Heat transfer coefficient of water and air with tank problem

In summary, the conversation revolves around finding ways to save heat from an uncovered steel tank filled with water at 50C. The evaporation rate and heat transfer coefficients for water and air are discussed, with suggestions to use the Churchill and Chu correlation for the outside tank walls and an enclosure natural circulation heat transfer coefficient for the interior of the tank. The use of reference lengths and the Grashof number are also mentioned.
  • #1
Unibond81
2
0
Hi all :)

I'm trying to figure out how much heat I can save from covering a tank which is open (uncovered). The tank is rectangular made of steel and is filled with water, heated to 50C, and is not insulated with any material. The outside temp and humidity is assumed to be 25C and 60% respectively with a very slight breeze.

I've managed to figure out the evaporation rate of the water using q=(25-19v)*A*(xs-x)/3600 where v=velocity, A is the surface area of the exposed water, xs =humidity ratio in saturated air at the same temperature as the water surface, x=humidity ratio in the air. and finally Q=2270*q where 2270 is the evaporation heat of water.

What I cannot figure out are the heat transfer coefficients for water and air to calculate U in the equation Q=U*A*deltaT. I know 1/U=1/h(water)+deltax/k(steel)+1/h(air).

In the class room you would usually be given these values but what does one do for real situations? Do I need to calculate h form the Nusselt number (only option I can think of)? and if so what equation of Nu do I use for the water in the tank and the air as well as the reference lengths to calculate the Grashof number?

Thanks in advance for any help

P.S. For completions sake I've calculated the radiation even though this can be neglected
 
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  • #2
For the outside tank walls, h(air), apply a natural circulation convection heat transfer coefficient for a vertical plate. The Churchill and Chu correlation is a good choice. Any good heat transfer textbook or handbook will have the details.

For the interior of the tank, h(water), you could apply an enclosure natural circulation heat transfer coefficient. See "Convection Heat Transfer" by Bejan or similar reference.
 
  • #3
Thx a lot edgepflow, thought of using the churchill and chu correlation for air but wasn't sure, guess should be more confident in myself. As for water hadn't read of enclosure natural heat transfer so a BIG thanks for that.
 

What is the heat transfer coefficient and why is it important?

The heat transfer coefficient is a measure of how easily heat can be transferred between two substances. It is important because it can help us understand how efficiently heat is being transferred and how much energy is required for heating or cooling processes.

How does the heat transfer coefficient differ between water and air?

The heat transfer coefficient for water is generally higher than that of air due to its higher density and specific heat capacity. This means that water can absorb and transfer more heat energy compared to air.

What factors can affect the heat transfer coefficient of water and air in a tank problem?

The heat transfer coefficient can be affected by several factors, including the temperature difference between the two substances, the surface area of contact, the type of material the tank is made of, and the flow rate of the substances within the tank.

How can the heat transfer coefficient be calculated for a water and air tank problem?

The heat transfer coefficient can be calculated using the heat transfer equation, which takes into account the temperature difference, surface area, and material properties. It can also be determined experimentally by measuring the heat transfer rate and temperature difference.

Why is the heat transfer coefficient important in designing efficient heating and cooling systems?

The heat transfer coefficient plays a crucial role in designing efficient heating and cooling systems because it determines how much energy is required to transfer heat between substances. By understanding and optimizing the heat transfer coefficient, we can create more efficient systems that use less energy and reduce costs.

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