Design Of Pipe Coils For Heat Transfer [1018

This procedure is specifically for helical pipe coils in vessels and tanks. Other designs are shown for illustrative purposes only. Helical coils are generally used where large areas for rapid heating or cooling are required. Heating coils are generally placed low in the tank; cooling coils are placed high or uniformly distributed through the vertical height. Here are some advantages of helical pipe coils.

1. Lower cost than a separate outside heat exchanger.

2. Higher pressures in coils.

.'3. Fluids circulate at higher velocities.

4. Higher heat transfer coefficients.

5. Conservation of plot space in contrast with a separate heat exchanger.

Manufacture

Helical pipe coils can be manufactured by various means:

1. Rolled as a single coil on pyramid (three-roll) rolling machine. This method is limited in the pitch that can be produced. Sizes to Sin. NPS have been accommodated, but 3 in. and less is typical. The coil is welded into a single length prior to rolling.

2. Rolled as pieces on a three-roll, pyramid rolling machine and then assembled with in-place butt welds. The welds are more difficult, and a trimming allowance must be left on each end to remove the straight section.

3. Coils can also be rolled on a steel cylinder that is used as a mandrel. The rolling is done with some type of turning device or lathe. The coil is welded into a single length prior to coiling. The pitch is marked on the cylinder to act as a guide for those doing the forming.

4. The most expensive method is to roll the pipe/tubing on a grooved mandrel. This is utilized for very small De-to-d ratios, usually followed by some form of heat treatment while still on the mandrel. Grooved mandrels create a very high-tolerance product and help to prevent flattening to some extent.

Coils are often rolled under hydro pressures as high as 85% of yield to prevent excessive ovalling of the pipe or tube. The accomplish this, the hydrotest pump is put on wheels and pulled along during the rolling process. End caps are welded on the pipe to maintain the pressure during rolling.

Stainless steel coils may require solution annealing after forming to prevent "springback" and alleviate high residual stresses. Solution heat treatment can be performed in a fixture or with the grooved mandrel to ensure dimensional stability.

Springback is an issue with all coils and is dependent on the type of material and geometry. This springback allowance is the responsibility of the shop doing the work. Some coils may need to be adjusted to the right diameter by subsequent rolling after the initial forming.

The straight length of pipe is "clogged" to the mandrel prior to the start of the rolling to hold the coil clown to the mandrel. Occasionally it may be welded rather than clogged.

Applications for grooved mandrel are very expensive due to the cost of the machining of the mandrel. Mandrels that are solution heat treated with the coil are typically good for only one or two heat treatments clue to the severe quench. Thus the cost of the mandrel must be included in the cost of the coil.

Design

There are two distinct aspects of the design of pipe coils for heat transfer. There is the thermal design and the physical design. The thermal design falls into three parts:

1. Determine the proper design basis.

2. Calculating the required heat load.

3. Computing the required coil area.

Physical design includes the following:

1. Selecting a pipe diameter.

2. Computing the length.

3. Determine the type of coil.

4. Location in the tank or vessel.

5. Detailed layout.

To determine the design basis, the following data must be determined:

1. Vessel/tank diameter.

2. Vessel/tank height.

3. Insulated or uninsulated.

4. Indoor or outdoor.

5. Open top or closed top.

6. Maximum depth of liquid.

7. Time required to heat/cool.

8. Agitated or nonagitated.

9. Type of operation.

The type of operation is characterized in the following cases:

1. Batch operation: heating.

2. Batch operation: cooling.

3. Continuous operation: heating.

4. Continuous operation: cooling.

Coils inside pressure vessels may be subjected to the internal pressure of the vessel acting as an external pressure on the coil. In addition, steam coils should be designed for full vacuum or the worst combination of external loads as well as the internal pressure condition. The coil must either be designed for the vessel hydrotest, externally, or be pressurized during the test to prevent collapse.

Pressure Drop

It is important that pressure drop be considered in designing a pipe coil. This will establish the practical limits on the length of pipe for any given pipe size. Large pressure drops may mean the coil is not capable of transmitting the required quantity of liquid at the available pressure. In addition, the fluid velocities inside the coil should be kept as high as possible to reduce film buildup.

There are no set rules or parameters for maximum allowable pressure drop. Rather, an acceptable pressure drop is related to the velocity required to effect the heat transfer. For liquids a minimum velocity of 1-3 feet per second should be considered. For gases "rho-V squared" should be maintained around 4000.

Pressure drop in helical coils is dependent on whether the flow is laminar or turbulent. Typically flows are laminar at low fluid velocities and turbulent at high fluid velocities. In curved pipes and coils a secondary circulation takes place called the "double eddy" or Dean Effect. While this circulation increases the friction loss, it also tends to stabilize laminar flow, thus increasing the "critical" Reynolds number.

In general, flows are laminar at Reynolds numbers less than 2000 and turbulent when Reynolds numbers are greater than 4000. At Reynolds numbers between 2000 and 4000, intermittent conditions exist that are called the critical zone.

For steam flow, the pressure drop will be high near the inlet and decrease approximately as the square of the velocity. From this relationship, combined with the effects of increased specific volume of the steam due to pressure drop, it can be shown that the average velocity of the steam in the coil is three-fourths of the maximum inlet velocity. For the purposes of calculating pressure drop, this ratio may be used to determine the average quantity of steam flowing within the coil.

Heat Transfer Coefficient, U

The heat transfer coefficient, U, is dependent on the following variables:

1. Thermal conductivity of metal, medium, and product.

2. Thickness of metal in pipe wall.

3. Fluid velocity.

4. Specific heat.

5. Density and viscosity.

6. Fouling factor (oxidation, scaling).

7. Temperature differences (driving force).

8. Trapped gases in liquid flow.

9. Type of flow regime (laminar versus turbulent, turbulent being better).

Notes

All of the following apply specifically to helical coils.

1. Overdesign rather than underdesign.

2. The recommended ratio of vessel diameter to pipe diameter should be about 30. However, it has been found that 2 in. pipe is an ideal size for many applications. Pipe sizes of 6 in. and 8 in. have been used.

3. Helical coils are concentric with the vessel axis.

4. Two or more coils may be used, with the recommended distance between the coils of two pipe diameters.

5. Seamless pipe is preferred. Schedule 80 pipe is preferred.

6. Limit maximum pitch to five pipe diameters, with 2 to 2I4 recommended. Physical limits should be set between 4 in. minimum and 24 in. maximum.

7. Centerline radius of bends should be 10 times the pipe diameter minimum. (1-in. pipe = 10-in. centerline radius).

8. It is recommended for bend ratios over 5% or fiber elongation greater than 40% that the coils be heat treated after forming. The bend ratio can be computed as follows:

100 tp R

9. Flattening due to forming should be limited to 10%. Some codes limit ovality to as little as 8%. Ovality may be computed as follows:

10. Wall thinning occurs any time a pipe is bent. The inside of the bend gets thicker and the outside ot the bend gets thinner. Typically this is not a problem because the outside of the bend that gets thinner will also experience a certain amount of work hardening that can make up for the loss of wall thickness. The tighter the bend, the greater the thinning. Anticipated wall thinning due to forming can be computed as follows:

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