The Trouble with R407c You might find this a bit long winded, and certainly a bit controversial. What do you think? Please click on the link at the end. Trevor Dann ThermaCom Ltd - June 2003
The Trouble with R407c Presently R407c has currently become the prevalent refrigerant available to supersede R22 in water chillers and indeed many other higher temperature applications. But as many are finding out, use of this gas is not without considerable problems, particularly if the plant has not been fully optimised for it s use. As a compressor re-manufacturer we expect certain basic boundary rules to exist to enable our clients to expect reasonable service from an overhauled compressor. Prime amongst these is the basic expectation the Refrigeration System and it s chosen refrigerant shall work properly in concert in the first place. However, initial experiences of compressors arriving from R407c systems are causing us to severely question this most basic premise... With a life expectancy of around ten years why is it we are already seeing machines just outside OEM warranty period arriving at our workshops? We believe the answer is very simple, but can the dilemma be solved! R407c is a blend of several other base components, most notably R134a. Like all blends utilising this base refrigerant, miscibility with more traditional Mineral oils is very poor, leading to oil recovery problems around the system. Early in R134a s development the now commonplace range of Polyolester oils was discovered to have an acceptable miscibility and by use of such oils the fridge systems were found to function correctly in terms of oil recovery much as they had upon the older gases R134a and it s derivatives, such as R404a, replaced. However, R407c requires a greater cooling surface area compared to R22, but the mass flow rate of the gas is much the same. The net result of this is a drastically reduced gas velocity through the evaporator. Failures ThermaCom are witnessing show all the hallmarks of classic hydraulic failure, indicating liquid return or slugging whilst running. However, most of these failures are very clean, with little bearing destruction and no sign of piston skirt scoring, this suggests liquid refrigerant return is not a significant problem. This point is particularly notable due to the great deal of attention given to the effect Glide. R407c does not evaporate a fixed point relative to the temperature and pressure of the refrigerant, instead the individual components flash off at marginally different rates, leading to the term Glide i.e. a band of superheat, rather than a fixed point. Even more attention is given to the reportedly deleterious effect of differential leakage. As field failures generally are not indicating major lubrication breakdown, perhaps these aspects have been overstated! Much has been made of this and certainly manufacturers seem very keen to push this point home very hard. Much less, however, is being said about the more acute problem of liquid (oil) slugging.
Also extreme valve wear seems a common problem. Those compressors suffering hermetic motor burnout seem to do so in a relatively heavy manner. Observations Hydraulic Failure Larger compressors seem to suffer more than smaller units, and the crucial difference here seems to relate to valve design. Larger compressors commonly use annular ring valves, traditionally held in the closed position by simple coil springs. The coil spring is a simple device to understand, with the valve at rest the spring loading is constantly set against the back surface of the valve. As GAS pressure beneath the valve increases it will eventually exceed the pressure above the valve, the valve however will remain shut until the pressure below exceeds the pressure above PLUS the pressure exerted by the action of the springs fitted. Once this is exceeded the valve shall commence lifting. The motion of the valve shall then gradually accelerate until it reaches the approximate mid point of it s travel, it will then gradually slow until it reaches it maximum opening point. Simple stuff so far. Following this action the spring and valve will then reverse as the pressure below the valve drops and the valve and spring will then accelerate and decelerate once more in the opposite direction. If traced on a graph the motion would be sinusoidal. This sinusoidal motion in vital for the correct function of the spring and the material from which it is made. All spring steels possess a distinct hysteresis, this provides ability of the metal forming the spring to remember it s original form, however in order to function properly the spring must be allowed to follow the sinusoidal action. If the spring is forced through it s range too fast it will lose it s hysteresis and instead will permanently deform. It may even crack and, exceptionally, may shatter. Herein lays the problem. During normal operation of the compressor the gas pressure changes either side of the valves are relatively gentle and constant. They are fast, for instance a typical compressor revolving at 1,450 RPM operates each valve some 25 times per second, but each operation will follow the sinusoidal pattern described above. When a liquid is pumped by a compressor the pressure change in the upper cylinder area is no longer gradual and constant, as a liquid cannot be compressed to a smaller volume any attempt to do this will result in an instant and immense increase in local pressure of the fluid. Any material in contact with the fluid shall immediately have this pressure exerted against it. In the case of a valve it will open in an instance, and the acceleration experienced will dramatically exceed the natural hysteresis of the valve spring. A moderate slug may only affect a partial area of a particular cylinder or valve, and invariably the item will survive, but it will be weakened. However, a major slug could cause the components in the vicinity to simply fail, or perhaps the weakening action of previous minor slugging, could mean even a moderate slug could cause a sudden and catastrophic failure. Case History
We have seen several machines from a group installation of chillers of a similar model. In each case failure was almost identical, although the extent of secondary damage did vary. Predominantly one cylinder in direct line with the suction gas path from the motor had suffered major piston and con rod failure. The pistons had basically flipped in the cylinder, but of course a piston viewed from the side is a square, and a square piston tends not slide too well in a round cylinder liner! The connecting rod had parted at or just below the small end. As is usual with most compressor failures, much of the cost of repairing these machines arose due to secondary damage caused by flying debris compromising critical clearances between moving components. What was unusual was the clean appearance of these failures. The aluminium alloy components used for Piston and Connecting rods of most compressors produces a fine black dirt when ground as a result of lubrication failure, and this tends to contaminate most internal component surfaces even in a moderate mechanical failure. These machines were in pristine factory condition, with the failures presented in a Clinical manner. We understand also that in each case the failure was precipitated by a substantial loss of refrigerant from the system. We suspect that as a result the oil return had failed due to reduced gas flow, prior to tripping on the system Low Pressure control. Thus when the machines were re-started the oil may have accumulated into a large slug, possibly exacerbated by the efforts of the Service Engineer to adjust (top up) the compressor oil level prior to re-starting. Of three machines returned, one was a write off because the piston ad actually flipped the cylinder liner within it s casting position and as a result the casting itself had failed. Even in this case the compressor was otherwise pristine. Valve Ring / Spring Failure At least one major compressor manufacturer has completely re-designed the valve operating mechanism in favour of wave springs to replace the older coil springs. However, the humble coil spring had not presented any particular problem for over twenty years. Why then should a major re-design be needed. Extreme Valve Wear Many valves have been seen showing very heavy grooving in way of the valve seating area. The term Wire Drawing has been applied to this condition, but the precise cause is not fully understood. What is certain is that valves that would normally give many thousands of hours of service are becoming heavily grooved within as little as a few months on some systems. Severe Motor Burnout Generally refrigerants do not burn very easily, mostly due to the total lack of oxygen to fuel a fire. During stator burnout localised temperatures can become very high. In severe cases even R22 would provide a pyrolysing agent in place of oxygen, normally from the Chlorine within the
refrigerant. R407c, however, seems to pyrolyse far more easily, with the net result that burnout s are generally more heavy and wider spread around the system. In one case the entire refrigeration system had become totally burnt. Thus the fire caused by the stator burnout in that case had spread burning it s way right around the system, effectively consuming all carbonaceous material within the system. Way forward Chillers in service on R407c may be giving perfectly adequate service, in which case they should be left to routine service routines. However, if you possess a Problem unit then perhaps a more drastic solution is needed. Several options arise 1 Hope. We can demonstrate this is an unsatisfactory approach once problems illustrated above have been witnessed. Following routine commissioning procedures, no matter how thorough, will not necessarily prevent recurrence. 2 Change compressor. Hydraulic failure due to the effect described will knock out the very best reciprocating compressor, and those of poorer or cheaper design will undoubtedly give up earlier. Thus changing to different make or model may at best delay a subsequent failure (ThermaCom are not available to advise on what compressor make may fall into which category!) 3 Change compressor type. Screw compressors on R407c seem bullet proof!. But then there is no comparison between a valve spring less than 0.5 mm diameter to the cross sectional area of screw compressor rotors several inches thick! 4 Change the gas. Many R407c chillers, particularly earlier models, were simply R22 units with a different expansion valve (TEV) and POE oil. Conversion back to R22 should be straight-forward and hopefully the problem will go away. With R22 phase out currently set at 2015, potentially some 12 years life would remain available to chillers converted in this way. It should be noted R22 has been banned for inclusion in new chillers supplied after 2000, although the actual law probably mentions nothing on retro converting back to R22! Perhaps a call to a lawyer may be needed before this is considered on newer units. R22 drop ins. More recently some very effective R22 Drop in gases have become available. As these are offered as long term replacement to HCFC refrigerant conversion to such gases should not upset the lawyers. Isceon now have Isceon 59 and Isceon 29 (optimised for water chillers). Early feedback concerning these gases seems very positive. Any takers for troublesome R407c chiller owners?
We expect many of the problems we have seen with R407c shall be engineered out, but at what cost. After all the chiller maker expects to make a profit. An over-engineered chiller is not likely to be economic to produce. We shall continue to monitor returns we receive from the market whilst we accumulate a knowledge bank. We shall be pleased to report our general findings further as we learn more. As a final note we would urge we are not trying to condemn any particular maker or indeed R407c as a refrigerant. Certainly some of our comments are controversial, but if this gas is to become the long term common option for R22 then the sooner any problems associated with it s use are in the open surely the better for all those concerned with it. Tell us what you think click here Trevor Dann ThermaCom Ltd - June 2003