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Ladder diagram for cooling system

  1. Dec 3, 2005 #1
    I need some help with this ladder diagram that is for a cooling system. Here is the assignment:

    The system has two rooms that need to be cooled. There is one evaporator in each room with one fan. The evaporators should have an electric anti-ice system (heat rods) that is to be controlled by a timer and a thermostat. The fans should have a delayed start after the anti-ice process finishes.

    The compressor has a capacity-regulator that is regulated with a low pressurestat (stops two cylinders when pressure falls). The system is to be controlled by the "pump down" principle with its own LP-pressurestat. The condenser has two fans. One fan should always run when the compressor runs, and the second one should engage via its own HP-pressurestat.

    The control board should have the following lights:
    - Running compressor
    - Compressor stopped by either HP-pressurestat or engine protection.
    - Anti-ice in any of the two rooms, two lights.

    There should be these switches:
    - Start compressor and condenser
    - Start switch for both rooms

    Now, the attachment shows a diagram that is supposed to be correct, but I need serious explanation to understand the concept of these diagrams. I have a list of questions I have:
    - The different squares labeled K1, K2 and so on, these are simply supposed to represent the compressor, the fans, and so on, right?
    - What is the difference between the type of switches at the red circles A, B and C?
    - What is the purpose of the magnet valve?

    Attached Files:

  2. jcsd
  3. Dec 4, 2005 #2


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    Yes, I believe you are correct on the function of K1,K2, and K3. It seems that K1 must have a relay since you A would be the contacts controlled by K1. So when the compressor has power the contacts K1 close, turning on the compressor ON light, H3.

    I think your B must be some sort of EMO (emergency Off) switch, Switches F4 and F6 would serve similar function.

    Your C is the temperature control switch.
  4. Dec 4, 2005 #3


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    The way I see it; F4(pressure switch) will be closed before starting the compressor with start switch S2. (note that S1 is the power disconnect switch which needs to be closed to power the system) When S2 is closed, power is supplied through K1 relay solenoid which closes relay contact K1 (your C) which starts the compressor. Lamp S3 will light indicating compressor is running. Also fan 1 relay solenoid K2 will be powered as long as the "low pressurestat"F5 (your B) is closed. Fan 2 relay solenoid K3 will be enengized when the HP switch closes. Now B2 (your A) is the thermostat that turnes on the heaters.

    hope that helps
  5. Dec 4, 2005 #4
    I really appreciate this help guys. I wonder about several things though;

    - A "relay" is is a device that causes a switch to open or close elsewhere, correct?
    - I don't get how F9, F10, and B3 works. What is it these do excactly?
    - I see that B2 is connected to the relay on the far right side, so this switch has three wires coming together instead of two. How does that work out?

    I am sure to come back with more questions, so just bear with me here...thx
  6. Dec 4, 2005 #5


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    It was a bit late when I posted the above, I see that there is a typo in the last sentence, A is your temperature control.

    A relay consists of a coil which when powered, changes the state of a set of contacts. I say a set because it is not uncommon for a relay to have multiple contacts. A relay can have 2 types of contacts. NO or Normally Open goes from an open state to a closed state when the coil is powered. NC or Normally Closed goes from closed to open when the relay is energized. Like any other switch relays can be classified by the function and number of contacts. You should become familiar with terminology like SPST, or single pole single throw, DPST, double pole single throw. See http://www.kpsec.freeuk.com/components/switch.htm" [Broken] page for examples of the terminology.

    Your B2 is a DPST switch.

    It is not clear to me how you can be learning this subject without understanding the workings of a relay you need to spend some time learning these basic concepts.
    Last edited by a moderator: May 2, 2017
  7. Dec 4, 2005 #6
    The subject is actually about heat pumps and cooling systems. These ladder diagrams are just a small part of the total syllabus.

    I understand most of the diagram now, thx, but still I'm confused with the Magnet valve, and its function. Could you explain that, if you know?
  8. Dec 4, 2005 #7


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    There is no way to know what its function is with the given information. All I can see is that it is temperature controlled. There is no way to tell if it is activated by high or low temp, there is no way to tell what it does when it is activated. This information would come with more detailed descriptions of the system.

    I am guessing that the funadmental knowledge of relays and swithes is assumed for your course. You may want to do some addional work to come up to speed on them.
  9. Dec 4, 2005 #8


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    yes, as Integral explained
    These are pressure switches. They use pressure to mechanically force the contacts to the desired state.
    The contacts are a part of the relay. In this case a double-pole single-throw contact configuration, thus three wires.
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