The Diagnostic Method

The Diagnostic Method

This diagnostic method is a generally accepted diagnostic method used by the HVAC industry. Measuring temperatures and pressures and doing calculations and comparing the outcomes of the calculations to what are acceptable operating ranges is what makes the technician skilled in refrigeration cycle diagnosis.

What are acceptable operating ranges for the important performance indicators, evaporating temperature, superheat, subcooling and condensing temperature over ambient? There is a wide variation in the outcomes of the diagnostic methods used by skilled technicians because of differing opinions as to what is an acceptable operating range, what is the meaning of an indicator and what are the standard service procedures used by the company or the technician.  There are technologies available to help.  Selecting a technology that has been tested by an independent third party is important.

Metering Device and the Evaporator

Metering Device and the Evaporator

In previous installments we discussed some characteristics of the refrigerant and how it is converted into its liquid form.  The liquid refrigerant leaves the condenser and flows through a small pipe, called the liquid line, usually through a filter drier and on to the metering device and evaporator. This is where the system will absorb heat from the air passing over it, making the air cooler and drier. 

The compressor pumps refrigerant out of the evaporator, reducing the pressure in the evaporator.  When refrigerant is at a reduced pressure, its saturation temperature is lowered.  The metering device is a restriction to refrigerant flow.  This maintains a pressure difference between the relatively lower pressure evaporator and the relatively higher pressure liquid line.  As the refrigerant is exposed to the lower pressure in the evaporator, it expands and becomes a vapor at the low side saturation temperature, also known as the evaporating temperature. The refrigerant requires energy to evaporate and it gets that energy by taking it from the air passing over the evaporator.  This is what cools the air, and when the evaporating temperature is below the dew-point of the air going over the evaporator, it condenses the water vapor from the air and dries it out some.

The Condenser

The Condenser

Once the refrigerant leaves the compressor, it flows through the hot gas line into the top of the condenser. The condenser is an important part of the refrigeration cycle where heat is rejected from the refrigerant. Three things happen to the refrigerant in the condenser. First, its cooled to its saturation temperature, the condensing temperature. Second, the refrigerant vapor is converted into its liquid form. And then, the liquid refrigerant is cooled more.

In an air cooled, comfort cooling system, the condenser is also the repository for the varying amounts of liquid refrigerant on the high hide of the system. It plays the part of the receiver.

Before we can think about the condenser in detail we must think about the importance of heat transfer from the condenser. Ask yourself do you accept that any system that is running is rejecting all the heat it is absorbing? This means all the heat that is absorbed in the evaporator, in the suction line, cooling the compressor and the heat of compression must be rejected, mostly by the condenser.

           

Do you accept the idea that any unit that is running, is rejecting all the heat that it is absorbing?  And that any unit that can’t reject all the heat it’s absorbing will shut down? When I present this in some groups this is accepted without further discussion and in others this seems to be controversial. It is certainly true; there is no heat warehouse available to the refrigeration cycle. If you were to try to test the theory, you might cover the condenser or shut off the condenser fan and watch what happens.

Refrigerant and the Compressor

Refrigerant and the Compressor

Every day technicians put their gauges on air conditioners and measure the high and low side pressures. Why do they do that? We know that we need the pressure information to make decisions about how well the unit is running and if there is anything we need to do to help it run better.  But did you ever stop and think about why you measure those pressures, and exactly which pressures you are measuring?

There are only few reasons to care about the refrigerant pressures in the system. One is to estimate the saturation temperature of the refrigerant.  This is because most of the calculations we make to determine the operational state of the refrigeration cycle compares the saturation temperature to the pipe temperatures or the ambient temperature. We need to measure the high and low side pressures in order to know the saturation temperatures, however once we have that, the measurements, calculations and rules will be in temperature and the pressure information has little further use.  There is another slight complication, the high-side pressure we need is the liquid pressure.  Sometimes in package equipment there may not be a convenient liquid port and you will have to use discharge pressure.

Important- The liquid pressure and the discharge pressure are not the same.  There is a pressure drop across the condenser coil.  When you have discharge pressure, you have to estimate the condenser pressure drop to arrive at an approximation of liquid pressure.  This guess will add an error to the subcooling estimate.  That error may be small or it may be great but it will be there.  We will discuss this further when discussing the condenser.