Porsche 911 Evaporator Comparison Test
Overview of Porsche Evaporator Test Results
We performed the Porsche 911 Evaporator Comparison Test comparison test of three different brands of Porsche 911/930 evaporators. The test results show the original Kuehl Serpentine Rev 4 brand outperformed the OEM (original equipment) and the “other” after-market knock-off unit, in terms of quickest response time and greatest drop in air outlet temperatures using a high latent heat, liquid medium test. The chart above shows the differences between all three units ( based on the chart’s Y numeric gradient, the vertical Fahrenheit numbers). The “medium” used in this early test had a higher content of “latent” heat as opposed to normal automotive refrigerant (R134a or R12). The rationalization for using this alternative medium is discussed below.
Testing Method
Ultimately the best test would include a particular 911 or 930, contained within an environmentally controlled chamber that has a controlled temperature, humidity and pressure, and a particular amount of refrigerant charge. Needless to say the cost to rent time in a vehicle chamber or construct the same is not inexpensive. Our objective is only to confirm field results and to compare similar components (an evaporator). The cost to setup, to maintain and to charge a test scenario using refrigerant is expensive in the same realm. However, there are other methods which will yield comparative data. And considering the large number positive field reports from customers already on hand for the Kuehl unit, we only wanted something more constant in terms of reducing field variables that obviously effect the results: variables such as the vehicle, evaporator fan motor (air flow), climates: evaporator air inlet temperature and the medium (or alternative for refrigerant) inlet temperature.
How the AC system works
In brief, the ac system removes heat from the car. Using refrigerant, the compressor increases the pressure (and temperature) of the refrigerant to approximately 180-230 psi (relative to the outside air temperature).
The high pressure gas moves through the condenser. Since the air surrounding the condenser is cooler, the heat migrates from the hot high pressure refrigerant through the condenser walls and is absorb in the surrounding air. As a result of losing heat the refrigerant goes through a change of state, from a hot gas to a warm liquid, typically somewhere between 130-150 F (again, relative to the outside air temperature).
The warm liquid travels through the receiver drier (in systems with expansion valves; in systems using orifice tubes typically the drier function will be after the evaporator) where any moisture (water) in the liquid is absorbed by a drying agent or desiccant. And, the drier acts as a filter as well as a storage tank for the refrigerant.
When the expansion valve is opened the warm high pressure liquid refrigerant is metered and atomized through the expansion valve into the evaporators coil. As the liquid enters the evaporator there is a pressure drop. The warm air from the cockpit, flowing over the exterior of the evaporator coils and fins, transfers the “heat”, (which can be stated in terms of BTU’s or British Thermal Units; or in calories) into the evaporator (warm molecules move toward cold molecules) resulting in cooler air coming out the a/c vents. When the atomized liquid refrigerant receives enough “heat” the liquid changes to a vapor at which point a change of state of the refrigerant medium takes place.
The cool refrigerant vapor exits the evaporator and moves into the low side or suction side of the compressor, where the cycle is repeated. Raising pressure, lowering pressure, removal of heat and absorption of heat.
The greatest amount of heat movement, or transfer, happens at the time of “change of state” (from liquid to gas or gas to liquid). However measuring the amount of heat with a thermometer or in terms of degree’s is not that simple for the following reason and we’ll use “water” as a simple example:
Assuming you have a known amount of water measured in terms of weight or pounds, 1 lb for example. And you raise the temperature to 212 degrees F (at sea level). You know that the water does not completely vaporize or steam off at one time. The 212 F water will absorb 970 BTU’s of additional “heat” without any change in the thermometer reading however the water will turn from a liquid state to a gas or vapor state at that temperature. The additional “heat” absorb is what is known as “latent” heat. Water as compared to refrigerant has a higher latent heat or capacity.
Why is refrigerant preferred over water as a medium to move the heat? A refrigerant’s change of state (when the greatest amount of heat can be transferred) can occur quicker and at the lower pressures as compared to water. To use water in an automotive application would require significantly larger “heat sinks” (evaporators, condensers) and an overall system that operates at a higher pressure.
As an alternative to using automotive refrigerant we chose to use simple water. Rather than use a refrigerant, such as R12 or R134a, as a testing medium, plain water was used. Naturally water has significantly different “cooling characteristics” (latent heat) in an evaporator as opposed to a refrigerant; based on the differences in pressures required to create a change of state. However we know that a lower temperature at the evaporator air outlet is the result of heat being absorbed by the medium (water) as it travels through the coil. And we can assume that if we can maintain a relatively stable water temperature as is enters the evaporator we’ll get an idea as to the performance or characteristics of the evaporator design. For relatively simple tests water is safe and easy to work with as a medium in a test. Other mediums, such as oils or gases could be used but for our comparison plain old “water” will do.
Testing Equipment
Thermometers – all thermometers were digital, calibrated and traceable to the NIST (National Institute of Standards and Technology) and had fresh batteries. Two identical fast reading thermometers, with a scale resolution of .1 and an accuracy of ±1°C between the range, monitored temperatures for air inlet and air outlet recording. Water temperature was measured using a VOM with a scale resolution of .01 and having a digital probe/VOM accuracy of ±½ of 1 degree or 0.5F. All three thermometers were tested in unison for verification of relative readings in both air and water. When voltage readings were monitored a VOM was used to observe the evaporator fan voltages and a clamp-on amp meter monitored current draw. Like equipment was used to measure water pump line voltage and current draw.
Evaporator Box – We chose to use a rather “used” OEM Porsche evaporator box taken from a 78-83 911. The fan motor inside was typical of what you would find in any given Porsche 911 or Porsche 930 in the 1978-1989 vintage driving on the road today (it is near its end of its life cycle, but it works for the moment).
Electric Water Pump – We used a 120 vac pump with manufacturers pump rating noting “5 gallons per minute up to 10 ft of lift. Actual lift for the position of the pump was 3 feet.
Water Tank – Four gallons of water were placed in a five gallon bucket and chilled to a temperature of 39 F for each individual test. Each of the three different evaporators used the same starting water volume and temperature for each of their own three tests.
Environment – The tests were conducted over a three day period inside a closed building having a temperature that ranged from 67-70 F. Relative humidity at the time of the tests was 35%.
Test Bench – The OEM Porsche 911 evaporator box, used to contain the three various makes of evaporator coils, was mounted upon a platform in a fixed position. Individual digital thermometers (noted above) were placed in the evaporator air outlet, air inlet, water tank and nearby the the test bench (to check changes in ambient near the test bench).
Operators – Two people managed the test equipment, procedures and results; observing the equipment and test constants and resulting data results. The data was logged into spread sheet. Though we could have automated the test sequence and data collection we determined from trial runs that small differences in timing or operational ranges did not impact the results enough to warrant the time and material.
Test Sequence Details
- All three brands (designs) of Porsche 911 evaporators were tested using identical test parameters or methods three times each (each evaporator was run for a full test sequence three times).
- Each of the three different Porsche 911 evaporators were mounted independently in the same OEM evaporator box which also had the OEM Bosch fan motor. The evaporator box was fixed upon a table in the same orientation for all tests. The fan speed was relatively constant for each evaporator and each of its three tests.
- The air inlet temperature entering the evaporator box was raised from an ambient of 67-70F to a nominal of 100F using an electrical heater. We chose 100F as a relatively common or average air temperature you will find within the cockpit of the car on during the course of a normal warm day. Though it is very common to find that the temperature in a closed (windows up) vehicle that has been sitting in the sun for the good part of the day will be 130F or higher, we used 100F as a constant. The temperature of the heated inlet air was measured and monitored during all test sequences at a fixed position in the passenger side inlet to the evaporator box. Both passenger side and driver’s inlet sides of the evaporator box where channeled in these test to receive the 100F inlet air. As a side note we had done earlier tests on all three units using unheated or ambient air and what we found was the higher the air inlet temperature, the greater the difference or resulting temperature drop between the winner and the losers.
- The air outlet temperature, exiting the evaporator box, was measured and monitored during all test sequences at fixed position inside the end of the round plastic air outlet.
- The voltage and current for the evaporator was set and monitored to equal what would be fan equivalent to fan “speed 3” or fastest, as noted on the right hand floor console switch.
- Rather than use a refrigerant gas, such as R12 or R134a, as a testing medium, plain chilled water was used (see notes above).
- Four gallons of water were placed in a five gallon bucket and chilled to a temperature of 39 F for each individual test. Each of the three different evaporators used the same starting water temperature for each of their own three tests.
- The water was pumped through the evaporator inlet (where the expansion valve would normally be attached) using the pump noted previously. The water coming out of the evaporator’s outlet was re-circulated into the same bucket. As a note, prior to running this group of tests we had pump the water into a separate bucket however the tests scores all appeared to follow the same trend.
- The running test time for each evaporator was the same, 30 seconds for each test. Timing was monitored on a simple electrical clock and was noted by one operator as “Start” when water started to exit the evaporator and “Stop” when 30 seconds had expired. As a note, prior to running this group of tests we had run longer tests (60 sec, 90 sec and 120 sec) however the the tests scores all appeared to follow the same trend, the air outlet temperatures all flattened at a given level after 30 seconds of continous water flow. Our decision to use 30 seconds is based on the objective to choose or pick an evaporator design and brand that meets two objectives: (a) time – how quickly the inlet air is cooled, and (b) temperature – how much the
- Prior to each evaporator’s test the following information was documented: starting water temperature, starting air inlet temperature, starting air outlet temperature, local ambient air temperature. At the end of each test the same items were documented.
- During each test the evaporator fan motor load and water pump electrical loads (amps and volts) were monitored for variations. As a note, prior to starting the these test comparisons we observed fan and pump during the course of a few days of preparation and noted the variations or range which could effect test results were very minor (the supporting line voltages in the building did not fluctuate greatly and the fan motor was holding up given its age).
Observations of Monitored Constants
The water temperature in the bucket was taken at the pump inlet. During the course of each test, for all three evaporators ( 3 evaporators tested 3 times each), a temperature variation of ½ of 1 or 0.5 degree F was noted. Meaning that the water temperature surrounding the test probe never varied more than one degree in total. We believe the change in temperature during the first half of any test time period resulted, logically, from currents of warm water exiting the evaporator and downward around the temperature probe. At the end of each test the water temperature was raised on an average of .5 to 1 degree F.
Evaporator fan motor current draw never varied more than .125 amps. These minor variations were shared by all three evaporators tested. The electrical water pump’s current draw and voltages swings were similar and shared alike by all three evaporators tested.
The heated air inlet temperature was constant 99-100F, and shared alike by all three evaporators tested (if a particular brand of evaporator had a test with 99 F air, then so did the others).
The average ambient air temperature around the test bench was 68 F. And the humidity was constant during all three days of testing for all units.
In summary of the monitored constants we do not feel that any minor variation of a constant or combination of the same would swing the results for any test scores for either evaporator as all evaporators shared the same constants and minor variations equally.
Outcome of Test Results
The original Kuehl Rev 4 brand outperforms the OEM (*see note below) and its competitors. The Kuehl unit has the capability to remove more btu’s, to lower your cockpit temperatures greater and faster than the OEM or “other” competitive aftermarket serpentine coils.
* All 911/930 OEM (original equipment) evaporators are a tube and fin construction. A tube and fin construction utilizes round tubing which has thin aluminum louvers or fins surrounding the tubing. Think of the aluminum fins as heat sinks. Up through 1985 the OEM evaporator utilized copper tubes to channel the refrigerant through the evaporator. There were two copper tube type constructions during that period. Around 1986 the tubing channels were changed to aluminum, where a copper main tube branched off to many small aluminum tubes or channels. Though the number of channels for refrigerant to pass through the evaporator increased significantly (a colder coil in practice) there may be a bonding issue with the connection of the main copper tube or header connects to the aluminum channel tubes; in simple terms we have seen many joint leaks over time (the reason why is not the subject of this presentation). It would be obvious to assume the 1986 aluminum tube version would yield better results than its previous all copper unit. However we feel, based upon these tests and the evolution of the serpentine type tubing construction, that the results would be no better than the inexpensive “other” brand aftermarket serpentine unit we tested.
