RTP Technical Bulletin
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1 RTP Technical Bulletin Category: REFRIGERATION REGISTERED TECHNICIANS PROGRAM Volume 1 Bulletin 3 This bulletin completes the overview of the air conditioning s y s t e m (refrigeration principles). It identifies the critical aspects of condensing and presents the phases of superheating, condensing and subcooling. REFRIGERATION In Bulletin 2 we discussed the pressure/temperature/superheat relationship of refrigerants on the low side of the system and we further discussed the implications this had on discharge line temperatures. As a brief review: The refrigerant enters the evaporator as a saturated liquid with a pressure temperature relationship that can be read directly off a Pressure/Temperature chart/graph or gauge (supplied as Technical Data Sheet 5). As the refrigerant changes to a saturated vapour (boils off) in the evaporator ITS TEMPERATURE DOES NOT CHANGE With R12 the identification and testing of these phases was not an issue, head pressures being the only test criteria, but with R134a the identification and testing of these phases is becoming the worldwide standard in fault diagnosis a n d performance evaluation. Just before exiting the evaporator the refrigerant SUPERHEATS - the vapour warms slightly. This superheat run is controlled by the TX valve which samples the outlet pipe temperature (internal sensing in a block valve) to ensure there is predetermined superheat occurring. If the superheat is too large it will open the valve to more effectively fill the coil. If the superheat is too small (or non existant) the valve will shut down to restrict flow into the evaporator. In Simple terms the TX valve controls flow IN EXACT ACCORDANCE WITH THE HEAT LOAD PLACED ON THE EVAPORATOR. SUPERHEAT CANNOT BE READ ON A PRESSURE GAUGE. IT IS INCREASED MOLECULAR ACTIVITY IN THE VAPOURS - NOT AN INCREASE IN PRESSURE WHICH IS ALSO CAPABLE OF HEATING THE VAPOURS. 1
2 The slightly superheated vapours then travel down the suction line where they may become further superheated. Most suction lines on modern systems are well insulated to ensure superheating does not reach unacceptably high levels where compressor cooling is jeopardized. The superheated vapours are then compressed in the compressor where their temperatures are dramatically increased. In many modern systems the discharge line temperature will be significantly higher than the high side pressure indicates (as per the Pressure/Temperature chart/graph), especially hot or hot/humid days (high heat load conditions). Now let s continue and identify the characteristics of the condensing phase of air conditioning operation. The Condensing Phase As Bulletin 1 and 2 indicated, the primary function of the condenser is to remove heat from the refrigerant to facilitate a change of state from a Saturated Vapour to a Saturated Liquid- but it has to do more than that. There are actually three phases that occur in the condenser in the modern system: 1: Removing Superheat 2: Condensing 3: Providing Subcooling. The condenser on a modern system has to do more than just condense, there are actually three phases. NORMAL CONDENSING ACTION (35º Superheat - 10º Subcooling) 2
3 Superheated vapours in the discharge line need to be reduced to condensing temperature in the first part of the condenser - this actually hapopens quite quickly. The 134a temperature/ enthalpy chart shown here indicates 20ºC superheated vapours contain only 16kJ/kg (4 kcals) of heat energy. Vapours superheat easily (small heat addition) but also dump off superheat easily - in the first portion of the condenser. Let s use an example: (Refer to condensing diagram page 2.) The high side gauge reads 1580 kpa (230 PSI) on a particular vehicle you are testing. By referring to the Pressure/Temperature chart the condensing temperature is 60ºC. On measuring the discharge line however it reads 95ºC (measured with a good quality thermocouple/thermistor). What this means is the vapours in the discharge line are SUPERHEATED by 35º. The portion of the condenser needs to remove superheat from the vapours before it can condense the vapours back to a liquid. Although 35º may seem a significant amount of heat in fact it occurs quite quickly in the first small portion of the condenser. This is to do with the dynamics of superheating of vapours. By referring to the Temperature/Enthalpy chart below, only 16kJ (4kCal) of heat drives the temperature of the vapours up to 20ºC (for R134a). 134a TEMPERATURE/ENTHALPY The second phase of operation of the condenser is the actual condensing. This is of course the major portion of the condenser since large quantities of heat need to be removed during this phase. Condensing will occur in accordance with the Pressure/Temperature relationship. In this example that is 1580 kpa (230 PSI) = 60ºC. If we measured the centre of the condenser with a good quality thermocouple/thermistor probe we would find a temperature of 57 to 58ºC (allow 2 to 3ºC for losses through the tubes although these losses are approaching nil in late design thin wall high efficiency condensers). In fact if we measure through the entire centre portion of the condenser we will find the temperature stable on 57-58ºC (60º refrigerant temperature). Back to the basics, this is the conversion phase from the SATURATED VAPOUR TO A SATURATED LIQUID. 3
4 If insufficient heat is dissipated at the condenser, then a complete change of state will not occur, thereby failing to supply the receiver drier with adequate liquid supply, even at full charge capacities. PRINCIPLES OF REFRIGERATION For a system to operate at peak efficiency complete condensation alone is not enough. Ideally, the refrigerant should also be subcooled in the last portion of the condenser. IMPORTANT NOTE: Excessive vapours exiting the condenser can severely limit system performance particularly under high heat load conditions. (High ambients/high humidities). Vapour feed to the TX Valve\Orifice Tube and subsequently into the evaporator directly limits evaporator performance. (See Retrofit Charging Guidelines) Subcooling Back to the ENTHALPY diagram (Bulletin 2). If the complete change of state from a vapour to a liquid occurs a small amount back in the condenser (due to the condenser being highly efficient) then the condensed liquids (still at 60º in this example) will cool down slightly. The correct terminology for this is the condenser has established a subcooling run. The Recognition and identification of subcooling is a vital indicator of condensing efficiencies and charge rates on modern automotive air conditioning systems particularly with bubbling in sight glasses which may be caused by incomplete condensing (See Charging/Partial Charging in Retrofit). Why Subcool? There are two reasons for providing a subcooling run in an air conditioning system. Firstly it provides a small safety margin to ensure that for all ambient/heat load conditions complete condensation can occur. If the condenser is only just efficient enough to dissipate enough heat to provide for complete condensation at low to moderate heat loads, under high heat loads, when the evaporator is absorbing large amounts of heat, the condenser may not dissipate adequate heat to permit full condensing at normal condensing temperatures. SERVICE TIP: If the high side gauge reads 60ºC (condensing temperature) an accuratge thermocouple will indicate 57º to 58º - this is the small loss that occurs through the pipes and in sampling. Comparing a gauge with an actual tube temperature verifies correct gauge reading. It is critical that condensation is complete to ensure the receiver drier is charged with liquid for adequate TX supply under all operating conditions. 4
5 Evaporator loadings increase not only as temperature increases but also with humidity. Enormous amounts of heat needs to be absorbed by the evaporator to condense water vapours present in the cabin air. This is the dehumidification role of the evaporator. Modern vehicles with aerodynamic styling and high under bonnet temperatures may have significant air pooling around the conderser. It is the TEMPERATURE of this air that is critical in evaluating acceptable head pressures if using a precalculated ambient temperature verses pressure chart. When considering condenser efficiencies there are a number of factors that need to be considered on hotter/high humidity days. Evaporator Loadings The evaporator will be absorbing large quantities of heat - the condenser needs to dissipate that heat to provide for condensation and a balancing of that system. The blower fan is more likely to be operated on high speed - an increased heat load per unit time. Higher humidity is an enormous heat loading on the evaporator/system. For every 1kg of condensate (water) that is drained from the vehicle cabin the evaporator has absorbed 539 kcal of heat energy (refer enthalpy diagram). Condenser Loadings Higher under bonnet temperatures may cause air pooling and increased air temperatures in the condenser area, hence increased condenser loadings - predominantly at idle with marginal airflows. Higher under bonnet temperatures causing excess suction line superheating of vapours and compressors - hence an increased superheat run in the condenser. Higher radiated heat levels from the radiator - predominantly at idle. With the TX valve fully open (under high heat load conditions) the flow rate through the system/condenser is faster. There is therefore less time to dissipate the absorbed heat as it flows through the condenser. The level of subcooling (recommended minimum of 3 to 5ºC) is a measure of the safety margin to verify complete condensation. The second purpose of subcooling is it actually increases system performance. If the liquid is subcooled it increases the performance of the system due to an increased capacity to absorb heat in the evaporator. Subcooled liquids fed through the TX valve into the evaporator are capable of absorbing larger quantities of heat. Subcooling Levels The advantage of using subcooling as a measure of condensing capability is that subcooling levels are relatively constant irrespective of ambient temperature and condensing temperature. Normal subcooling levels are 5ºC 5
6 to 16ºC for automotive systems. With repeated practice subcooling checks become relatively simple to perform. For a full explanation of subcooling and testing refer to System Charging in Retrofit (this Bulletin). The subcooled liquids are then fed to the receiver drier for storage as a ready supply for the TX. The capacity of the receiver drier must be sufficient to feed the TX for maximum heat load conditions. IMPORTANT NOTE: Subcooling checks are an additional test to be used in conjunction with normal test procedures (gauge readings, frost lines etc). Do not use subcooling as a stand alone test. UNDER NO CIRCUMSTANCES SHOULD THE RECEIVER DRIER BE SUBSTITUTED FOR ONE OF SMALLER CAPACITY. THE RECEIVER FUNCTION OF THE DRIER IS CRITICAL FOR OPTIMUM SYSTEM PERFORMANCE. SERVICE TIP Subcooling checks are simple to perform and with some practice become second nature. Comparative data can be established to identify system dynamics - particularly in systems of unknown charge rates and efficiencies. Summary The three Bulletins presented have given an overview of system operation past what was required for R12 and older systems. The critical aspects of reading gauges to identify both pressure and temperature (by referring to the Pressure/Temperature chart or reading directly off the gauge) cannot be overemphasised. The comparison of temperature of evaporation and condensation with suction line, discharge line and liquid line temperatures give us the total system dynamics of superheating and subcooling. The gauges with good quality thermometers allow us to look into the system to identify either correct system operation or limitations. Gauges and good quality thermocouples allow us to look into the modern system to identify its characterictics. 6
7 7 PRINCIPLES OF REFRIGERATION
8 Critical Points Checklist The gauges indicate change of state pressures and temperatures - the conditions that exist in both the evaporator and the condenser. Vapours heated in the evaporator and suction line after the completion of change of state are known as SUPERHEATED VAPOURS. Liquids cooled after the completion of condensation are called SUBCOOLED LIQUIDS Both SUPERHEATING and SUBCOOLING CANNOT be registered on gauges. They are changes in molecular activity conditions - not pressure sensitive conditions. Measure superheat levels and subcooling levels with accurate thermocouples/thermistors. Some superheating must occur in the evaporator and/or suction line to guard against liquid flood back. Excessive superheat, caused by the evaporator not being full is undesirable since we lose efficiency plus compressor cooling. Excessive suction superheat may/will cause excessive compressor temperatures and discharge line temperatures especially under high heat load conditions. Subcooling of liquids in the last portion of the condenser is a desirable condition. Subcooling checks verify condensation is complete. Subcooling levels are charge sensitive (see charging in retrofit). 8
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