Experimental Study On Flow Resistance Of The Horizontal Tube Bundle With Falling Film Flow

Low temperature multi-effect distillation is one of the primary desalination methods because it has small temperature differences, good water quality and can utilize low-grade waste heat. Horizontal tube falling film evaporation is widely used in LT-MED desalination plant because it has high heat transfer coefficient and is fit for small temperature difference evaporation.This thesis is about flow resistance of tube bundle, which is based on the falling film evaporator used in the large-scale desalination. The experimental system for flow resistance is established, which contains a1500mm length test part. Outer diameter of HAL77-2A aluminium brass tubes is25.4mm, length of tubes is500mm and tubes are arranged by regular triangle. During experiment, steam flow resistance through a plain tube bundle is measured, then steam flow resistance with falling film is measured. Both the steam and the sprayed water are in saturation conditions at the same temperature. By changing experimental conditions such as steam temperature, spray density on topmost row tube and steam velocity, etc., parameters such as temperature, pressure and pressure drop are obtained and then come out the variation laws of steam temperature and pressure at the entrance and exit to tube bundle as well as effects of steam temperature, spray density on topmost row tube, steam velocity and tube column number on the steam flow resistance through horizontal tube bundle.Experimental results indicate that, when steam temperature is in the range of50℃-70℃, spray density on topmost row tube is in the range of0-0.08kg/(m-s) and steam velocity is in the range of1-13m/s, the steam temperature and pressure at the exit to tube bundle are lower than that at the entrance because the influence of flow resistance. Steam temperature, spray density and steam flow velocity have great influence on flow resistance. The steam flow resistance through horizontal tube bundle increases with the steam temperature, spray density on topmost row tube and steam velocity. In the entrance and exit section of tube bundle, resistance increases rapidly with the column number because the influence of local resistance; in the middle section, steam flow steadily and resistance increases linearly with column number. The steam flow resistance with falling film is quite different from that through a plain tube bundle, especially in high spray density and steam velocity, the steam flow resistance with falling film is far greater than that through a plain tube bundle.