PID Control Loop

[Rastus 0]

Hello All,

I have a question about heating a tank of water. Its not really Allen Bradley specific, but i am using a ControlLogix unit...

Anyway, I'll attach a basic drawing of the layout.

I need to heat water to 65deg using an existing heat exchange. Once it is hot, then the water is diverted to a machine for cleaning. 

At the moment i am using temp sensor 1 to continue the cycle until it reaches temperature. 

Meanwhile Temp sensor 2 has three levels (x<80, 80<x<85, x>85) to open the proportional valve to regulate the steam to the heat exchange at three levels.

This process takes a long time, so I'm wondering if i could improve the process anywhere? Besides hardware changes. 

Thankyou.

Edit: Because of the layout of the sensors etc. The water at sensor 2 reaches temperature much quicker. I was using sensor 1 to ensure the entire tank was hot enough. Maybe i just need to do some trials with sensor 2 as the process variable.

Edited 4 Jun 2020 by Rastus

[BobLfoot 301]

OK putting on my "Engineer's Hat" for a moment let me make some observations.

1. Assuming that Water Flow and Pressure through the HE is constant then the Delta-T you can achieve between Input and Output of the HE is going to depend on the setting of the proportional valve.

2. If you can codify the formula so that the Exit temp of the HE is between 80 and 85 then you'll heat the tank at it's fastest reasonable rate.  I say this because going over 85 you risk hot spots in the HE and calcification of the coils or plate/frame assembly.  

3.  One other thing to consider on this scenario is your inflow and outflow rate of the tank.  You might improve the process by bringing the tank to 65 and once you start cleaning add cold water immediately, knowning the HE can being it up to temp as it is only a portion of what's coming out.

Just some observations.

[Michael Lloyd 85]

You're not vaporizing or condensing on the water side so M*Cp*Delta T can used to calculate the amount of heat being input and removed in the system. The steam side is a little more difficult and only useful if you're trying to evaluate the heat exchanger.

M = mass flow rate, #/Hr

Cp = Specific heat of the fluid. Water is easy. It's 1

Delta T is obvious

Hopefully you have flow measurement. Most of the time (in my industry) liquid flow rate is displayed in GPM (gallons per minute) or BPH (barrels and hour). If GPM it's pretty easy to get #/Hr. #/Hr = GPM * 500 * Sg (specific gravity). Water (in your temperature range) has a specific gravity of 1.0-ish, which makes the calc easy. Know how much heat the exchanger is capable of producing (Bob's warnings about hot spots and calcification are spot on) vs how much heat is required is a good start to analyzing whether or not you can speed the process up and it will tell you if the exchanger is fouled. I doubt if you can. You may even have a fouled heat exchanger if it's been in service for a while and the steam system water isn't treated properly. There should be an exchanger spec sheet available. Sometimes (rarely these days) the nameplate has the info that you need.

Sensor 1 is your PV since you're are blending cold water with hot water and the desired temperature of the blend is your target.

[Rastus 0]

Thanks guys, thats given me something to go with.

Is the warning about calcification in deg C or deg F?

I believe the steam service is well maintained. There is a treatment system in use.

I dont have a flow meter in the system, and i was running the pump at 30Hz because any lower produced ground loop faults with the lower voltage and the excessive moisture.

I believe slower water flow results in more heat transfer?

I think i will check out the heatexchange, are these serviceable? or does calcification mean replacement?

Thankyou.

[BobLfoot 301]

Rastus being you were doing "Hot Water Cleaning" and you were from "Down Under" Austrailia according to the post, I assumed the 65/85 were Celsius.  Cleaning with 65 F water would accomplish little as tap lines run 55F.  In my former life we cleaned with 155F water or about 68C.  My ehat exchangers were configured to SCRAM or CRASH at 165F or 75C.  Meaning if the HE Exit water reached 165F then the steam valve was slammed shut as a precaution.  Hope that answers your question.

[Rastus 0]

Thanks BobLfoot,

I should have assumed that you assumed this.

[TrevorD 1]

On 05/06/2020 at 8:18 AM, Rastus said:

Thanks guys, thats given me something to go with.

Is the warning about calcification in deg C or deg F?

I believe the steam service is well maintained. There is a treatment system in use.

I dont have a flow meter in the system, and i was running the pump at 30Hz because any lower produced ground loop faults with the lower voltage and the excessive moisture.

I believe slower water flow results in more heat transfer?

I think i will check out the heatexchange, are these serviceable? or does calcification mean replacement?

Thankyou.

Maybe i am wrong how ever i was always taught that running the pump at 50hz (higher speed) assuming the system can handle the higher flow rate would result in a more efficient transfer through the HE. 

As far as i can see from your diagram PV would be temp senser 1 until after valve 1 closes, then PV would have to be moved to Temp sensor 2 to allow for the temp increase in the HE before going to the machine. 

 

[Rastus81 0]

17 hours ago, TrevorD said:

Maybe i am wrong how ever i was always taught that running the pump at 50hz (higher speed) assuming the system can handle the higher flow rate would result in a more efficient transfer through the HE. 

As far as i can see from your diagram PV would be temp senser 1 until after valve 1 closes, then PV would have to be moved to Temp sensor 2 to allow for the temp increase in the HE before going to the machine. 

 

Thanks TrevorD,

 

I will try the faster speed on the pump. Thats an easy test. Whats the logic behind the idea?

As for the process variables. They are separate at this point. Sensor 1 only controls the water valves, and sensor 2 controls the steam valve.