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10-31-2009, 08:40 AM #1
What controls the Temperatures???
This probably sounds stupid but what controls the temperatures in a walk in cooler? In residential hvac the thermostat controls that. If your to hot you turn the t-stst down. Is it a PSI switch that keeps the pressure set to maintain the temp? So different but fun learning it.
10-31-2009, 08:46 AM #2
No offense, But you really need to get some training before you try and tackle commercial. This is ABC stuff.It's what you do that defines you.
10-31-2009, 09:16 AM #3
No kidding!lol. I just started a new job that works on refrigeration I'm a helper when it comes to this kind of work.
10-31-2009, 09:58 AM #4
Yes, walk-in cooler temperature can be controlled by a low pressure switch. It's one of the most basic methods of temperature control. It controls temperature indirectly by sensing suction pressure, cutting out at a suction pressure just below where system is running at the desired box temperature and typically cuts in at an off-cycle suction pressure which corresponds to a temperature slightly above the desired temperature.
This methods does have its limitations. One of which is when the condensing unit is located in an ambient colder than the desired box temperature, the system won't cycle back on because the LP control sees the ambient temp, not the box temp. Another problem has to do with control of refrigerant migration during the off-cycle and flooded starts.
For most walk-in coolers however, the temperature is controlled directly by a thermostat of some kind. On smaller systems up to about 1 HP, the thermostat may be switching the compressor on and off directly by breaking on leg of the compressor power feed. When the compressor amperage draw exceeds the rating of the tstat a contactor is required and the tstat becomes a pilot control wired in series with your safeties and other operating controls like a defrost timer.
This method, while solving the lower ambient problem above, still doesn't address the flooded start issues. A crankcase heater will help, but there's a better way.
One of the more common and better methods of control (IMHO) is rather a modified combination of the ones described above...ie, pump down control. This method has the thermostat controlling a liquid line solenoid valve typically located just before the expansion valve (TEV) and uses the low pressure control, set at about 5-10 psig cut-out to cycle the compressor.
That's it in a nutshell. There's a lot more to it, but that'll give you a pretty good start at understanding how walk-in cooler temps are controlled.
Get you post count up and Apply for Pro Membership. Then you'll be able to access the Educational Forums, where we have loads of good stuff to help you learn more about it. (Shameless Plug for the EF )
10-31-2009, 10:20 AM #5
The refrigeration system takes away the heat
1. So-called "air conditioning" is refrigeration. So you have been working on "refrigeration" all along.
2. Your question doesn't sound stupid. Nobody starts out knowing everything. In fact; everybody starts out knowing nothing. The better ones work continually away from that point.
3. The refrigeration system moves the heat from inside the space to outside the space - just like it was a small house. It does this by creating the correct pressure range which results in the inside coil being colder than the inside air temperature
4. When enough has been removed the refrigeration system stops.
5. When enough heat has re-entered the space the refrigeration system starts operating again.
This can be done with a thermostat.
Because the inside (evaporator) fans run all the time, it can also be done with a pressure control which stops the refrigeration when the pressure setpoint is reached. Because the evaporator fans are on all the time - the suction pressure is an accurate representation of the temperature in the box.
There are numerous little odds & ends details which can also be applied to the temperature and pressure controls system - but the above is the basic deal.
BTW: Because these guys will break your balls doesn't mean they don't love you. Always picture that they are smiling as you read whatever they write.
10-31-2009, 11:16 AM #6
That's great information! So the Pressure switch is set on a certain pressure of the refrigerant depending on the application? When the psi on the suction goes up the psi switch closes completing the circuit turning on the compressor? When the psi goes down below the set point the compressor shuts down? Can you change the set point on the psi switch if you had to? How do you know what the psi and temps should be set at in the first place?
10-31-2009, 12:01 PM #7
10-31-2009, 12:12 PM #8
Thanks for all the help. I just got a Lincoln Tech book Modern refrigeration and ac. About 1000 pages to go over plus some Jim Johnson videos that go over refrigeration as well. I'll keep you posted Thanks again God bless.
10-31-2009, 12:18 PM #9
I see you have 15 post so go and apply for pro membership then you'll have
access to the Educational Forums tons of great info in their and always adding to it.
10-31-2009, 12:35 PM #10
A lot of like using a chart like this to set up a low pressure control to maintain temperature.
Use it to get yourself in the ball park, then tweak the settings until you get the desired temperature.
I have always found it helpful to think of these settings in terms of the refrigerant's saturated temperature instead of the saturated pressure. For example, pick one of the types of systems listed on the chart and compare the cut-out pressures for all the different refrigerants listed, then compare their saturated temperatures corresponding to those pressures. They should all be roughly the same.
Thinking temperature rather than pressure gives you a much better feel of what's going on in a system.
10-31-2009, 12:59 PM #11
10-31-2009, 03:23 PM #12Professional Member
- Join Date
- Nov 2008
- Altamont, IL
another neat thing about pump down control or pressure sw. control is that you don't have wires runnning to the remote condensing unit.In GOD We Trust
10-31-2009, 03:44 PM #13