Sensible and Latent Cooling

When you look at manufacturer’s tables showing the capacity of their units, you will notice terms such as latent capacity, sensible capacity, or total capacity. The sensible capacity expresses the unit’s ability to reduce the air temperature. In most conditions, only part of the system cooling capacity goes into reducing the air temperature. The name comes from the idea that this change in heat can be sensed, or measured with a thermometer. I have had students tell me they remember it because it makes “sense,” and is therefore “sensible.” The word latent means hidden. Latent cooling capacity is used to take water out of the air. It is latent, or hidden, because you cannot measure it through temperature change. Latent changes involve a change of state. The water in the air is changing state from a vapor to a liquid. To accomplish this, the heat that went into the water to vaporize it must be removed. A system’s capacity is not fixed – it changes with the temperature, relative humidity, and volume of the air blowing across the evaporator coil. System capacity used for latent cooling is not available for sensible cooling. So as you increase the percentage of latent cooling a system performs, you decrease its sensible capacity. Here in the southeast, latent cooling is just about as important as sensible cooling. We MUST remove water from the air to be comfortable. In other parts of the country, such as the southwest, taking water out of the air may be undesirable. We can have some control over this by controlling the airflow. As a general rule, as you increase airflow across the evaporator coil, you increase sensible cooling and decrease latent cooling. In a humid area it makes sense to set the system airflow to a level that will increase latent cooling. For example, 350 CFM per ton rather than 400 CFM per ton. In a dry area, you may want to increase airflow to minimize latent cooling. For example, 450 CFM per ton. Although most systems will operate at 400 CFM per ton with no mechanical problems, that may not be the ideal airflow for your application.

Sacrifice

It looks to be a beautiful Memorial Day weekend, in Athens Georgia, at least. The weather is perfect for grilling out and downing a few cold beverages of choice with your friends and family. Maybe take in a concert, go to the lake, hike in the mountains – a perfect time to enjoy the beauty and bounty of our country. At its heart, Memorial Day is not about grilling out, mega sales, or a day off from the regular grind. It is a day to honor the men and women who sacrificed their lives in service to their country.

I believe it is also a good time to remember what sacrifice means. It means there are parents who will not spend another Christmas with their son. It means there are boys who will never toss another football to their dad. It means there are girls who will never again feel the comforting embrace of their mother. It means the families left behind have a harder road ahead because a loved one is forever gone. These are hard, real, human sacrifices. A soldier’s sacrifice is not limited to their death, but extends through their family and friends. It is fitting that we should take time to remember the fallen. It is just as important that we take time to remember and help the families left behind because their sacrifice continues.

Jaroy Roberts Story

After posting the article about cutting out compressors because of the safety hazard, Jaroy Roberts wrote me describing an accident he had while changing out a compressor. I am grateful to Jaroy for sharing his story. I believe a real story is far more effective than anything I can say. Jaroy's story and photos showing the injuries he sustained follow.

"Hello, my name is Jaroy Roberts and I am the Instructor for Midland College's two year HVACR program.  On Oct. 16, 2001 and was severely burned by just such an incident.  I was called in to change a compressor for another technician who had diagnosed the bad compressor but did not have the skills needed to replace it.  I showed up and put my recovery bottle in a big trash can full of ice water.  I then hooked up the recovery machine and hoses and started pulling refrigerant out of the system(or so I thought) and once the recovery was started I proceeded to get all the rest of my tools and parts on the roof.  Once I had everything I needed to complete the job, I looked at the recovery equipment, and all gauges showed to be in a vacuum.  I shut everything off and started to take out the compressor by de-brazing it.  The Txv capillary had a hole rubbed into it, and the sensing bulb had lost it charge.  This had the txv shut down completely, and with no low pressure switch in the system the compressor had run until it burned up and caused a bad compressor burnout.  The burnout had filled the high side service port full of trash as well so the true pressure could not be read.  The failure of the txv plus the valve on the discharge of the compressor had the refrigerant trapped in the high side.  All 14 lbs. of it.  When I de-brazed the discharge line out of the compressor the full charge left in the system started blowing out and coating everything will oil that ignited.  The gas itself was extremely hot from having been heated with the torch.  Now this compressor was the fourth one back inside of this unit which was a York 20 ton that had four five ton compressors.  So I was up inside the unit at the back of the compressor compartment surrounded by condenser coils and fan motors and shrouds above me.  The fire was between me and the exit out of the unit.  I was completely covered with ignited high pressure oil spraying all over me like a blow torch.  I now stress the importance of cutting open every system before lighting a torch.  It was and experience that was extremely painful and took a long time to recover from.  I have included a few pictures taken many days after the incident."

Are You Ready for R-32?

Are you ready for R-32? R-32 is one of the two components found in R-410A. Some major air conditioning manufacturers have been studying R-32 for use by itself. Why? Primarily because R-410A has a high global warming potential (2088) – even higher than R-22 which it replaced. On the other hand, R-32 has a much lower GWP of 675. R-32 is an HFC compound, not a blend. So it has none of the problems of vapor charging, fractionation, and glide found in the zeotropes. It is also a very efficient refrigerant and operates at pressures similar to R-410A.  So what is the catch? Flammability.

R-32 has a 2L flammability rating. This means that it will burn, but it will not explode. There is no flash point. It will burn when ignited, but it self extinguishes when the ignition source is removed. Still, it burns. Safety in design and handling must be a major priority when handling flammable refrigerants. As global warming potential becomes more of an issue, the industry will be moving toward flammable refrigerants. In general, the more flammable a refrigerant is, the lower its GWP. Most non-flammable refrigerants all tend to have a high GWP while extremely flammable refrigerants such as propane, R-290, have a very low GWP.  (Note that this is not always the case. CO2 has the lowest GWP of 1 and it is not flammable at all.)

Daikin, the world’s largest air conditioning manufacturer, is already producing R-32 systems for sale in other parts of the world. Danfoss is also studying R-32. For more information on R-32, check out these links.

Danfoss

Daikin

AIRAH

Cut it Out!

The safest way to remove a refrigeration component is to cut it out, rather than de-brazing it. In the case of filter driers you don’t want to heat the drier and drive moisture and contaminants back into the system. With all components, there is the hazard of fire caused by vaporizing the refrigeration oil in the system. Heating the joint enough to render all the brazing material molten increases the temperature and pressure. When you pull the part loose, a stream of gasses comes out and is ignited by the torch flames and/or the red metal. Even if the refrigerant is non-flammable, the oil will burn. People have been seriously injured when large amounts of flaming oil discharged on them after de-brazing a refrigeration component. The most serious cases involved people de-brazing a system that was still fully charged. Accidentally cutting a line on a charged system would not be nearly as hazardous as de-brazing one.

In addition to the flame hazard of the oil, all fluorinated refrigerants create nasty, toxic fumes when exposed to flames even if they don’t burn. If you have ever gotten a strong snort of this stuff you know it is an experience to be avoided.

One more reason to be cautious now is the presence of so many refrigerant blends with small quantities of hydrocarbons in them. Although the quantity of hydrocarbons is small, the hydrocarbons can concentrate in the compressor oil when charged into a system with mineral oil. Even after recovery, there can still be volatile hydrocarbons in the compressor oil. You heat it to de-braze the compressor, and a flammable gas comes out when you pull the pipe loose. I have heard one story where the technician was de-brazing a compressor and was engulfed in a ball of flames for a short period of time after pulling the line out of the compressor. It was a brief flash fire that burned off his eyebrows and gave him a sunburn, nothing too serious – but definitely scary. The refrigerant was an R22 replacement rated A1/A1. Also, consider that with so many replacement refrigerants you really don’t know exactly what is in the system. Although the highly flammable R22 replacement refrigerants are illegal, they are widely available on the internet and they are sold to anyone with a credit card. Just on the off chance that the system you are working on was topped off with propane, you should avoid de-brazing. Make sure and purge the system with nitrogen before brazing the new compressor in. Not only will you avoid oxides inside the lines, you might also avoid fires in the area you are working in. I believe refrigerants with some degree of flammability will become more common. To be safe, we need to start treating all systems as if the refrigerant could burn. So the next time you have to remove a refrigeration component – CUT IT OUT!