8 replies to this topic

I am designing a cooling system for product storage tank (chemical) where the product need to be maintained at about 20 degree C to prevent degradation. The atmospheric temperature of plant site will generally range from 25-35 degree C. The tank will be insulated. The dimension of the tank is 15m diameter and 16m height with a capacity of 2500m3.

Current proposal (Option 1 in the attached schematic drawing) is to circulate the product to a heat exchanger, whereby cooling water from chiller unit will act as the coolant.

However, is it possible to apply the concept as heating coil, where the coolant from chiller unit will be circulated directly into the tank (Option 2 in the attached schematic drawing). As such, I might be able to save extra cost for the chiller unit and additional product circulation pump.

Can anyone help to comment if this is workable?

Thank you.  20120131141227046.pdf   140.83KB   166 downloads

Edited by 88blue, 31 January 2012 - 01:20 AM.

[S.AHMAD]

1. Tecnically, both options are ok.
2. Should decide base on cost effectiveness.
3. Option 2, need tank mixer.for more effective cooling
4. Cooling surface required for option 2 will be much bigger since the U is smaller than that of option 1.
5. Coil in tank reduces storage capacity especially if the surface area of coil is big
6. Option 1 maintenace of the cooler is easier. Do not require tank out of service. Repair for tube leaks, for example.
7. You need to find more tangible and intangible benefits for each option. The above are some foods for thougth.

Edited by S.AHMAD, 31 January 2012 - 03:02 AM.

[Dacs]

I noticed you provided a dedicated pump for this purpose.

For value engineering, why don't you just utilize the existing product pump (which is not shown, but I assume exists), and get a slipstream from the discharge and utilize it for the proposed cooler?

[Art Montemayor]

88Blue:

The answer to your basic question is: yes, you can apply an internal cooling coil, where the cooling depends on induced convection currents created within the tank’s fluid.

However, I would not apply an internal, convection coil in this application because:

• This is a relatively large tank and will require a large internal coil. This coil will not be available for inspection and routine maintenance unless the tank is completely emptied, cleaned, and subjected to confined space entry. I will never subject my personnel to confined space entries unless there is absolutely no options. I consider this a serious safety issue.
• Any internal coil is subject to potential refrigerant leaks (and subsequent product contamination) – basically because of the lack of available routine maintenance and inspections. Although it can be argued that an external heat exchanger s also subject to leaks, it is always available to routine maintenance and inspections – without shutting down production storage and forcing confined space entry.
• What Dacs has pointed out is exactly true. It is practical and common to employ the storage product pump-out pump as the means to circulate the product around the external heat cooler and the tank. This is a net savings in capital cost and maintains a pump in steady, running condition – a net advantage for a pump that normally sits idle for about 90 -95% of the time.
• The external cooler area will be drastically much smaller than the required internal convection coil. This is so because you can employ forced convection, high Reynolds Number, controlled flow rates, and a resulting high heat transfer coefficient.
• Comparing both systems conceptually (and based on personal field experience in the same application) I can state that the external cooler system will be much more economically and operationally attractive. The only differences between the two will be that the cooler will be smaller than the internal coil and that you will need a special, refrigerant circulating pump for the internal coil – all net positive advantages for the external cooler. I consider this comparison a “no brainer”.

I hope this experiece is of help to you.

Thank you all for the useful reply.

I do understand that the internal cooling coil system poses some disadvantages. However I do realize that it is quite common for heating system. (correct me if I am wrong as I am new to this industry ), the same possible disadvantages (leakage, maintenance, inspection) will happen to the heating system, why it is still being utilized?

[S.AHMAD]

88blue
1. You are comparing an apple with an orange. Nevertheless, your lateral thinking is appreciated.
2. The right things to do is that you do preliminay economics for the two options
3. The heat transfer coefficient for condensing steam is fairly constant and independent of tube diameter. Most of all it is very much high as compared to the cooling water in the tube. Therefore, overall heat transfrer coefficient is higher as compared to cooling water.
4. Temperature of steam utilized is also high this gives higher LMTD as compared to cooling water
5. Higher overall heat transfer coefficient coupled with higher LMTD gives smaller surface area for steam system
6. Therefore, you should not compare steam heating with cooling water in coil.
7. It will be interesting to do the economics and appreciate very much if you could share the outcome with members.

[Dacs]

You have a point there. I presume that a condensing steam with a higher temperature delta would just entail a (much) smaller coil to achieve a similar duty as compared with a chiller that only does sensible heat change working on a smaller temperature delta.

By that alone, usage of internal heating coil will be more justified since it's smaller and I would presume easier to maintain.

[Art Montemayor]

88Blue:

You started this thread with the statement: "I am designing a cooling system for product storage tank (chemical) where the product need to be maintained at about 20 degree C to prevent degradation."

Why are you now changing scope and direction and arguing about internal steam HEATING coils? This makes no sense to your query and wastes a lot of time and effort to try to help you by addressing your basic query.

If you really need to find out why some people employ an internal steam heating coil, the answers I have found in the field after 52 years are:

• People are ignorant of what they have had installed by others who were motivated by low capital investment and a quick profit for construction;
• Others have been saddled with the installation and have no reources (or engineering) to change to a more efficient and economical method;
• There is no basic need to economize or to be efficient because the scope is unlike yours - a sensitive product that must be kept in a narrow temperature range.
• Others are just either lazy or totally ignorant of the potential savings in energy and effficiency and simply don't do anything.

Repeating my prior post: relying on simple and inefficient, natural convection currents inside a stationary, un-agitated storage tank for efficient and accurate temperature control is simply dumb engineering - devoid of the basic understandings of heat transfer and control.

[DB Shah]

Dear 88blue
internal heating is employed in number of places, instead of giving you direct answer let me put some points for further thinking. View the installation of internal coils considering points like- pour point, viscosity of stored material.