In order to choose the coil length and diameter, a temperature must first be chosen upstream of the choke; the higher Th the longer the coil L] and the shorter the coil L2. In Heat Transfer Theory we showed that the greater the LMTD between the gas and the bath temperature, the greater the heat transfer per unit area, that is, the greater the LMTD, the smaller the coil surface area needed for the same heat transfer. The bath temperature is constant, and the gas will be coldest downstream of the choke. Therefore, the shortest total coil length (Lt + L2) will occur when LI is as small as possible (that is, Tj is as low as possible).
Although the total coil length is always smaller when there is no upstream coil (L1 = 0), the temperature could be so low at the outlet of the choke under these conditions that hydrates will form quickly and will partially plug the choke. In addition, the steel temperature in the choke body may become so cold that special steels are required. Therefore, some guidelines are necessary to choose T1 for an economical design.
It is preferable to keep T2 above 50°F to minimize plugging and above -20°F to avoid more costly steel. With this in mind the following guidelines have proven useful. For a water bath temperature of 190°F:
1. Set T2 = 50°F. Solve for AT and calculate T1. If T, is greater than 130°F, L1 will become long. Consider going to the next step.
2. Set T1 = 130°F. Solve for T2. If T2 is less than -20°F special steel will be needed. Consider lengthening L1 instead and go to the next step.
3. Set T2 = -20°F. Increase T1 as needed.
4. If L1 becomes too long, consider using glycol/water mixture or another heat medium liquid and raise the bath temperature above 190°F.