARGING Most problems of an overcharge need no introduction to a majority of VASA technicians. Likewise the appreciation that R134a is much more charge critical than R12, with +20 grams being a tolerance for many small charge systems. Some items of overcharge however may need some clarification. When head pressures rise above normal levels the temperature of the condenser rises with the refrigerant temperature. This causes the formation of flash gas which may be the bubbling we see in the sight glass with an overcharge. This flash gas, which is really vapour, when entering the TX will cause a decrease in evaporator efficiency and can lead to failure to fill the evaporator on hot/humid days with the subsequent generation of superheat. Subcooling levels checks become invalid with any rise in pressure/ temperature above the normal levels. 15

16 If existing TX valves are retained the flow rate is 20% higher than with R12. The TX valve normally shuts down to compensate but during the stabilisation phase, whilst the TX is shutting down there is an increased risk of liquid flood back to the compressor. This of course means our charge rates are more critical. given we are already working close to the danger zone with normal charge rates. UNDERCHARGING The problems with undercharging are not as well documented and in fact have largely not been a problem with charge tolerant R12 systems. Interconnected with this is the belief that annual servicing is finished. The reality is late model systems are far more critical to charge rates and system integrity and an annual service check (as opposed to a full service - which would be done as necessary) is probably more critical than ever before. Let s look at the problems with undercharging: First and foremost is the inability to fill the evaporator coil under high heat load conditions. Many technicians fail to realize the evaporator can easily be filled on a 25º day (with the TX fully open to compensate for the low charge) but on a 35º day (examples only) there is no way the evaporator can be filled for maximum efficiency. Further to point 1, if the evaporator is not filled the long superheat run, plus suction line superheating that will result may generate excessive discharge line superheat with subsequent thermal switch activation or compressor failure (lack of compressor cooling). It is important to realise this may only occur on hot days (high heat loads). No superheat testing on cooler days will indicate the problems that will arise under high heat loads. Our role as professional service technicians is to ensure a 90% charge rate (or close to it) during service. We then know the evaporator will be capable of being filled under all heat loads. Thirdly is the question of oil return that will be addressed in the next RTP. At this point it is important to realise the amount of refrigerant circulating directly controls the oil circulation rate, with oil return back to the compressor being critical especially under highway conditions. At lower charge rates oil circulation rates reach critically low levels. Excessive flow rates through the TX valve may exist before the TX shuts down if it is oversized- which is technically the case when an R12 valve is used in an R134a Most people do not appreciate the inability to fill the evaporator coil at high heat loads, resulting in lost This Retrofit Bulletin has covered some basics and some advanced concepts of charging in retrofit. We would encourage all VASA technicians not to immediately go into worry mode over this information but rather to think more carefully when charging systems, and to relate this information in simple terms to the customer to encourage professional service as our agenda. 16