hydroponic system 8

[daviddor]

Hii everyone .
i am designing an hydroponic system , and there is one calculation that really gives me a hard time , i will be very happy if someone can help me with this. I simplified the equation that i need to develop down below. thank you very much for any help.

Question
An isolated tank contains M amount of water. A pipe connected to the tank with a pump, delivering the water in a close circle back to the tank through the atmosphere. Assuming the water temperature distribution is equal in the tank at the beginning . Create an equation of the temperature in the tank relative to the time .

Given :
water flow through the pipe 3M/h (three times the water tank per hour)
Initial temperature at the tank T1
Atmosphere temperature T0
Length of the pipe L
h0 - convective heat transfer coefficient of the air
hW -convective heat transfer coefficient of the water
k- pipe thermal conductivity (W/m·K)
d – pipe diameter

*Can use any other obvious data that is needed and can be reached easily online .
Please explain every step, equation and assumption that is used.

 

[pierreick]

Hi,
Homework problem?

Pierre

 

[daviddor]

Absolutely not homework, it just that I wrote it in a way that it would be clear what kind of answer I need

 

[LittleInch]

Heat loss from the tank?

Temperature difference?

Wind or air velocity?

Length of pipe/ area.

If the pipe is long then your DT of the pipe will be variable from one end to the other.

This is a transient thing so you won't find an equation, but a series of steps.

You're not asking for much!
Tell us how you're going to do it and we'll comment is the way it works. We're not teachers.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[daviddor]

Thank you for your response.
I assumed isolated tank. Since I want to create this device for different conditions, weather and screen houses. I need to know if it would be profitable to me to cool down the water, and how much energy I would need approximately. That's why I want to use variables in my formula. Yes I agree it's gonna varies from the beginning to the end. But maybe we can use some assumptions to predict the overall change in the tank per time , which is what I need, thank you for your help

 

[pierreick]

Hi,
Is that the type of system you are talking about?

https://www.epicgardening.com/hydroponic-systems/

Pierre

 

[daviddor]

Yes, the water going out from the tank through the pipes and going in again to the tank, everytime different conditions of growing. I want to calculate the overall temperature drop, to see how much energy do I need to invest to cool down the water in the tank

 

[daviddor]

Yes something like this

 

[pierreick]

Hi,
To me it's much more a mass balance issue, you need to keep the volume of water constant in your tank, probably using a float system to manage the make-up water. To keep your reservoir homogeneous, you need to blow some air in the tank; together with the make-up water this will help you to keep the temperature stable. BTW we have no indication about the flow rate of water you are thinking of, pump similar to one use in an aquarium, the pipe length, the diameter and the nature of the pipe.

Good luck
Pierre

 

[IRstuff]

I want to calculate the overall temperature drop, to see how much energy do I need to invest to cool down the water in the tank

You could start by identifying what you think is causing the temperature drop, although your statement is confusing, since you then talk about actively cooling the water.

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[3DDave]

I guess it depends on the plant. One year the outside unit blower motor on my AC failed - apparently along with thousands of other people, so the company had a 2 week waiting list. I took the oscillator off the fan-spray sprinkler and set it to gently rinse the coils. I ended up with a marsh of a back yard (but a cool house) with water in the 90-100F range and zoysia grass that was the greenest and healthiest it had ever been.

Anyway, you don't need to chill the tank, you need to chill the return. You know the rate and the temperature of the return, so why bother caring about the tank temp when what the goal is is to take the excess heat out. You only need to cool the return to whatever temp you want to pump back out. Whatever that temp is, the tank will get there. Add more insulation to the tank if heat gain in the tank is a problem.

 

[georgeverghese]

See page 11-18 in the subsection for batch heating and cooling; Chapter 11 in Perry Chem Eng Handbook 7th edn. Expression specific to your application would be equation 11-35e. You've got to be conversant in engg heat transfer to use this expression.

 

[daviddor]

Thank you all for your great answers,
I believe my temperature drop is caused by first :the pump power inside the tank, and then the exchange with the air outside through the pipe.

Thank you very much for this source @georgeverghese , if you can just draw a little sketch of this reactor process please it would be great! i understand all the variables , but not sure about how it is really looks like .

I also tried to develop a formula, if anyone has some thoughts about , please , i will add it as a file. thank you again all , you are really helpful .

 

[pierreick]

Hi,
Similar to Perry an extract from Kern: Process heat transfer.
Good luck
Pierre

 

[IRstuff]

I believe my temperature drop is caused by first :the pump power inside the tank, and then the exchange with the air outside through the pipe.

So, you actually need a heater? How much heat? A simple measurement of temperature and water flow would at least get you that far.

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[georgeverghese]

So the heat getting into this tank is from the nutrient recirc pump.
In your attachment derivation, page 1, the heat into the tank should be ((1-η).P, where 0 < η < 1, and P is the operating power draw of the pump
What is your initial temp diff T1-To? If this diff or the mean dT is low, you may be struggling to get any heat exchange.
As suggested, bubbling some cool ambient air into the tank may be a more practical way to get rid of this heat.

 

[daviddor]

The difference in the temperature for example can be between 22c in the tank to 30c in the atmosphere. basically we testing a new device for cooling the water. But please if you can give a link to an example with such bubbles system maybe i can suggest it also to the team.

I am also wondering which heat generation would be bigger or if any of them is negligible.
First from the pump and then from the air outside when the fluid goes through the pipe.

Thank you very much for your help,

 

[IRstuff]

8C is a pretty large climb; is that a steady-state? What in the environment is 22C; is it the air temperature, or source water temperature?

If the air temp is sufficiently cool, you could just run the water through a radiator.

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[MedicineEng]

If water is cheap and you are living in a dry temperate area, you can consider cooling your water using evaporative cooling. You might struggle to get 8C drop unless you have very dry/hot weather pattern but you might get close to it.

 

[LittleInch]

My understanding from this rather convoluted thread is that you are starting with a cool tank of water at say 15 or 20C, then by circulating this water round through your system at a rate of three tank fills per hour in ambient air that can reach 30C the water temperature increases and could damage the plants or reduce growth?

What you want to do is to keep the water at a lower temperature of (?? 20C?).

The thing that seems to be confusing people is this sentence "I want to calculate the overall temperature drop, to see how much energy do I need to invest to cool down the water in the tank"

I THINK you mean you want to calculate the temperature RISE of the water??

Why don't you just measure the temperature of the tank over time and then work out how much heat is being transferred to make it do this?

Bubbling air at 30C through water at 22 C isn't going to cool it down, nor is running it through a cooling tower.

If you want to cool something to below the ambient air temp then you need some sort of mechanical cooling or possibly use an evaporative cooler if the air humidity allows.

In a system like this ALL the pump shaft power will convert to heat, either as an initial jump in temperature at the pump or friction in the pipe. Then yes, your system will absorb heat from the air if that is hotter than your tank temperature. But you have so many variables that trying to calculate this is never going to be accurate.

Depending on how big this thing is why don't you just chuck some ice in every now and then?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.