Cooling temperature control using compressor & cooling coil

Hi guys,

I've an application to maintain the temperature of the water tank at 15 deg Celcius by using an compressor with cooling coil (in the tank). I've a temperature transmitter (4~20mA) to the PLC. The compressor on/off will be control from the PLC.

To control the compressor (1.5Hp, 230V AC), can I use a SSR or I just need a contactor + overload? Can this be done?

Thanks.

Ok so you are going to use a refer plant to cool the tank by placing evaporator in tank. Make sure evap is made of material compatible with refrigerant and for cooled contents. Also make sure it will drain lube oil out. Oil trapped in evaporator will lead to compressor starvation and failure.

Yes you should definitely have overload protection on this motor. Refers are hard starting especially so when short cycled.

Ovesize conductors (I would use # 10 and maybe #8 if total run is 100 feet or more) to allow for starting current.

You can use SSR of course one each for each line conductor. Larger SSRs should be used for more cooling. Oversize some more if you are using single phase compressor with start capacitor.

Anti short cycle is a good thing and should be easy to do with PLC.
In essance a time delay is set after unit turns off. After 15 to 30 minute unit is allowed to restart.

Dan Bentler

hi,

Thanks for your reply, the compressor is just located besides the water tank, it's a small tank with 150litre of water. For a 1.5Hp, 230V AC = 5Amps, if I size in the 15Amps SSR should be good enough, am I right?

I'm afraid using contactors + overload will damage the if it's on/off too frequent, will it?

flyers said:

hi,

Thanks for your reply, the compressor is just located besides the water tank, it's a small tank with 150litre of water. For a 1.5Hp, 230V AC = 5Amps, if I size in the 15Amps SSR should be good enough, am I right?

I'm afraid using contactors + overload will damage the if it's on/off too frequent, will it?

Click to expand...

For comparison I built a 120 volt 1 ton 1 HP water water heat pump. run current was about 10.5 amps and on a real hard start about 60 amp for approx 20 to 30 seconds. You need over current protection on compressors.

Starting and stopping frequency will be determined by the heat load of what you are cooling with chilled water. Contactors have worked well on these for eons - yes they did wear out but if properly sized seemed to last for years.

Dan Bentler

If you are providing the motor drive (ala contactor/ssr) then another thing to consider is the use of a VSD. This can provide better regulation of temperature and also some energy savings.

The cost of a cheap VSD is still much more expensive than a contactor, but where cost isn't so important then it can be beneficial. This would especially be true where reliability is more of an issue. Constantly starting and stopping motors isn't good on them, nor the contactors. Using a VSD is another option, albeit more expensive.

flyers said:

hi,

Thanks for your reply, the compressor is just located besides the water tank, it's a small tank with 150litre of water. For a 1.5Hp, 230V AC = 5Amps, if I size in the 15Amps SSR should be good enough, am I right?

I'm afraid using contactors + overload will damage the if it's on/off too frequent, will it?

Click to expand...

Frequent starting and stopping will not only wear out a contactor, but also the motor and the compressor. I would be sure to include minimum on and off times in your logic that are matched to the recommendations by the compressor mfg.

You may also want to look into some other way of regulating the cooling so that you can leave the compressor running and use positioning valves to load and unload it according to the cooling demand.

I am not a refrigeration expert, so that is about all I can add.

Good Luck!
Paul

Water chillers used in the Semiconductor Industry regulate the Freon. It has a solenoid valve on the liquid line, and a Hot Gas Bypass valve to route a little gas back to the input side when the valve is closed. Maintains 1-2 PSI on the low side when valve is closed.
Neslab is probably the most popular. Look for manuals on Neslabs. One of them has a diagram of the Freon circuit. I'll dig more tonight if you don't have any luck.
I suspect a refrigeration expert could help with solenoid controlled design.

I suggest you take a look at this post:
http://www.plctalk.net/qanda/showpost.php?p=209322&postcount=4

Take the short cycling issue very seriously. If you don't then I'm sure the compressor manufacturer could use the economic boost it will get from the second and third replacement compressors you will buy.

And do a forum search on "compressor" There are quite a few threads on the subject.

Controlling a chiller is not difficult, but it is also not nearly so simple as it seems on the surface. There are fundamentals of the physics involved and the behavior of compressors that have to be factored in, otherwise your going to destroy your compressor in short order. Its not something to be taken on by someone who does not have very much knowledge about refrigeration. If you don't have that know how, then find help from someone who does.

For a 1.5Hp, 230V AC = 5Amps, if I size in the 15Amps SSR should be good enough, am I right?

Click to expand...

Compressor motors are notoriously difficult to start and it will probably draw 40 or more amps during start-up. Check with the compressor manufacturer. As has already been stated, you need a motor starter with an overload relay. It can be a contactor/overload or a soft starter using SSRs with overloads designed for motors, but using a simple SSR w/o overloads is a bad idea.

Is your tank calm? If not you need to find a way to make it somewhat turbulent. Immersion evaporators don't work very well in a calm tank. You need about 2.6 liters/min of flow over your evaporator per kilowatt of heat removed from the water, dictated by the laws of physics, or you're going to have decreased performance and may even damage the system. Making a guess on your chiller capacity based on the size of your compressor motor you need somewhere in the neighborhood of 14 lpm of flow around the evaporator at a minimum. In a typical tank the flow to/from the process needs to be several times the required evaporator flow in order to keep enough fluid moving around the evaporator. Often a supplemental recirculation device is required - it could be as simple as a motorized propeller in the tank. Based on the size of your compressor and extrapolating that out to the kind of process heat load it could be cooling, my bet is that your 150 liter tank is too calm without supplemental recirculation. But thats just a guess made from the available info.

bevanweiss said:

If you are providing the motor drive (ala contactor/ssr) then another thing to consider is the use of a VSD. This can provide better regulation of temperature and also some energy savings...

Click to expand...

Determine if your compressor has a built in positive displacement oil pump, utilizes high to low side refrigerant pressure to pump lubrication oil or if it has a centrifigal oil pump. If it's the later, you may have problems getting sufficient oil to the bearings depending on the loading/ operating conditions and have a seized compressor in a short period of time.

This looks like an application for a Reduced Voltage Solidstate Starter (RVSS). A number of manufacturers build these - Cutler Hammer, Allen Bradley, all the majors really. Contact them for details. You should also consider Klixon or similar motor winding temperature thermostats in the motor - cheap protection.