Yejin Lee
72111587
Chemical and Process Engineering

Evaporator Experiment

University of Canterbury
Department of Chemical and Process Engineering

Summary
An analysis of the vertical tube type with fifty 1’’ diameter 16 swg tubes evaporator was carried out with the aim of determining the overall heat transfer coefficient of the system and to perform a heat balance over the evaporator. The experiment was carried out by measuring the mass flow rate of the inlet water and the condensate, temperature of the inlet water, condensate before it passes the steam trap and the surface area of the evaporator. The collected data were used to find the enthalpies of the steam and water at suitable conditions from the steam table. The values were used to calculate the heat flow rates and finally the overall heat transfer coefficient of the water. The value found for the heat transfer coefficient was 6400 ±400Wm­-2K-1. This value was concluded to be inaccurate as it was not within the range of the desired value of 7700 ±700Wm­-2K-1 with experimental uncertainty. The discrepancy percentage was 16.25%. The errors mainly came from the unsteady state of the system and inaccurately collected data.

Contents

1. INTRODUCTION

1.1 Aim
The aim of the experiment is to find out the overall heat coefficient of the evaporator. From the mass balance and energy balances, the heat coefficient is calculated. A sub aim is to develop skills in the collections, manipulation and calculating the variables from the collected data.

1.2 Apparatus
Evaporators are used to vaporize solvent from a solution to give a more concentrated solution. This equipment is usually used to do before crystallizing or drying the material, or reduce the volume of transported fluid. The evaporator in University of Canterbury is a vertical tube type with fifty 1’’ diameter 16 swg tubes and central downcomer of 154 mm inside diameter. The solution to be evaporated is fed into the bottom of the evaporator. The level of the solution is maintained manually by adjusting the valve that control the solution flow. The flow rate is measured by the rotameter. The heat source of this evaporator is steam from the building’s steam main. The heat source enter the shell side around the tube. The condensate after the steam transfer the heat drains out through a steam trap.

1.3 Theory
The steam from CAPE boiler transfers the heat to the water in the heat exchanger. The rate of the heat energy transferred by the steam is calculated by the equation: (1)
QE is heat flow used to heat and evaporate the feed water. U is overall heat transfer coefficient. A is heat transfer area and T is the temperature difference between condensing steam and boiling water.
This transferred heat energy can be described in the other form of equation: (2) mw is the mass flow rate of feed water. Hg,Tb is enthalpy of steam at the boiling point of water at tube side pressure and Hf,Tin is the enthalpy of water at feed temperature. The enthalpies of the water and steam can be found in a steam table.
This value is used to give the heat balance equation. The total heat flow that steam has is equal to the heat flow to the water to evaporate it and the amount of heat lost. (3)
QS is the heat flow given up by the steam and QL is the heat flow lost to the surrounding. If the system is at steady state which means all variables of the system remain constant, this heat flow equation must be balanced.
The QL can be calculated by the equation: (4)
H is the convection heat transfer coefficient. This value is 15Wm-2K-1 in this experiment. A is the inner surface area of the evaporator. is the temperature difference between the temperature of the evaporator surface and temperature of surroundings. The surface area of the evaporator can be calculated by using the sum of hemisphere equation and rectangular surface area equation: