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Check Those Condensers

Pressure enthalpy (p-e or p-h) charts will definitely tell the whole story. I was referring to them in a post by Jeff Lawrence a bit earlier on THE WALL, and spoke about the benefits of plotting a pressure-enthalpy chart.

Most often used as a desing or engineering tool, I use the pressure enthalpy charts in all of my classes to evaluate and teach air conditioning and refrigeration theory. THese charts are great troubleshooting tools and provide a snapshot of the refrigeration system.

When I was a younger lad, I used to plot customers systems on these charts whenever one of our techs was on the job. This was I had a visual history of the system as was able to predict system malfunctions even before the customer was aware of a problem.

With the chart, you can calculate cool stuff such as NET REFRIGERATION EFFECT, TOTAL HEAT OF REJECTION, HEAT OF COMPRESSION, HEAT OF WORK, THEORETICAL HORSEPOWER PER TON, MASS FLOW RATE PER TON, MASS FLOW RATE OF THE SYSTEM, COEFFICIENT OF PERFORMANCE, COMPRESSION RATIO, CAPACITY OF THE EVAPORATOR, CAPACITY OF THE CONDENSER, CAPACITY OF THE COMPRESSOR, and SYSTEM EER.

All of my students have grown to know and love the pressure enthalpy chart! Just ask 'em!

Comments

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    High Head Pressure

    With all of the hot weather here in the northest, I spent the last few days helping out a company owner friend of mine who is out of town. He asked if I could supervise his guys and put out any fires that may arise.

    No problem.

    I decided to visit one of the technicians on a job to see how my friends guys conduct themselves (technicaly) in the field. Of course I asked the guys if they minded having me tag along and it didn;t seem like they really cared one way or another.

    I met John (not his real name) on the job. It was a residential system and the customer was complaining that there was not enough cooling. The system had a fixed-bore metering device.

    After touching base with the customer, John checked the setting on the thermostat and then went to the condensing unit where he proceeded to gauge up on the system.

    It was about 93 degrees outside and the system pressure readings were 320 psig (discharge) and about 80 psig (suction). After taking the readings, John went to his truck and retrieved a recovery tank.

    I asked him what the recovery tank was for and he told me that the system was overcharged and that the evaporator saturation temperature was too high.

    I asked him about taking superheat and subcooling readings and also about verifying airflow through the condenser and evaporator coils. He mentioned that doins so would not be necessary, since both the suction and discahrge pressures are higher than normal, the system is overcharged.

    Not saying a word, but thinking a whole lot, I watched as John began to remove refrigerant from the system. After what seemed to be an eternity of watching John struggle to get the system pressure close to being correct, I finally interjeted and suggested that he check the condenser coil airflow.

    The condenser coil was absolutely filthy!

    After hosing the coil down, the system pressures were very low (as a result of John having removed nearly the entire refrigerant charge), the superheat was very high and the subcooling was very low. A reading of the temperature difference across the evaporator coil yielded 8 degrees. All of these conditions indicated a drastic system undercharge (what a surprise!).

    John then proceeded to add refrigerant back in to the system and, would you believe, the amount of refrigerant added to the system was exactly the same amount of refrigerant that he removed an hour before.

    ALl of a sudden, as if mby magic, the delta-t across the evaporator coil, the pressures, the subcooling and the superheat were all within acceptable ranges.

    Guys, be sure to check airflow through the condenser and evaporator coil BEOFRE removing or adding refrigerant from the system. You will be happier in the long run and also safe a ton of time.

    This is a true story and the events depicted here are true in every respect with the exception of the technician's name. I spoke with the technician prior to posting this as it is my intention to help others learn, but not at the expense of others. He told me that mess-ups this good need to be shared so that others don't make the same mistakes I made.

    I think John learned more than he bargained for that day.
  • Steve Ebels_3
    Steve Ebels_3 Member Posts: 1,290
    The obvious

    The obvious is overlooked so often that its scary. This applies to any HVAC device in my humble experience.
  • Bryan_16
    Bryan_16 Member Posts: 262
    First

    thing I do on any call. If I haven't washed the condensor myself recently, it gets done before anything else. You cannot troubleshoot any type of system with the coil partly plugged or completely plugged. I treat it like CO testing on a Munchkin boiler, you have to do it in order to know what is going on.


  • > thing I do on any call. If I haven't washed the

    > condensor myself recently, it gets done before

    > anything else. You cannot troubleshoot any type

    > of system with the coil partly plugged or

    > completely plugged. I treat it like CO testing

    > on a Munchkin boiler, you have to do it in order

    > to know what is going on.



  • A related subject on the

    heating side. Warm air furnaces that are condensing are able to do this by allowing flue gases to pass through a secondary heat exchanger (same as a condensing coil if you will) this will take the water in a vapor state and allow the latent heat to be removed and as it does so it condenses. What happens on a lot of these furnaces the techs do not pull the blower out and check the underside of the condensing coil (secondary heat exchanger)which acts just like a plugged filter reducing air flow through the unit.

    Another thing that heating only tuype guys are not used to is cleaning the condensate line which can get plugged or build up nasty green stuff as it sits all summer.

    I have run into these type problems on numerous occasions on condensing warm air systems.
  • Empire_2
    Empire_2 Member Posts: 2,343
    Eugine

    I love how you handled this this senario with the Tech. Being a 22 yr tech. I have found that bosses yelling, asking why you didn't find the problem, and what took you so long never works. Everyone has to learn from their mistakes espicially when you are under the gun and honestly are trying to solve the problem. Although I would have intervieened sooner to avoid the dreadid reclaimer, I think your way was better to show him what was actually happening, why it was happening, and how to tell. The goal of any Technician is to do the job right make sure it works as designed, and to not make more work for yourself. Almost like Having a car stall and pulling the engine first before you check the gas situation..... Also I think plotting a "PE" chart would have told the whole story.. Been so long since I did one, but I think that's what it's called...LOL........

    Thanx..Mike T.
  • Jeff Lawrence_24
    Jeff Lawrence_24 Member Posts: 593
    A lot of them!

    This is the first summer in a couple of years that it's been hot for more than a short time. The condenser coils on a lot of stuff is plugged up bad!

    I carry a 1300 PSI pressure washer on the van. I use it quite often these days on any non-spline-fin coils and it does wonders.

    Yeah, I've lugged the recovery drum around the house and recovered charges before I thought to check the condenser coil. I have learned to check the filter and blower wheel on the air handlers to give me an idea of the condition of the evaporator coil.

    "John" won't do that again, I'll almost guarantee it.

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • bob_44
    bob_44 Member Posts: 112
    Eugene

    how do you recommend checking condenser airflow? I have found fan blades with the wrong pitch, motors with the wrong rpm, motors running backwards due to strong prevailing winds,motors running backwards due to air short circuiting on units with multiple fans, fan blades that aren't in the right position on the shaft and consequently not in the right relationship to the veloute, ratchets missing on multiple fans, not to mention the obvious stuff. Short of measureing the average coil face velocity times the area or useing a hood I don't know how to check it for sure. I don't believe a lot of the mfg. even publish the spec on small equipment. bob
  • Bryan_16
    Bryan_16 Member Posts: 262
    another thing to check

    My assistant and I cleaned a coil yesterday that was double layered. We washed it and were not satisfied with the results. When we split the coil we found that the outside layer was still plugged. With large units and high SEER ratings, this will have to be watched very closely.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Double-layered Coils

    Good point. I have seen what looks like felt in between the two coil layers.

    Be very careful and diligent and your customers will appreciate it.
  • Mitch_4
    Mitch_4 Member Posts: 955
    K.I.S.S.

    I have made it a habit to lok for all the obvious problems first..saves me more headaches than not.

    IE: When I worked for a large service company, we had a customer with a GSW powervented HWH. no hot water, been there 7 years. 1st tech.Pressure switch not making. Change switch - leave (no test), next tech (same day)pressure switch not making..knows switch replaced, replace inducer. leave..no test), 3rd tech (me), check vent terminal - cust had covered terminal with window screening to prevent bugs coming in. Too restrictive, remove, cycle, customer educated and happy. Never pulled out the tools..

    Mitch
  • Kal Row
    Kal Row Member Posts: 1,519
    check those evaporators too!!!

    and dont assume that the people that installed a hydronic heating coil, in the return, figured on the heating coil's air pressure drop - been burned by that one
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