Let's Talk Refrigeration By David Bisiker
After a discussion with a friend yesterday it prompted me to write the following. These are just a collection of views I have gained in over 45 years in the industry. I have been asked either for advice over the phone or have visited sites where there appears to be an intractable problem. The mechanic concerned has been to site once or on many occasions and cannot diagnose the problem. Sometimes the conversation starts with “You probably think I’m stupid ”. I always have the same reply. If you have a problem that you can’t fix and you ask for help, you are not stupid. When you don’t know and just keep changing parts in the hope you finally fix the problem then you are stupid. In short, smart people ask and the stupid people worry more about their ego than fixing the problem by not asking for help.
Here is my advice. Firstly, KNOW YOUR THEORY! Understanding the theory on how the refrigeration cycle works is crucial. If you don’t or you have forgotten PICK UP A BOOK AND REREAD! ALWAYS use both gauges even if you have to break into the system. The extra work may take more time initially but will save you time in the long run and it will be there for use next time. One gauge only tells you half a story. Many times a mechanic has called me after fitting one gauge and I have insisted he fitted the other, resulting in a quick diagnosis of the problem. Don’t get fixated. A problem on the low side does not mean the low side is the origin. Keep an open mind. Think holistically. Imagine the system as a whole and use your knowledge to follow the cycle around the system to trace the fault. Many do that as a matter of course on an electrical faults but don’t when it gets to the refrigeration side. The same logic works for both. Once we dismiss the more common faults such as gas leaks iced up/blocked coils blocked condensers and faulty fans most system faults will come from one of the following and good understanding will help with diagnosis.
Understand when pressure drop is good and when it is bad.
All systems need pressure drop (PD) for refrigerant to flow around the system. Pressure drop is a function of flow. When the compressor stops the pressure will eventually equalise throughout the system. PD is needed for the TXV to lower the pressure and temperature in the evaporator. Basically every stroke of the compressor is causing a pressure drop which forces the gas to travel from the evaporator to the compressor. Flash gas is good through the TXV and bad almost everywhere else and to be avoided especially on the high side. It is the reason why in some systems the sight glass never fills. Many mechanics have overcharged systems in an effort to have a full sight glass. This causes liquid bank up in the condenser and effectively decreases the condensing area available to the discharge gas resulting in very high discharge pressure and overloading of the compressor can result. Because of pressure drop we need to introduce sub cooling on the high side to avoid the problems of pressure drop and the resulting flash gas. Excluding critically charged systems, sub cooling must be undertaken after the receiver. Whilst fitting a large condenser adds more sub cooling at the condenser outlet, this is lost in the receiver where the refrigerant exists in two states. (A properly sized condenser that operates over a range of conditions is best. Sometimes an oversized condenser just adds more problems and more instability to the system) As the liquid refrigerant enters the receiver be it warmer or cooler, the receiver liquid will either evaporate or the vapour will condense to try to reach equilibrium with pressure and temperature. This becomes an ever changing dynamic as cooling demand causes the receiver to rise and fall. Once the refrigerant now at its saturation temperature/pressure enters the pickup tube, changes direction through the receiver valve and flows into the liquid line, this introduces pressure drop and if there is insufficient sub cooling will show up as flash gas in the sight glass.
There needs to be full liquid at the TXV for it to operate correctly. Flash gas cause the valve to starve and wears the needle and seat prematurely. Understand how the TX valve works and know the theory of operation.I have been told many times by mechanics /technicians “I like (Brand) TX Valves because you can just put them on and not adjust them. If that were the case the valve could’ve been made much cheaper by not fitting a spindle. Of course that is wrong and shows a lack of understanding of TXV theory and operation which in my experience, is quite prevalent in the industry. It is the go to part to change when a tech is unsure of the problem. Occasionally there is a system blockage either at the TXV or the drier which causes the problem. Another that happens occasionally is the liquid line solenoid valve that only partially opens. Whilst rare on a direct acting type the servo operated valve can cause this problem if oversized. Again a valve that requires pressure drop to operate. Some techs make the mistake of sizing a servo solenoid by the pipe size rather than its’ capacity. This can cause insufficient pressure drop to open the valve where it partially opens and the resultant flash gas caused by the solenoid caused the valve to operate sporadically. In many cases the solenoid may partially open intermittently and the TXV is seen as the culprit. Always check the solenoid is fully open before changing a TX valve.
Many reciprocating compressor are wrongly diagnosed as inefficient by technicians and are often found to be operating correctly upon testing. Many reciprocating compressors are not designed to run into a vacuum especially hermetic compressors. If the compressor runs down to a low positive pressure and will hold it is unlikely to be faulty. In my experience compressors, diagnosed as not pumping when tested, only a small minority are found to be faulty. Semi and fully hermetic compressors can spot burn the stator on vacuum and can burn out and should be always run at a positive pressure for that reason.
Written By David Bisiker