Abstract

Indirect line heaters are used in gas production, storage, and transmission systems to boost gas temperature in order to facilitate transmission efficiency and reduce problems from water condensation and hydrates. Heat transfer efficiency in these systems is governed by conduction and convection forces, which are determined by the thermophysical properties of the fluids. An ethylene glycol/water mixture is the most common heat transfer fluid used, but it and propylene glycol suffer from less than optimal heat transfer efficiency. Glycols have low thermal conductivities and high viscosities, which degrade thermal performance, and they can have environmental and toxicity issues which limit their use in some applications.

A novel heat transfer fluid has been developed that offers both better heat transfer efficiency and a safer environmental profile than glycol systems. This fluid has individual thermophysical properties that are better than glycol systems, and may provide reduced fuel usage and higher throughput in line heaters. In addition, this new fluid is environmentally safe, being highly biodegradable if discharged, but having a low oxygen demand, reducing the chance of fish kill. The fluid is inhibited for prevention of corrosion, can be formulated for low freeze points, and is expected to be readily serviceable and stable for long life. This paper presents an analysis of the heat transfer process, a comparison of the novel fluid with glycols, and data from an initial pilot test of the system.

Introduction

Heat transfer has always been an integral part of natural gas production and transmission processes1. The compression of natural gas generates a tremendous amount of heat, which must sometimes be dissipated to protect the process. Conversely, the decompression for distribution requires a tremendous amount of heat to restore the gas temperature to initial levels. Heating minimizes the tendency of moisture to condense from the gas on cooling. Liquid moisture in the pipeline is undesirable not only because it can facilitate corrosion but also because it can freeze or form hydrates that cause blockages or resistance to flow. Operators of gas production, transmission or storage systems frequently use line heaters to increase the gas temperature to avoid these problems. A line heater assists in moving natural gas through successive pumping or compression stations by adding energy to it, reducing the density, and utilizing the tendency to expand to assist in the forwarding of the gas from station to station.

Line Heater Design.

In a line heater, an example of an indirect-fired heater, Figure 1, heat is added to a bath of heat transfer fluid. This heater contains a fire tube and a number of gas tubes, called the process coil. The fire tube is the heat source for the fluid bath, and the gas tubes are the heat sink. When the gas runs through the heated tubes, it will take on enough heat to avoid the problems associated with the cooling effect of the pressure drop.

Line Heater Design.

In a line heater, an example of an indirect-fired heater, Figure 1, heat is added to a bath of heat transfer fluid. This heater contains a fire tube and a number of gas tubes, called the process coil. The fire tube is the heat source for the fluid bath, and the gas tubes are the heat sink. When the gas runs through the heated tubes, it will take on enough heat to avoid the problems associated with the cooling effect of the pressure drop.

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