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Thread: (help)Figuring out head and suction pressure

  1. #1
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    (help)Figuring out head and suction pressure

    Whats up guys im almost done with school and got a job doing commercial hvac and refrigeration, I was on the job today and i wasnt to sure what my suction and head pressure should have been for the unit(med temp reach in r-134a) So i think i understand how to get suction pressure by taking the temp the evap coil should be and using a pt chart to get the psig, but what about head pressure? i read how to do it on a AC system but cant find anything for refrigeration. Can anyone help me out here?

    thank and god bless

  2. #2
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    I seriously hope this is a reflection of the 'schoolins' you got versus the energy you put into yer 'schoolins'.

    This is an open forum, so I will be careful.

    One thing I teach guys I am training, after beating them in the head for wasting money on 'schoolin', is that you learn the temp you need, versus the pressure you need.

    Why? Temps don't change, but the refrigerant does.

    So, I'll give you some information, then some homework.

    You said medium temp. So I am -assuming- a 35deg F box/case/room temp.

    You did not specify the condenser ambient conditions, so I am going to, again, -assume- 80deg.

    Now, generally, you need a 10-15deg TD (temperature difference) between the evaporator and 'box' temperature. (not superheat, just TD)

    (I am assuming this is a small unit, and not digging too much into superheat at the moment, as you need to understand pressure/temperature correlation first)

    Same goes for the condenser. You would like to see 10deg subcooling (in other words, the liquid is 10deg cooler than ambient around the condenser)

    So, given that information, pull out your PT chart for R134a, and tell me what the suction and discharge should read for the above conditions?


    Now, bonus questions..

    1. What about R407a in the same conditions
    2. What about R410a in the same conditions
    3. What would your discharge (head) pressure be if the room was 95deg?


    *jeopardy music surrounds us all*
    Last edited by BadDaddy420; 04-27-2021 at 12:27 AM. Reason: I'm country... 'schoolins' is a reflection on me, not you.

    I can't fix it if it won't stay broke..

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  4. #3
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    Quote Originally Posted by Jesussaves View Post
    Whats up guys im almost done with school and got a job doing commercial hvac and refrigeration, I was on the job today and i wasnt to sure what my suction and head pressure should have been for the unit(med temp reach in r-134a) So i think i understand how to get suction pressure by taking the temp the evap coil should be and using a pt chart to get the psig, but what about head pressure? i read how to do it on a AC system but cant find anything for refrigeration. Can anyone help me out here?

    thank and god bless
    LOL. Where were you that day when the school taught that info?

    Start by re-reading your schooling stuff, then try studying,

    "Commercial Refrigeration for Air Conditioning Technicians" by Dick Wirz.
    RSES,SAM # 620-111 ,"Temperature-Pressure Values of Common Refrigerants at Saturation" for "Determining Commercial Refrigeration Head Pressure" by Nick Reggi,CMS
    RSES,SAM # 620-32A,"Condensers" by Otto J. Nussbaum,CM
    Copeland, AEB #AE-1234-R4, "Low Ambient Compressor Operation"
    Copeland ,AEB # AE4-1334,"Low Condensing Temperature Bulletin"
    Sporlan, "Specific Requirements for Extreme Low Ambient Conditions"- The Low Ambient Kit and Heated, Insulated Receives
    Parker, "Head Pressure Control for Supermarkets"
    "Understanding The Fundamentals Of Head Pressure Control" Part #1&2.by Dave Demma, Sporlan Valve Co.
    TRENTON Refrigeration, "Head Pressure Control Application Guide"
    The NEWS, "The need for better head pressure control" June 1,2000.by John Tomczyk.
    The NEWS, "The Professor: Low Ambient Controls Reduce Head Pressure", January 6,2014, by John Tomczyk.
    "Servicing Outdoor Air-Cooled Condensing Units", Controlling head pressure is key to maintaining a properly operating refrigeration unit, by Joe Marchese,CMS.
    For starters!!!LOL.

    ALL a Good Read!

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  6. #4
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    Here's a tip. Never check pressures on these small systems until you've confirmed with a flashlight that the condenser is completely clean.
    Having even a slightly plugged condenser will throw your readings way off and you'll try to compensate with charge.
    The diagnostics go down hill from there
    You don't squat with your spurs on.
    And you NEVER put the torches away before pressure testing.

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  8. #5
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    Why on Earth would someone go from school right to commercial ?
    Wouldn't residential be a softer start ?
    Just a light commercial Lennox rtu with 80 plus trouble codes could be mind-boggling 😳

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  10. #6
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    I just went right to commercial then school.

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  12. #7
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    Quote Originally Posted by VTP99 View Post
    Why on Earth would someone go from school right to commercial ?
    Wouldn't residential be a softer start ?
    Just a light commercial Lennox rtu with 80 plus trouble codes could be mind-boggling 😳
    Maybe it's because (generally speaking) residential sucks more and pays less? At least that's why I got into commercial.

    Why would some fancy lennox rtu be any worse to figure out than one of their top of the line residential furnaces? Residential stuff can be just as complicated. Plus, you have the added bonus of having to be right next to the overflowing cat litter box while you're trying to figure it out.

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    Quote Originally Posted by ammoniadog View Post
    Maybe it's because (generally speaking) residential sucks more and pays less? At least that's why I got into commercial.

    Why would some fancy lennox rtu be any worse to figure out than one of their top of the line residential furnaces? Residential stuff can be just as complicated. Plus, you have the added bonus of having to be right next to the overflowing cat litter box while you're trying to figure it out.
    I'm not young enough, to know everything...

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  15. #9
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    I have been teaching commercial/industrial refrigeration for 15 years. I can tell you first hand school is not for everyone. There are ALOT of students that don't graduate or do get through somehow and don't retain squat. Alot of techs don't need degrees to be super sucesssful, just motivation. I myself do not have any mechanical or hvacr degree, 100% field experience. Im not trying to knock the OP, in fact I commend you for asking questions, lesser techs would just mess it up or give up and go back to bagging groceries. A D- score will get you a piece of paper in some schools. That means theres ALOT of stuff not learned. An A grade doesn't reveal much more. The college credit-hour structure does not work well for highly skilled trades like refrigeration. The more insightful thing to do is talk to the instructor that taught them.

  16. #10
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    "Same goes for the condenser. You would like to see 10deg subcooling (in other words, the liquid is 10deg cooler than ambient around the condenser)"

    Defined liquid line approach, not subcooling. Compare liquid temp to saturated for subcooling.
    Approach readings (condenser, evaporator & liquid line) can vary a good bit based on application and mfg.

    I started in nuclear power and chemical industry straight out of high school, first chiller was 8,000 hp. Residential learning curve came 16 years later. Hired top students with associate degrees who did not know how to clean a coil or properly log a unit, but turned out well.

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  18. #11
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    Quote Originally Posted by ehsx View Post
    "Same goes for the condenser. You would like to see 10deg subcooling (in other words, the liquid is 10deg cooler than ambient around the condenser)"

    Defined liquid line approach, not subcooling. Compare liquid temp to saturated for subcooling.
    Approach readings (condenser, evaporator & liquid line) can vary a good bit based on application and mfg.
    Bah. I get approach and subcooling twisted all the time. Thanks


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    I always get screwed up on dew or bubble point for figuring subcooling & superheat.

    Need to remember that subs bubble.....

  20. #13
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    Now I go by ,Where in the system do you see "bubbles" ? The hiside SightGlass , so SC.
    And I go by, Where in the system do you see "dew"? The loside evap coil so,SH.

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  22. #14
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    Quote Originally Posted by pecmsg View Post
    I just went right to commercial then school.
    I went straight to commercial , 8 years of following my dad . no school needed , not until they put computers on everything .
    Gun Control: Belief that violent predators willing to ignore laws against
    robbery, kidnapping, rape, and murder will obey a law telling them that they
    cannot do so with a gun.

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    Quote Originally Posted by ehsx View Post
    I always get screwed up on dew or bubble point for figuring subcooling & superheat.

    Need to remember that subs bubble.....
    Subs bubble and dewperheat

    Sent from my SM-G965W using Tapatalk

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  25. #16
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    Quote Originally Posted by BALloyd View Post
    Subs bubble and dewperheat

    Sent from my SM-G965W using Tapatalk
    Bubbles in the sightglass


    Sent from my iPhone using Tapatalk

  26. #17
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    So Ive never taken a HVAC tech class, but I had good exposure to psychometrics, ASHAE design principals and refrigeration cycle in college. I think I would have have never figured it out if not for the book learning of the physics. Of course (just my experience) non of it matters 90% of the time because it’s an electrical problem, and 5% of the time its a plumbing problem (icemaker solenoid, condensate drain line), and maybe 1% its a totally plugged condenser.

    I got a call to a restaurant at 10pm- a couple of weeks ago WIC is 50 deg, OAT is 60... Go up on roof- have to replace one condenser fan motor, and the cap on the other motor (new #1 motor made old #2 motor spin backwards.). Owner gets the bill the next day, and says it must have been a lot of Freon I added. Ive done maybe 20 jobs for him... never added Freon. Ever. His WIC is spraying water everywhere and he calls saying it needs Freon.

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