Temperature Controlled PLC Suggestions

Hello all, I'm a newbie so take it easy on me . I am currently experimenting with a solar water heating system. I would like to set up a PLC to control the system, basically monitor temperatures of the water and turn pumps and/or solenoids on and off depending on temperatures, time of day, etc. I will need to monitor around 3 sensors and the runs to each sensor may be about 25 feet at the longest. Readings do not have to be that accurate (+/- a few degrees). I will also need a calendar built into the PLC, I believe some models offer this. I took PLC programming in college, I have to refresh my memory but programming it shouldn't be too much of a problem as I did real well in the class, but I have no experience with actually purchasing a PLC for a specific application and what the different models do.

What do you guys recommend I use as far as the PLC, sensors, etc?

Cost is definitely a factor, if this is going to cost a lot I will have to go about it a different way. I was looking at the Eaton Cutler-Hammer systems that offer the temperature module add-on but do I even need this? I see Panasonic and Honeywell offer similiar systems too. What would be the cheapest and simplest for my application? Any help is much appreciated!!

If cost is a major factor, ask yourself if you really need the plc to monitor the temperature?

If you want the plc to open one two or three valves depending on the temperature then use pt100 probes and the temperature module, but if you simply want to open a valve when the water is at 40 degrees (For example) i would be tempted to use a simple capillary type thermostat. £15 ish cost. You can feed the stats back to the plc so the plc still makes the descisions.

If you can tell us exactly how many devices, and your budget, we can point you in the right direction. Cost may be to much. Since you do not have PLC programming software, this will limit your choices also.

Thanks for the replies already. As far as the capillary thermostat, how do they function? Are they adjustable?

Unfortunately the way these systems operate is not as easy as turning solenoids on or off at a set temperature, there are more variables then that, like temperature differentials, turning the system off at night, unloading in below freezing temps, etc. Plus I could tailor the system to be much more efficient if I was able to have all of the options and adjustments that a PLC would give me.

You mentioned the PT100 probes, what advantages do they have over thermocouples and is anything special needed for a long run such as 25 ft?

I am probably looking at around 3 input sensors in different areas of the system and on the output side, at least 3 solenoids to control flow, and 1 pump. As far as cost, to run a generic but high quality solar water control, it will cost me anywhere from $200-$500 for the controller itself. I was looking at the Eaton PLC's that factorymation.com offers because of the low price and free software but wasn't sure if that will fit my needs. I take it software is expensive?

anybody??

iaknown,
PT100 sensors are really nothing more than accurate temperature variable resistors, so extending them a distance is as easy as adding wire. The wire will add a small amount of resistance, but since the total length of wire is known, as well as its resistance, this "offset" can be compensated for in code. However, I don't thing the small inaccuracy will really make a difference in your application.

If you have to extend thermocouples, you need to use thermocouple cable. TCs rely on electrical potential generated by dissimilar metals. Every time you make a connection that combines dissimilar metals (like using a normal terminal block or wire) you really create another TC. TC cable contains the same type of metals used in the TC itself - make sure you hook up the wires the right way, and don't reverse them. TCs have advantages in high heat environments, but for this application, I think they'd be more trouble than they're worth.

Look at the offerings form AutomationDirect. They have the DL-06 (20 discrete inputs and 16 discrete outputs) for right around $200, depending on input and output options. AD has a 4-input PT100 card, P/N: F0-04RT, for $199. I'd recommend a PLC with DC inputs and DC outputs. You can then use either standard or solid state relays to run your solenoids and pump. If cost is really an issue, you might look at the DL-05. It has 8 discrete inputs and 6 discrete outputs, so it should work for you, but without much room for future expansion.

AD's software costs $395 and a cable about $30. Their tech support is very good.

Good luck, and let us know how your project ends up.

-rpoet

Several years ago I put together a pilot system to extract heat wasted from a laundry setup here at the marina. Total hot water consumption was about 500 gallon per day supplying both laundry and showers.

I asked a PLC guy about using a PLC to control this. What I had in mind was an algorythm to calculate temp difference between hot wasted water and cold incoming water to preheat. This differential temperature would be used to control. His advice to me was to do this with regular temperature switches. I would not get the fine level of control I could get with PLC. AFTER testing and pilot project his advice was to maybe put in PLC during final construction.

Capillary switches are good and very reliable and very cost effective. Yes they are adjustible. You can get them with a long capillary ie 15 or 20 feet.

If you go with electrical temperature sensing you will probably be better off with resistance based units either RTD or thermistor. Thermistor are simpler than RTD. I used thermocouple (TC) in my project and had little trouble. I would not recommend TC because they can be a good antenna and pick up EMF. Had troubles with this in another project.

I looked hard and long for stand alone controls that could operate on a temperature differential. I did not find any.
PLC should be a good option, I was not able to make it work because of problems with thermocouples picking up EMF. It was a PLC class project - accepted my grade of A, remembered lessons learned and moved on.

You want precision and you want low cost - it does NOT work that way. Numbers are arbitrary and just for comparison
10% = $100
1% = $ 1500
.1% = $ 5000

It sounds like you do not have this engineered out to 4 decimal places but is more of a design build. That is fine - a lot of fine machines and processed were developed this way.
Here is my advice
1. Get the system on line with simplest (and less efficient and accurate) and least cost controls.
2. Put your cost savings from reduced heating costs in a piggy bank
3. Revise the system (both mechanical ie piping and pump changes) and controls to improve performance
4. When you think you are near final design and have this running at an acceptible performance
5 Take the savings and buy better controls including a PLC.
6. Use PLC to get more precise controls and better algorythms ie temp difference etc, and better performance and achieve more cost savings.

Dan Bentler

iaknown said:

Thanks for the replies already. As far as the capillary thermostat, how do they function? Are they adjustable?

Unfortunately the way these systems operate is not as easy as turning solenoids on or off at a set temperature, there are more variables then that, like temperature differentials, turning the system off at night, unloading in below freezing temps, etc. Plus I could tailor the system to be much more efficient if I was able to have all of the options and adjustments that a PLC would give me.

You mentioned the PT100 probes, what advantages do they have over thermocouples and is anything special needed for a long run such as 25 ft?

I am probably looking at around 3 input sensors in different areas of the system and on the output side, at least 3 solenoids to control flow, and 1 pump. As far as cost, to run a generic but high quality solar water control, it will cost me anywhere from $200-$500 for the controller itself. I was looking at the Eaton PLC's that factorymation.com offers because of the low price and free software but wasn't sure if that will fit my needs. I take it software is expensive?

Click to expand...

Just my 5 cents, think u should look at ddc rather than plc for this application, just about all have 365 calender built in, low cost & 10k thermisters for temp around $20 each ~.1 deg accuracy.

The HVAC market uses direct digital controls, DDC, as opposed to the industrial world. HVAC components are generally less expensive because the market is bigger (more commercial buildings and large apartment buildings than process factories) and the requirements are not as stringent: all people like A/C around 70°F so the thermostats have low ranges, compared to industrial.

I only dabble on the fringes of HVAC, so I'm no authority, but the recommendation to look at HVAC DDC's might be well worthwhile.

100 ohm industrial RTD elements tend to be expensive, but use copper wire as lead wire.

Thermocouples are inexpensive, but require thermocouple lead wire, which in the US is about $0.50/foot. If I were doing it with thermocouples, I'd use type T because of its inherent greater accuracy over J or K. If appearance is not a factor, then thermocouple wire by itself could be used, with the junction end pair just twisted together. For operation in a residence, that's more than adequate.

My dad did solar house control with conventional thermostats, relays and a manual winter/summer switch. I'll see if he needed differential temperature.

Dan

Danw has a good point regarding using HVAC type controls. I found in my projects that HVAC controls were more easily found and used. That makes sense now that I think about it. Hydronics (hot water heat systems) generally operate at 200 F or less which is the same temp range as the solar system. Johnson had a modular type temp and pressure switch system which allowed you to add more modules to allow controlling more pumps etc at different setpoints on the same sensor.

Dan Bentler

One additional reason to look at HVAC controls for residential heating/cooling - property resale value and marketability.

It is very common nowadays for residential house buyers to hire a 'inspection service' prior to purchase. Conventional thermostats are not likely to raise an eye-brow, whereas a one-off PLC running the heating/cooling is something that screams "who can maintain this?" Can it be maintained by "Mr.Somebody's Heating/Cooling Service" found in the yellow pages of the phone book? Very doubtful, because the HVAC market is not PLC literate.

Unless you're in a housing area where if service is needed, a buyer would say, I'll just replace it with whatever, regardless of cost, custom PLC controls on heating cooling could be a big negative for resale.

The solar heating my Dad installed did not need a differential temperature switch (DTS). I did a web search for differential temperature switch'.
1) One ebay listing incorrectly identified a UE121 switch as differential; it is not.
2) There is an article here regarding a custom circuit for a DTS.
http://www.electro-tech-online.com/...ws/85899-temperature-differential-switch.html

A web search for 'differential temperature controller' brings up dozens of solar specific sites, with all sorts of packaged solutions

1) $85 http://www.arttecsolar.com/retail.html
2) http://www.siliconsolar.com/solar-hot-water-differential-temperature-controller-p-18001.html $120

Food for thought.

Dan

danw said:

I only dabble on the fringes of HVAC, so I'm no authority, but the recommendation to look at HVAC DDC's might be well worthwhile.

Click to expand...

leitmotif said:

Danw has a good point regarding using HVAC type controls. I found in my projects that HVAC controls were more easily found and used. That makes sense now that I think about it. Hydronics (hot water heat systems) generally operate at 200 F or less which is the same temp range as the solar system. Johnson had a modular type temp and pressure switch system which allowed you to add more modules to allow controlling more pumps etc at different setpoints on the same sensor.

Dan Bentler

Click to expand...

Here is an example of a HVAC item. Many years ago I used these units for measuring differential temperature. The original concept is a Wheatstone bridge, using a thermistor and a potentiometer. All that is needed to make these into differential controllers is to not use the potentiometer that comes with the unit and to use a second thermistor. The manufacturer uses a jumper on the terminal strip to use the potentiometer all that is needed is to remove the jumper and wire in the thermistor.

Note they are now discontinued but are still available on Ebay.



Do not be concerned about the temperature range listed. The thermistors are good to 300 degrees and the temperature only applies to the potentiometer value. But since you do not need the potentiometer any way, it may work for you.

Here is an installation PDF, that explains how to use a remote setpoint. All you do is put another thermistor in place of the potentiometer or resistor.

http://yaunco.com/pdfs/1ne_sp32d_inst.pdf

.

rpoet said:

iaknown,
PT100 sensors are really nothing more than accurate temperature variable resistors, so extending them a distance is as easy as adding wire. The wire will add a small amount of resistance, but since the total length of wire is known, as well as its resistance, this "offset" can be compensated for in code. However, I don't thing the small inaccuracy will really make a difference in your application.

If you have to extend thermocouples, you need to use thermocouple cable. TCs rely on electrical potential generated by dissimilar metals. Every time you make a connection that combines dissimilar metals (like using a normal terminal block or wire) you really create another TC. TC cable contains the same type of metals used in the TC itself - make sure you hook up the wires the right way, and don't reverse them. TCs have advantages in high heat environments, but for this application, I think they'd be more trouble than they're worth.

Look at the offerings form AutomationDirect. They have the DL-06 (20 discrete inputs and 16 discrete outputs) for right around $200, depending on input and output options. AD has a 4-input PT100 card, P/N: F0-04RT, for $199. I'd recommend a PLC with DC inputs and DC outputs. You can then use either standard or solid state relays to run your solenoids and pump. If cost is really an issue, you might look at the DL-05. It has 8 discrete inputs and 6 discrete outputs, so it should work for you, but without much room for future expansion.

AD's software costs $395 and a cable about $30. Their tech support is very good.

Good luck, and let us know how your project ends up.

-rpoet

Click to expand...

Thanks for all the replies guys! Now lemme hit you with more questions I checked out the controllers at AutomationDirect. I was just wondering why you recommend the DC in and outs. Not doubting you, just trying to learn more about this. Also, pricing is close so how do these compare do the Eaton-Cutlers? Just a matter of preference?
I also see AutomationDirect offers the same software but free for a limited size program, do you think I would be able to fit within those specs for a pump and say 3 or 4 sensors and a few different circuits?
It looks like Factorymation also offers free software (ELC Soft) with no limit but didn't know how it compared. I am sure customer support would mean a lot to me since I am new to all of this.

danw said:

The HVAC market uses direct digital controls, DDC, as opposed to the industrial world. HVAC components are generally less expensive because the market is bigger (more commercial buildings and large apartment buildings than process factories) and the requirements are not as stringent: all people like A/C around 70°F so the thermostats have low ranges, compared to industrial.

I only dabble on the fringes of HVAC, so I'm no authority, but the recommendation to look at HVAC DDC's might be well worthwhile.

100 ohm industrial RTD elements tend to be expensive, but use copper wire as lead wire.

Thermocouples are inexpensive, but require thermocouple lead wire, which in the US is about $0.50/foot. If I were doing it with thermocouples, I'd use type T because of its inherent greater accuracy over J or K. If appearance is not a factor, then thermocouple wire by itself could be used, with the junction end pair just twisted together. For operation in a residence, that's more than adequate.

My dad did solar house control with conventional thermostats, relays and a manual winter/summer switch. I'll see if he needed differential temperature.

Dan

Click to expand...

Excuse me for my lack of knowledge but what exactly does a DDC do? And where can I learn more about them and where to purchase them? Sounds like it may do the trick also. I know that a lot of the current solar controls use what seems to be a standard 10k Thermistor, is this similiar to an RTD just less accurate?

milldrone said:

Here is an example of a HVAC item. Many years ago I used these units for measuring differential temperature. The original concept is a Wheatstone bridge, using a thermistor and a potentiometer. All that is needed to make these into differential controllers is to not use the potentiometer that comes with the unit and to use a second thermistor. The manufacturer uses a jumper on the terminal strip to use the potentiometer all that is needed is to remove the jumper and wire in the thermistor.

Note they are now discontinued but are still available on Ebay.

View attachment 10157

Do not be concerned about the temperature range listed. The thermistors are good to 300 degrees and the temperature only applies to the potentiometer value. But since you do not need the potentiometer any way, it may work for you.

Here is an installation PDF, that explains how to use a remote setpoint. All you do is put another thermistor in place of the potentiometer or resistor.

http://yaunco.com/pdfs/1ne_sp32d_inst.pdf

.

Click to expand...

Unfortunately I think my system is going to be a little too complicated for just a differential controller (or a few of them). As far as the general operation of the system a differential controller would probably be fine, as was mentioned, many solar setups use this type of control. However, I am experimenting with a few different options on mine, for example, the ability to dump the water in the solar circuit when freezing temperatures are encountered, or at night, or when the temperature of the solar collector goes too high. In addition I want to add a separate circuit where my heating system will utilize the hot water when above a certain temperature by turning a pump on and off. It is probably possible to do all this with differential controllers somehow but I'd have so many of them to do what I need to it would be crazy, plus I won't be able to have the calendar option which I need. An average system with some of these options may be able to be controlled that way but I just think mine will be too complicated, that's why I was leaning towards the PLC (plus it'd be cool to play with). Unless of course you were refering to using the differential controller along with the PLC, but still I don't think it would be cost effective because of how many I would need.
I realize the capabilities of a PLC can be overkill in this application but I need a lot of flexibility with controls because of what I am doing. It seems like the DDC may work too, I just need to find out more about them...

I was just wondering why you recommend the DC in and outs. Not doubting you, just trying to learn more about this. Also, pricing is close so how do these compare do the Eaton-Cutlers? Just a matter of preference?
I also see AutomationDirect offers the same software but free for a limited size program, do you think I would be able to fit within those specs for a pump and say 3 or 4 sensors and a few different circuits?

Click to expand...

I recommend the DC INs and OUTs for the simple fact that 24 VDC is low voltage, so that means no line voltage near the operator controls (switches, PBs, etc...) and the DC outs can be used to drive solid state or standard relays. Personal preference, really. There are plenty of reasons to use 120/240v for control, but I think 24 VDC is a little safer, all things considerd. If you use standard relays for your control, make sure to use a "flyback" diode across the relay coil to prevent damage to the PLC output form the inductive kick.

I like AD as a matter of personal preference too. I learned on AD and I'm very comfortable with it. Their tech support is free, friendly, and very helpful.

As for the free version of the software, 100 words is probably a little tight for what you'd like your program to do. It also limits your expandability in the future as your system grows. The full version of the software is a good investment.

Good luck!

rpoet

I am experimenting with a few different options on mine, for example, the ability to dump the water in the solar circuit when freezing temperatures are encountered, or at night, or when the temperature of the solar collector goes too high.

Click to expand...

I installed a solar water heating system in my house in 1978 (31 years ago now!), and the best way to prevent the water from freezing is to use a drain-back tank. This is a small (10 gallon) insulated tank that sits on top of the main hot water tank. When the temperature of the solar collector is not more than 10 degrees above the hot water tank, the differential temperature controller turns off the pump and all the water in the collector loop drains back into the drain-back tank. Simple, and there are no valves or other controls to worry about. The main thing is to arrange your piping so it all slopes downhill back to the tank.

As for having excess solar heat to dump into your heating system, you probably will not have any extra unless you buy several extra collectors. Then you will have way too much solar heat in the summer with no place to use it.

If you do need some more collectors, I have one 4" X 8" that I will sell cheap. Yes, I don't use them anymore, and the glass on one collector got broke by a fallling limb.