Transient thermal performance of ice slurries being pumped through pipes

The paper examines the transient melting rates of ice slurries being pumped through pipes whose walls are initially warmer than the freezing temperature of the slurry. The application for this work is the new innovative pipe cleaning method known as ice pigging. Ice pigging consists of introducing a length (typically much greater than 50 diameter lengths) of ice slurry into a pipe. The ice slurry displaces the existing fluid and forms a semi-solid plug whose temperature is at the freezing temperature of ice at the prevailing operating contentions of pressure and freezing point depressant concentrations. The plug cools the pipe walls, which in turn results in some phase change within the slurry. The plug is then propelled along the pipe by a pressurised fluid introduced behind the plug. As the plug moves along the pipe, its front continually sees ambient temperature pipe wall, which it cools. The ice slurry half way along the plug also sees pipe wall, but this part of the wall, having been chilled by the front end of the plug, is much cooler and so is less able to supply heat to the ice slurry. A fixed position along the pipe wall sees a large temperature change when the plug first passes it, the surface of the pipe wall rapidly cool, however as its temperature decreases the rate of cooling decreases, even though the temperature of the passing ice slurry remains constant at its freezing temperature. Once the ice slurry plug has passed the fixed wall position, the wall is exposed to the relatively warm propelling fluid, it will be heated and eventaully attain the temperature of this fluid. The paper adopts the classical transient conduction of Carslaw and Jaeger to estimate the energy removed from the pipe wall as a result of the passing ice pig. This energy estimate is used to develop equations predicting the amount of phase change occurring in the pig and hence estimate the distance it can travel before it has melted. These equations are solved analytically and numerically to obtain a predictive model which can be used to estimate the amount of ice slurry required to viably pig given pipe lengths.