The Basics: Superheat

   Vital to the operation of the refrigeration cycle, superheat must be maintained and stable to protect the compressor.  We all know that the heart of the system cannot compress liquid (slug) without bad things happening.  This would represent low/no superheat.  But, did you know high superheat can cause valve and compressor problems as well?  No matter the reason for attaching your gauges to the system, you must check the superheat, in the air-conditioning cycle.
Q:  What is the Saturation Point of R-410A @ 118#?
A:  40F
  Imagine you are barbecuing this weekend and decided to boil some water on that side burner.  If we were at sea level, say Boston, MA for instance, that water would boil at 212F.  Bostonians learn this at a wicked early age, but what we neglect to remember is this is at an atmospheric pressure of 14.7# (0 psig).  If we boiled the same water in Denver, CO, the boiling (Saturation) point would be approximately 202F, due to the lower atmospheric pressure at higher altitude.  This represents the Pressure/Temperature relationship, which holds true since the discovery of Boyle's Law in the 1600's.
  On the flip side, if we contained all the boiling water in a pressure cooker, we are able to manipulate the boiling point higher by driving up the pressure.  This relationship in the refrigeration cycle is measured on our gauges, yet found in the middle of the evaporator and condenser coils on a properly charged system.  Fortunately, the most used tool in an A/C Technician's arsenal can no longer be left in the van as the P/T chart is built into the Digital Analyzers these days.
   Now, let's say we are able to contain all of the steam being boiled off the pan on the grille.  Once all of the water is boiled off at 212F and we make that steam 213F, 214F, 215F; this becomes 1F, 2F, 3F of superheat.  Superheat is the amount of sensible temperature we add above the Saturation point. We, as technicians, measure this sensible temperature on the suction/vapor line at the condensing unit. When making the measurement at this popular location, remember that this represents the total superheat.  Total superheat is different than the Evaporator Superheat, which would be measured at the outlet of the Evaporator.  Pressure drop and temperature gains of a line set can give you a false sense of high superheat on a high load day.
  Once you have measured the Sensible Temperature of the Vapor Line, deduct this from the Evaporator Saturation Temperature (Low-Side Gauge Pressure converted to Temperature on your P/T Chart), and the result is your Superheat!  Generally speaking, Thermostatic Expansion Valves (TXV's) and their Electronic counterparts (EXV's) maintain decreasing superheat as the efficiency of the equipment rises.  Therefor, proper valve operation is critical to prevent compressor failures.  TXV's  typically maintain between 5F - 25F of superheat. Yet very high efficient system that employ EXV's could be as low as 2F!  This gets dangerously close to 0F, representing the possibility of liquid getting to the compressor.  Of course, Superheat is how we adjust the refrigerant charge with a fixed metering device (i.e. piston, capillary tube).  System airflow and heat content of the air can also play a role in the superheat.
  The only reason to attach your gauges on an operable system is to find the Saturation Temperatures of the refrigerant cycle, used to verify proper refrigerant charge.  Why does this not get recorded and monitored?  Is it as simple as lack of understanding of the single most important aspect to the refrigerant cycle?  I know if I did not fully understand something as critical to my job as this, I would be embarrassed and avoid it at all costs.  Maybe we just need a little refresher, become a little more confident, and then we will begin protecting compressors thereby avoiding call-backs and warranty losses!

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