Heat Transfer Process in Vessel

[hamidun]

Hi all,

Let's get straight to the point. I need help regarding heat transfer in vessel. I have a vessel exposed to the surrounding. The liquid temperature (the liquid comes from pipe) in the vessel is lower than the ambient/surrounding temperature. I have been searching, but I couldn't find any example of heat gain by ambient/surrounding temperature on the vessel, there is only always heat loss against ambient temperature.

1. My thought is, based on heat transfer fundamental, temperature will go from high to lower temperature. So, in my case, the temperature vessel should increase, am I right?

2. Maybe, I'm mistaken about surrounding and ambient, is there any difference of them? What is sky temperature? Because I also think if this case involving solar radiation.

3. I have been thinking, this case is just only natural convection, but then my surface temperature is lower than ambient temperature whereas usual in natural convection case the ambient temperature is always lower than ambient temperature

4. The last thought that I have is, is this forced convection by considering wind speed of the surrounding?

Please help me figuring out this confusion. I need to know heat transfer process occur in this case.

Thanks for any help!!

[JAlberts]

I am not aware of any examples of the caculation you are seeking but offer the below answers to your basic questions.

1. Yes, actually the heat transfer will be both ways in the region of your tank your tank will be heated and it surrounding air will be cooled. If your tank were to be confined to a very small confined and insulated region then this process would continue until both the tank and the surrounding air achieved the same temperature. Under open region conditions, the (downward) convection of the tank's surrounding air will raise the temperature of the tank to the ambient temperature of general surrounding region.

2. First, an example of the difference between "surrounding" and "ambient" temperature is that the air in the immeadiate region surrounding your tank will be lowered by contact with the cooler tank wall and therefore cooler than the "ambient" temperature of the general region surrounding the tank. Second, when exposed to the sun, the tank will experience radient heating independent of the convective air heating.

3. Convection can be either upward or downward although the term is not generally thought of in that manner. Adiabatic winds in mountainous regions whereby air is cooled by snow at the high regions of the mountains flows down the mountian sides are an example.

4. If there is any significant forced (wind) flow around the tank then it will increase the heat transfer rate.

In all cases the rate of transfer is going to be a combined effect of tank vs. air temperature differential, combined air flow and radiant heating effects: plus, in a large tank, the convective and conductive heat flow of the tank liquid. The tank shape and orientation are also factors. In other words, you are dealing with a very complex combination of factors that to get a truly accurate solution requires a computer heat flow program analysis.

[hamidun]

Hi JAlberts,

Thanks for the response. I really appreciate it.

1. Yes, actually the heat transfer will be both ways in the region of your tank your tank will be heated and it surrounding air will be cooled

Does it mean after the radiation heat the tank, at the same time (or after quite few time) the cooled surrounding temperature will cooler back the tank?

2. Second, when exposed to the sun, the tank will experience radient heating independent of the convective air heating.

So, can I conclude that my case is exactly radiation by the solar?

3. Convection can be either upward or downward although the term is not generally thought of in that manner.

This is what I have been thinking as well, in the most cases the surface temperature usually lower than the ambient temperature and we get positive value, but when we swap the value of the temperature, we will definetely get negative value. If we get negative value, can we interpret that the heat transfer is just the other way round?

4. If there is any significant forced (wind) flow around the tank then it will increase the heat transfer rate.

With additional wind speed, so my case would be combination of radiation and forced convection? Is there any minimum wind speed that can be neglected, I mean what is the minimum value of wind speed that must be taken into account which would be significant for the heat transfer calculation.

This would be my additional question, if we have a loop system with surface temperature is lower than ambient temperature, does it mean that the system would experience temperature rise continuously? Or we need also to consider as well the amount of heat that can be released by the system, so the temperature must not rise continuously.

Another hopefully last question, where can I get convective heat transfer coefficient value? Do you know any reliable website or reference for it?

Thanks and sorry for the trouble again.

[JAlberts]

hamidun:

In response to your last post:

1. If a metal tank is exposed to direct sunlight it is most likely that the sun's radient heat will raise the tank's temperature to a level that is higher than the temperature of the surrounding air; which, at that point the surrounding air will start cooling the tank either by natural convection or by forced flow (wind) cooling.

2. I am not sure I understand your question; but, if your tank is exposed to the sun it will experience radiant heating.

3. If I understand your comment, you are correct. Heat flow will always be from high to low regardless of other factors.

4.Since convection flow over a short vertical surface is generally at a very low velocity, any amount of wind will have an effect.

5. The rate of heat transfer will reduce as the tank and air approach the same temperature but, as long as the temperature of the air surrounding the tank is different from the temperature of the tank there will continue to be heat flow. The heat exchange between the tank and its surrounding air will only cease when the tank and air reach exactly the same temperature.

To clarify one point, any heated surface (other than an insulated one) is capable of radiant heat radiation. If the tank temperature is increased above the temperature of the surrounding air the surface of the tank will begin to exhibit both conductive and radiant heating of the air next to its surface.

Finally, any college level text book on heat transfer will provide the principals of conductive and convection heat flow; but, a rigorous solution of your problem must, in additon to the above factors we have discussed, take into consideration the heat conduction rate and convection effects of the fluid within the tank and the conduction rate of the the tank wall including the conduction rate of any coating(s) used on the internal and/or external surfaces of the tank. In a practical sense, before attempting any even basic analysis consider exactly how important the temperature of the fluid in the tank is to your application. In some cases, for example storage tanks, as long as the temperature of the fluid in a tank is not sufficient to cause vaporization of the contained fluid or is not likely to effect the downstream processes of the system then a heat transfer analysis is not really required; or, it simply easier to shade and/or enclose the tank ot reduce the heating factors. Of all of the elements we have discussed, the most likely significant one for outside storage of products is, depending upon your geographical location, the radiant sun heating of a metal tank, in which case the best solution is simply to shade the tank from direct sun exposure.

[hamidun]

JAlberts,

Thanks for your explanation. But, I am still a bit confused.

1. I don't understand how actually this happens, because I think that if the tank temperature is raised by the surrounding, the surrounding air should've already been hot, am I right?

Wow, this is really confusing.

Yes, you're right, actually at the first stage I just consider natural convection and conduction, then after I realize, I need to consider also radiation, convection by wind force etc.

One additional question, can you help me differentiate between surrounding, ambient, surface temperature?

Thanks again.

[Kelly_Bramble]

Don't know if these webpages will help the op but here they are. The heat energy is flowing in the same direction being discussed, different however the application and concepts are similar.

Heat Loss from Piping Chart

Heat Loss from Pipe Outdoors

Heat Loss Insulated Pipe Equation and Calculator

Heat Losses From Bare and Insulated Pipe

[hamidun]

Thanks Kelly,

I think http://www.engineersedge.com/heat_tr...ator_12921.htm this link has exactly the same for the piping calculation, but I have a problem to get surface temperature of the pipe. Can I just use conduction in the pipe?

[jackieann000]

This is a great information about the heat treatment process. Very clear and sophisticated ideas are given here about the heat treatment and techniques.