Temperature sensors in a digester with mixer to control the boiler 1

[cdxx139]

Hello,

I am trying to determine how to control the boilers for a Primary/Secondary digester building. The new mixers being installed do not allow temperature sensors, so we will locate them. Does anybody have suggestions or references they can point me at?

The location should be below the digester tank level sensor. I am thinking multiple sensors and take an average. The mixer will be a Draft Tube Mixers.

Any help is appreciated.

knowledge is power

 

[MartinLe]

What heating system do you have, HX?
Why do you want to control the boiler, this seems odd. You mean heating water temp in the HX, right?

 

[cdxx139]

We have a hot water boiler with its main fuel being excess digester gas and back fuel oil #2. We are replacing the digester mixers that will have a built in heat exchanger. We will provide hot water to the HX to maintain the digester around 95 degF. I have discussed this with the mixer manufacturer and they do not have any temperature sensor option, and quite frankly, dont seem to be involved in this portion of the project.

As a newbie in digester projects, I assume I would put multiple (3?) temperature sensors throughout the digester tank, and take an average to control when the boiler system will energize. My question is where and how. I assume on the mixers, but the manufacturer will get back to me, but they dont sound confident. I was told there is a recirculation line, which may be an alternative.

Just checking iof anybody has experience or thoughts?

knowledge is power

 

[MartinLe]

You havent told me where you heat the digester, what is replacing the hx in the old mixer?

 

[cdxx139]

The new mixer is coming with its own new heat exchanger. Oridninally the designed for inlet HW temps of 170 degf, but somebody said that may cake the digester material near the heat exchanger? So they have since reduced the inlet HW temp to 140 degF.

knowledge is power

 

[cdxx139]

The mixer manufacturer confirmed he did not want to put the temperature probe on the mixer.

Locating it from a manhole at the cover of the digester would prove difficult for maintenance.

I have since found an existing sleave into the digester, that I will re-use as the temperature probe to control the boiler.

Now just looking for a manufacturer to provide a temperature probe for digesters.



knowledge is power

 

[danw2]

Temperature sensors are custom for each application.

Find out the info below and call a controls and instrumentation supplier/distributor/vendor to get a quote on some temp sensors.

Sheath or thermowell material - Is 316SS suitable? What is the tank material? What is the Hx material?

Does the point of ingress into the tank see any pressure, negative or positive? If so, a thermowell is usually used to contain pressure and allow installation/removal of the sensor element without affecting tank pressure.

Placement/insertion length: Where can the thermowell or element be placed so that it will not come into contact with anything in the tank that could physically damage it?

Can the sleeve have a threaded coupling welded to it for mounting the thermowell well or the sensor sheath?

Sensor: the temperature range is about 160Deg F max, which can be handled by an RTD or a thermocouple sensor. Pick the one which the receiver can handle (next item) and which the maintenance people are most familiar with. If RTD, make sure the sensor and the receiver can handle a 3 wire RTD (not 2 wire). If thermocouple, thermocouple wire is needed for the run from the tank to the receiver.

What is needed for a signal ? What kind of temperature signal can the receiver (PLC/DCS/controller/indicator) handle? Raw thermocouple or a raw RTD or is a 4-20mA signal needed? If 4-20mA, a transmitter is needed.

Is it a hazardous area? If so, an Intrinsic Safety (I/S) barrier is needed for any of the signals and a transmitter should be approved for I/S service.

 

[cdxx139]

Instrumentation is not my strong suit, thank you for the primer.

knowledge is power

 

[MartinLe]

I'm curious, what type of mixer with integrated heating do you use?

 

[cdxx139]

It is what they call a "draft tube mixer". That portion is selected by another group, My scope is to provide utilities. It has a jacketed heat exchanger dead center, and pumps the sludge (10,500 GPD) from the bottom of the tank, through the center HX and up to the cover so it pushes out to the edges to mix and repeat.



knowledge is power

 

[bimr]

The temperature probes are typically installed after the heat exchanger in the heat exchanger recirculating loop.

There is no reason to install a temperature sensor in the digester.

The temperature of the digester does not change rapidly because the digester tank is large in comparison to the heater. It is also important to control the temperature output of the heat exchanger.

 

[cdxx139]

Thank you for providing insight bimr.

That is how I would typically control an HVAC equipment such as chiller or hot water system, but I am having trouble understanding the correlation between the return temp from the HX and the digester temperature.

To put real numbers into the equation. The HX has an inlet temperature of 140 degF, assume a 10 degree delta T, so 130 degF exiting the heat exchanger and returning to the boiler. The digester needs to stay between 85 and 95 degF. If I control with the HX return water temp, could I overheat the digester sludge? I think so. There is also the concern of "caking" the sludge near the HX with 140 degF (but I am still looking into that).

When would I start and stop the pumping and boiler system with return temp control? I would need the pumps to run 24/7? And when would the boiler energize and de-energize?

I am an experienced HVAC engineer doing my first industrial wastewater project.

knowledge is power

 

[bimr]

The digester heater does not have to run at all times. In the summer for example, you may wish to recirculate without heating.

Once the digester is operating, the heat load is the heat required to bring the influent up to temperature plus the heat losses through the digester walls.

The residence time in the digester is 10-20 days. If you assume that it will take 20 days to heat up the fluid in the digester, that will be in the ballpark.

Designers are concerned with struvite formation and other issues. Not sure if this is a problem for you.

 

[MartinLe]

I think bimr meant to control for the sludge temeprature on the outlet of the HX. Which I also strongly suggest.

I'm still a bit confused on the mixer, by draft tube you mean something like this?

http://www.ovivowater.com/product/m...gestion-mixing/eimix-mechanical-sludge-mixer/

or is the draft tube in the digester? Sounds so.

Either way control for sludge temperature on outflow of the tube. Which you can't do because mounting the sensor from the manwhole is problematic.

If I finally understood your problem, the hard part is to get the temp. probe near the outflow of the draft tube in the digester. Can you show us a sketch or somthing?
Maybe make an extra hole in the digester roof for a long thermo well?

danw2 has covered a lot of ground for the sensor and installation.

 

[cdxx139]

Thank you all for your feedback.

Yes the draft tube is internal to the digester. See attached photo from the same manufacturer. You were looking at the EDT model, and I am talking about the RDT model, which you suspected.

My concern with temperature sensor from a manhole of the roof, will be maintainability. Which may be the same reason the mixer manufacturer does not provide a sensor option, because then you would have to empty the digester to get to it, or be removed from the digester cover, which is inconvenient; therefore probably won't be done.

So I think the temperature sensor thru the side wall is the best option. It will be midpoint of the flow of digester sludge to get an average temperature, and have easy access from inside the building for maintenance and calibration.

knowledge is power

 

[cdxx139]

Also attached is a quick sketch, showing the internal mixer, with the motor located over the cover. The Heat Exchanger is built in the center of the mixer pole with propeller built in. Hot Water is supplied to and from the HX.

Again, I don't see the need to control off of the heating water loop. I plan to provide the hot water the mixer manufacturer requires, and when the digester temperature is satisfied, the boiler and hot water pumps will de-energize at 95 degF (ADJ.), and then energize again if and when the digester fluid drops to 85 degF (ADJ.)

I am open to being wrong, please convince me of a better way.

knowledge is power

 

[cdxx139]

I think what I am missing is the importance of the boiler is for the pickup load, when they first fill the digester tank with potentially 50 degF sludge, and the boilers need to pick that up to 95 degF within 24 hours.

So I will size the boiler as small as possible to meet that load, so it is not so oversized, when it only has to maintain the tank at 95 degF.

knowledge is power

 

[bimr]

The water loop should have its own temperature control with a maximum temperature of 150 [sup]o[/sup]F

Attached are some pages from a text.

You probably should consult with the particular manufacturer that you plan to purchase this equipment from as to the specific location of the temperature sensor. You will be buying a process as well as the equipment.

http://www.pncwa.org/assets/2012Con..._Digestion/mossinger_digester_heating_101.pdf

 

[MartinLe]

Here's the problem I see with the location of your temp. sensor:

The sludge must never be above 42°C/108°F, biological considerations.
Your sensor location is practically the inlet of the HX, the sludge will be far cooler (compared to outlet) especially in the heating up phase
The temp. you measure changes long after a change in HX operation (the sludge has to move all the way down)

This makes for a bad value to control with, how can you ensure you never hit 42°C

With a thermowell/sleave (as danw2 sugggested) you can remove the sensor without opening the tank, so specifically what is your maintanence problem?

I would also question the heat up in 24h requirement, is there a specific reason for this? A digester that will be in operation for years without beeing emptied, why the hurry?

Please confirm: There will be a dedicated boiler heating only the digester, and the boiler will be in on/off operation (no modulation down to the expected continous heat demand of the digester)?