ACHRNEWS

Mastery Of Basics 1: Evaporators And Boiling Refrigerant

November 2, 2003

Introduction

Many entering technicians feel like they can never absorb all the details that are involved in the HVACR industry. It is much like looking over a large Thanksgiving meal and wondering how you can consume it all - you do it one bite at a time.

It is encouraging to know that you do not have to know all of this information at one time. Mastering the basics, one at a time, will make you fully prepared for this industry. It does take time to master the basics, but average people have done it and so can you.

I will use my story as an example. I was a very average student in high school, dropped out of college, and then went to a technical college where I found an interest in the subject matter. I was a good student in technical college because of my interest in the subject. I had been advised after much testing to never go into anything mechanical, as I did not have the aptitude for it. This is where interest can overcome a handicap.

After completing two years and receiving a degree in gas fuel technology from Southern Technical Institute in Atlanta (which is now Southern Polytechnic Institute located in Marietta, Ga.), I launched my career. It took two years in a dead-end job with the state of North Carolina for me to decide to change. I became an instructor for Coosa Valley Vocational-Technical Institute in Rome, Ga., and remained there for four years. This was the best of experience. And one of the most valuable lessons that I learned from teaching was you really do not understand a subject or procedure unless you can explain it to someone else. Some people are not good at expressing themselves and should work on that area by explaining simple, understandable things to other people. Helping young people understand basics is a good start.

Now for an example of how Mastery of Basics can help you. I personally went from one subject to another studying everything I could find about this industry. I started with the four basic components of the refrigeration cycle: the evaporator, compressor, condenser, and expansion device. I am not a fast learner; in fact, I am a very poor reader. However, if I read enough on a subject from different people, I begin to really understand the subject.

For instance, we will start this series with the evaporator and explore it until we have an understanding of it. Then we will move on to the compressor and so on. This is a huge project, but by taking it one step at a time, we'll get to the finish line.

I would like to add a personal story to emphasize this point. While teaching my first four years, I was only a step ahead of my students. It required me to do more studying than they ever did. I started reading The News in 1962 and have been a subscriber for most of my life. I gained a wealth of knowledge while reading it weekly. I had an advantage in having access to other instructors that had some field knowledge of refrigeration. I had some field knowledge of heating, so we shared and worked together. I began to work on household refrigerators until I was doing a side job for a local appliance store making in-warranty factory repairs after hours. I also did some work on residential air conditioning systems for local people. My confidence was building.

One day after four years of teaching, learning, and doing side work, I read an ad in The News for a job in North Carolina doing industrial air conditioning work for a manufacturer. It involved starting up and servicing air conditioning equipment up to 1,500 tons in capacity. I applied and got the job based on being really well rounded in the basics.

The company provided an experienced technician to help me with the first job. I walked into the room and there were two chillers, 850 tons each. I had never seen a system larger than 40 tons before. These systems were huge in comparison to anything I could imagine. The suction line was about 2-1/2-feet in diameter and the liquid line was about 4 inches. The training technician asked me what I knew about one of the chillers. I quickly identified the four basic components and he said this was going to work out just fine.

Working with large equipment is just like working with small equipment. It still has the same four basic components that perform the same functions. You just have to scale up your thinking. Do not let a large system overwhelm you. In fact, I think it is harder to scale thinking down than up. I have learned that one of the hardest systems that I have ever worked on is a residential heat pump because it changes its role from cooling to heating and back. It still has the same four components, with one additional component that lets it both heat and cool.

Evaporators And Boiling Refrigerant

The job of the evaporator is to absorb heat into the refrigeration system. The term evaporator explains what happens inside the coil. Refrigerant is changed from a liquid to a vapor. This is called evaporation. When we see water on the sidewalk after a rain and it goes away when the sun shines on it, the water has evaporated. Heat is required to evaporate a substance. The terms evaporation and boiling are used interchangeably in our industry.

Everybody knows that it takes heat to boil water, but everybody doesn't know that water boils at 212 degrees F at only one condition - that is at sea level conditions when the atmosphere's pressure is at standard conditions. We hear the weatherman talk about atmospheric conditions and pressure. Standard atmospheric conditions are at sea level when the atmosphere's pressure is at 29.92 inches Hg (inches of mercury). This is the same as 14.696 psi (pounds per square inch). If you change the atmospheric pressure, you change the temperature at which water will boil. This is going to be a very important point. You can add more heat, but the water will only boil faster, it will not get any hotter.

For example, you can boil the water slow on low fire and you can boil the water faster on high fire where the water will boil away to a vapor faster. The temperature will remain the same. If you take the same pot of water up on a mountain, it will boil at a lower temperature. In fact, you cannot cook food like pinto beans or cook boiled potatoes on the mountain. The water may boil at 200 degrees F or lower and this will not cook these foods. Someone invented a pressure cooker to cook these foods faster. The pressure cooker encloses the food in a tight vessel and more heat will cause the pressure to rise in the vessel. Most pressure cookers cook at 15 psi above the atmosphere's pressure. This will raise the boiling temperature of the water to about 250 degrees F, which enables you to reduce cooking time.

Why do we need to know this? If you understand that you can control the boiling temperature of water by controlling the pressure of the vessel, you have cleared the first and one of the most important steps in understanding how an evaporator works.

If we enclosed water in a vessel and reduced the pressure of the vessel, we could reduce the boiling temperature of the water. We could actually reduce the pressure to where the water will boil at 40 degrees F. That is a significant number because that is the temperature at which a typical air conditioning evaporator coil operates. We could actually use water as a refrigerant, except it has some qualities that make it impractical. Water is actually classified as a refrigerant, but is not used in common applications. The important thing to remember here is that by controlling the pressure we can control the temperature, and this is true for all refrigerants.

Figure 1. This coil is using water as the refrigerant. These are the same conditions that a typical coil would operate at, 75 degree F room air passing over a 40 degree F cooling coil providing 55 degree F refrigerated air.
Figure 1 shows a coil that is using water as the refrigerant to cool air. Notice the low pressure that must be maintained in order to boil the water at 40 degrees F. It is a very low vacuum, 0.28 inches Hg absolute. That is 0.28 inches above a perfect vacuum. These pressures are not obtainable with a conventional compressor, plus water creates such a volume of vapor that the compressor would be huge. These are the reasons that other refrigerants are used.

It is important to realize that all refrigerants have what is known as a pressure-temperature relationship. When liquid refrigerant is present in a closed vessel, the temperature of the liquid refrigerant will determine the pressure inside the vessel. Just as important is the fact that if you start removing vapor from the vessel, the temperature of the remaining liquid refrigerant will reduce to correspond to the pressure. This is called the pressure-temperature relationship of the vapor and liquid. The refrigerant will boil. It takes heat to boil it and the heat comes from the remaining liquid refrigerant when you remove vapor. This is why when you charge vapor refrigerant into a system, the cylinder temperature begins to drop. If you keep taking vapor from the cylinder, the cylinder will start frosting. Eventually, you can take enough vapor refrigerant from the cylinder that it will become so cold that the vapor pressure will drop to the system pressure and no more vapor will transfer.

Now, let's apply this to a modern refrigerant. Any refrigerant will do, because all we have to do is realize that by controlling the pressure in the coil, which is the vessel, we can control the boiling temperature. We will use R-22 because it is such a common refrigerant. This refrigerant is much more efficient for an air conditioning system than the water example; the pressure will remain above atmospheric pressure and the boiling refrigerant will not create large amounts of vapor, so a small compressor can be used. It is important that you have a pressure-temperature chart with R-22 on it to follow the rest of this train of thought. In fact, you should carry a pocket version of this chart with you for ready reference.

The chart shows that R-22 boils (evaporates) at 40 degrees F when the pressure in the coil is maintained at 68.5 psig (pounds per square inch gauge). As shown earlier, when air that is 75 degrees F passes over the 40 degree F coil, heat will move into the 40 degree F refrigerant, which reduces the temperature of the air and boils the refrigerant.

Figure 2. This illustration provides a view of what is happening inside an evaporator that is using R-22.
Figure 2 shows an R-22 evaporator coil with the above conditions. Notice that this illustration shows what is happening inside the coil all along the way. It is a vessel, just like a closed pressure cooker, only it is a pipe. This is called a direct expansion or dry-type evaporator. (This is different from a flooded evaporator which is the subject of another article.)

There are several things that must happen in all evaporators to accomplish our goals:

1. The pressure must be maintained at the correct level to provide the correct coil temperature.

2. The liquid must all be boiled away to a vapor before it leaves the coil because the next component is going to be the compressor. A compressor can only work with vapor; liquid will devastate it.

3. After the liquid is boiled away to a vapor, enough heat must be added to the vapor to ensure that there is no liquid entrained within it. The quality of the product leaving the coil must be pure vapor.

Maintaining The Correct Pressure

This part of the process is controlled by the design engineers. They choose the correct coil area to maintain the correct pressure and temperature under design conditions. In rare cases, the equipment selection will not be correct and the system will not cool correctly or remove enough moisture while the system is doing all it can. If the system has cooled a structure on the hottest day, the equipment is sized correctly unless an additional load has been imposed on the structure. The coil and compressor selection must be correct within the bounds of the system. This part of the process is not within the technician's control, but the technician may oftentimes need to determine that the system is performing to its capacity and will not carry the load. Checking the efficiency or working characteristics of an evaporator will be saved for a future article. For now, we are just trying to understand the operation of an evaporator.

Making Sure All The Liquid Is Boiled To A Vapor

The technician has some control over this part of the process. It is similar to boiling water on top of the stove; the heat must be able to move into the evaporator for it to boil the liquid. Air filters must be clean, the fan must be moving the air, the coil must be clean, and on systems where there is an adjustable refrigerant control, it must be correctly applied and installed and adjusted. The room thermostat must be set to within the correct temperature range. As we will discover later, the refrigerant control has a part to play. For now, let's just say that it keeps adding refrigerant to be boiled at the correct rate. This would be much like boiling water in a pot. As it boils, it will boil dry unless you keep adding water. If you add it too fast, the pan will run over. The water temperature in the pan may also fall below the boiling temperature if water is added too fast. More about the refrigerant control in a later article about refrigerant controls. For now, we are trying to master the basics of the evaporator.

Adding Heat To The Vapor To Ensure No Entrained Liquid

Heat added after all of the liquid in the coil has boiled to a vapor is called superheat. As mentioned before, you can add as much heat to the liquid as you want and as long as the pressure remains the same, the liquid will only boil faster. When all of the liquid has been boiled away, the vapor rising from the liquid is called "saturated vapor." This vapor is saturated with heat to the point that if it is cooled, it will condense back to a liquid and fall back into the liquid. If heat is added to it, the temperature will rise to a temperature higher than the boiling liquid temperature. The saturated vapor cannot be heated to a temperature higher than the boiling liquid unless it is moved to another place and then heat is added.

Look back at Figure 2 and notice that when the liquid was all boiled away there is still 40 degree F vapor moving in the coil towards the coil outlet. This 40 degree F vapor can still absorb heat from the 75 degree F air passing over the coil. The saturated vapor at 40 degrees F begins to pick up heat until the end of the coil, where it leaves the coil at 50 degrees F. It has 10 degrees of superheat (50 - 40 = 10). This 10 degrees of superheat is our insurance that no liquid is leaving the coil, only vapor.

Review:

The evaporator absorbs heat into the refrigeration system.

The temperature at which the evaporator operates can be controlled by controlling the pressure of the evaporating liquid.

The evaporator must boil or evaporate all of the liquid to a vapor before the end of the coil.

Bill Johnson has been active in the HVACR industry since the 1950s. He graduated in gas fuel technology and refrigeration from the Southern Technical Institute, a branch of Georgia Tech (now known as Southern Polytechnic Institute). He taught HVAC classes at Coosa Valley Vocational & Technical Institute for four years. He moved on to become service manager for Layne Trane, Charlotte, N.C. He taught for 15 years at Central Piedmont Community College, part of this time as program director. He had his own business for five years doing installation and service work. Now retired, he is the author of Practical Heating Technology and Practical Cooling Technology, and continues as a co-author of Refrigeration & Air Conditioning Technology, 5th Edition, all published by Delmar Publishers. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or bmj@myexcel.com.

Publication date: 11/03/2003