Introduction

Welcome to my blog. 

My name is Dale Rossi.  I have worked as a technical person in the commercial HVAC service industry my entire career, something like 35 years at this point.  I am writing this blog for several reasons.

1. I remember when I first started working on air conditioners.  I knew almost nothing about the refrigeration cycle.  I remember trying to figure it out.  I remember working with fellow technicians and learning from them.  I now know that much of what I “learned” turned out to be wrong or partially true.  It took me about 15 years of working on commercial HVAC equipment to begin to understand.  I work with technicians still.  I see evidence every day that there are many technicians working under the same disadvantages I experienced. I would like to help.
2. There are several factors that I think allows me to be of help to my brothers and sisters in the HVAC service industry.  These include the fact that I spent a career working on commercial HVAC equipment and that I have been associated with my younger brother who has a PhD. In this subject.  I believe this allows me to be a bridge or a translator between the engineering world and the technician working on equipment every day.  I still learn new things all the time, but I am reasonably confidant that the knowledge I have gained will stand up to examination.
3. I think I have the skills to communicate this type of information effectively.  For the past couple of decades some of my work has been making documentation and developing training materials and delivering training to technicians.  I have been thinking about, and practicing, how to present these sometimes complex ideas to people who, like me, have very practical intelligence but may not have the benefit of an extensive educational background.

To sum up, I want to help; I think I know this subject and I think I know how to explain it. Because of this, I feel a responsibility to help.

There are many basic concepts that must be explained and then brought together to have a clear understanding of refrigeration cycle diagnosis. I will go through these in detail over the course of this blog.  I invite comments and questions at Dale.T.Rossi@gmail.com.

I would like to start however at the end.  Where is this all going?

What do people want from an air conditioner?

• They want reliable comfort
• They want the machines to last a long time and do not require too much service work.
• They want the machines to not use more energy than they have to.

Where are the bigger opportunities?

• We can avoid premature compressor failures by maintaining proper suction superheat.
• We can sustain design capacity and efficiency by maintaining clean heat exchangers and proper charge.

Why does maintaining proper suction superheat avoid premature compressor failures?

• High superheat does not provide adequate compressor motor cooling. High motor temperatures eventually lead to shorted or open compressor motor windings.
• Low superheat allows liquid refrigerant to wash away the lubricants from the moving parts and breaks valves and pistons.

What needs to be done on the job to get us there?

There are six important measurements that are needed. A complete diagnosis is not available with less information. The six important measurements are:

• SP - suction pressure
• LP - liquid pressure
• ST - suction temperature
• LT - liquid temperature
• AMB - outdoor ambient temperature
• RAWB- return air wet bulb temperature

From these measurements we can calculate these performance indices:

• COA (condensing temperature over ambient)

COA measures how hot the condenser needs to get to transfer its heat to the outdoor air.

Cooler condensers provides the roughly the same cooling capacity with less energy consumption.

• ET (evaporating temperature)

ET measures how cold the evaporator needs to get to transfer heat from the indoor air.

Nominal ET is about 40F; if it gets much colder (below 32F), the water that condenses from the air will freeze on the evaporator coil. If it gets much warmer, then water will not condense out of the air stream and the equipment will not reduce indoor humidity.

• PD (pressure drop across the metering device)

The lower the pressure drop, the lower the energy consumption.

Thermal expansion valves (TxV) require a minimum pressure drop to control refrigerant flow. If the minimum pressure drop is not maintained, the TxV will not control superheat.

Low outdoor temps can cause the pressure drop to be too low. Fan controls are needed to maintain a minimum pressure drop when running air conditioners in cold weather.

• SH (suction superheat)

Superheat is an indication of the amount of refrigerant on the low-pressure side of the cycle.

High or low SH leads directly to premature compressor failures.

SC (liquid subcooling)

Subcooling is an indication of the amount of refrigerant on the high-pressure side of the cycle.

SC is an important indication of the charge level in the unit. It is the primary indicator of charge level in TxV units.

We analyze the performance indices to determine:

• Is the unit running properly?
• If not, what corrective action is needed to fix the problem?

There are other measurements that are useful:

• AOC - air-off-condenser
• RA - return air temperature
• SA - supply air temperature

From these measurements we can calculate additional performance indices:

• CTD (condenser air temperature difference)

CTD is needed to distinguish between two possible faults when the COA is high.

High CTD is an indication that the high COA is caused by a high side heat transfer problem, most likely a dirty condenser coil or a problem with the condenser fan.

• ETD (evaporator air temperature difference)

ETD is a direct indication of the cooling capacity being delivered to the building. It is useful to confirm that capacity is being delivered.

High humidity is a big load on the air conditioner; ETD will be lower with humid return air.

What are the problems a comfort cooling refrigeration cycle can have? Here are the problems a working technician sees most commonly.

• There can be too little heat absorbed into the low side.
• There can be too much heat absorbed into the low side
• It can be too hard to reject heat from the high side.
• There can be too much heat rejected from the high side
• There can be too little refrigerant in the system
• There can be too much refrigerant in the system
• There can be too little refrigerant flow through the system
• There can be too much refrigerant flow through the system
• There can be contaminants in the refrigerant
• The compressor can be pumping less than it was designed to