Coil and jacketed tanks keep the fluid inside them hot or cold: edible oil, chocolate, milk, chemicals, bitumen and heavy fuel. The heat transfer area is calculated from the amount of heat to be transferred, and the type of coil or jacket is chosen by the fluid, pressure and cleaning needs.
The heat the coil must transfer comes from three figures: the fluid's mass, its heat capacity and the difference between the initial and final temperatures. If this heat must be transferred in a given time, for example heating oil from 20 to 60 degrees in two hours, the required heat duty follows.
The coil area equals the heat duty divided by the product of the heat transfer coefficient and the mean temperature difference between the two fluids. The heat transfer coefficient depends on the type of fluid, the flow velocity and whether there is an agitator. A viscous fluid without an agitator has a very low coefficient and needs a large coil.
The basic relations for calculating a coil:
Adding an agitator is usually cheaper than enlarging the coil, because it multiplies the heat transfer coefficient.
| Feature | Internal coil | Full jacket | Half-pipe jacket |
|---|---|---|---|
| Heat transfer area | High | Medium | Good |
| Internal cleaning | Harder | Easy | Easy |
| Heating fluid pressure | Medium to high | Limited | High |
| Common use | Bitumen, chemicals | Dairy, food | Steam and thermal oil |
In the food and pharmaceutical industries, anywhere the fluid can linger is a place for bacteria to grow. An external jacket keeps the inside of the tank smooth and makes clean-in-place (CIP) possible. The internal welds are smooth and polished, and the stainless steel is passivated after fabrication.
For these uses, a stainless 304 or 316 tank with a jacket or half-pipe, and a hygienic agitator where needed, is the common choice.
From calculating the coil area to separate testing of the coil and tank
Many fluids can only be stored or processed at a specific temperature. Edible oil and chocolate harden in the cold, milk must stay cool, and bitumen and fuel oil cannot be pumped without heating. A coil tank controls the fluid's temperature by passing hot water, steam, thermal oil or chilled water through a coil or jacket.
The coil size is not a guess. The amount of heat to be transferred in a given time, the temperature difference between the two fluids and the heat transfer coefficient set the area needed. In viscous fluids, without an agitator the heat only reaches the layer near the wall, which is why coil tanks often have agitators too. The body of these tanks is usually a stainless 304 or 316 tank, and the coil and nozzle welds are checked by liquid penetrant testing. If the fluid is viscous, a mixer tank with a coil or jacket is the better choice.
Each method has its own advantages and limits
A spiral pipe inside the tank, immersed directly in the fluid. It has the largest area and the fastest heat transfer, but makes cleaning the inside of the tank harder.
A second shell around the body, with the heating fluid flowing in the gap between the two shells. The tank's internal surface stays completely smooth and is easy to clean.
A pipe split lengthwise and welded in a spiral around the body. It has faster flow and better heat transfer than a plain jacket and withstands higher pressure.
The main design parameters of a coil tank
From heat load to the final test
The coil or jacket usually has a higher pressure than the tank itself, especially when it works with steam. That is why each is designed and tested separately. In food tanks, weld quality and internal surface finish are as important as the thermal design.
Wherever the fluid temperature must be controlled
Tell us the fluid, required temperature, heating time and volume to receive the coil type and a tank price.
The most common questions about coil and jacketed tanks
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