As shown in Figure 5-2, the wellstream enters the first coil at its flowing tubing temperature and pressure. Alternatively, it could be choked at the wellhead to a lower pressure, as long as its temperature remains above hydrate temperature.
There is typically a high-pressure coil of length L1, which heats the wellstream to temperature, T1. The wellstream at this point is at the same pressure as the inlet pressure, that is P1 = Pin. The wellstream is choked and pressure drops to P2. When the pressure drops there is a cooling effect and the wellstream temperature decreases to T2. This temperature is usually below the hydrate temperature at P2. Hydrates begin to form, but are melted as the wellstream is heated in the lower pressure coil of length L2. This coil is long enough so that the outlet temperature is above the hydrate point at pressure, P2. Typically, a safety factor of 10°F higher than the hydrate temperature is used to set Tout.
In fire tube type heaters, the coils are immersed in a bath of water. The water is heated by a fire tube that is in the bath below the coils. That is, the fire tube provides a heat flux that heats the water bath. The water bath exchanges heat by convection and conduction to the process fluid. Instead of a fire tube, it is possible to use engine exhaust or electrical immersion heaters to heat the water bath. Fire tubes are by far the most common source of heat.
Since the bath fluid is normally water, it is desirable to limit the bath temperature to 190°F to 200°F to avoid evaporating the water. If higher bath temperatures are needed, glycol can be added to the water.
In order to adequately describe the size of a heater, the heat duty, the size of the fire tubes, the coil diameters and wall thicknesses, and the coil lengths must be specified. To determine the heat duty required, the maximum amounts of gas, water, and oil or condensate expected in the heater and the pressures and temperatures of the heater inlet and outlet must be known. Since the purpose of the heater is to prevent hydrates from forming downstream of the heater, the outlet temperature will depend on the hydrate formation temperature of the gas. The coil size of a heater depends on the volume of fluid flowing through the coil and the required heat duly.
Special operating conditions such as start up of a shut-in well must be considered in sizing the heater. The temperature and pressure conditions found in a shut-in well may require additional heater capacity over the steady state requirements. It may be necessary to temporarily install a heater until the flowing wellhead temperature increases as the hot reservoir fluids heat up the tubing, casing, and surrounding material.
It is perfectly acceptable for a line heater to have an L1 equal to 0. In this ease all the heat is added downstream of the choke. It is also possible to have L2 equal to 0 and do all the heating before the choke. Most frequently it is found that it is better to do some of the heating before the choke, take the pressure drop, and do the rest of the heating at the lower temperature that exists downstream of the choke.