Heat recovery in a drying process

Abstract

This project has the aim of find different designs to optimize the energy usage in a drying process located in Hofors. The company is called Sorption AB and produces pellets using the waste material of the paper factories situated around it. The idea is to introduce some technology innovations to recover the heat contained in the outgoing air of the process. The implementation of these new ideas implies an initial investment and a consequent decrease in the energy usage. The best option will be chosen after an energy and economic study of each design. In order to have precise results, all the calculations will be done with the software Engineering Equation Solver (EES), which has an internal data base with the properties of different working fluids. In the first part of this project a brief introduction of the actual situation and the different possibilities to decrease the energy usage is done. The waste heat recovery is also explained in this section in order to introduce the reader in this topic. When a company find its optimum production range, it should start to think how reduce the energy usage to improve the efficiency of the company. The objectives and the limitations of this study are also included in the introduction part. Once the problem is explained, the necessary theory to understand the project is exposed. The heat pump is introduced as the most important improve of the system. The idea of the heat pump is introduced and all the possible modifications are explained. The working fluid (refrigerant) and the different components needed to build up a heat pump are also defined. Each working fluid has its own properties and each component has its own function in the system, so it is necessary to understand these facts properly before being introduced in all the calculations. Another alternative explained in the theory part is the air recirculation. It consists, as its name indicates, in the introduction of part of the outgoing air in the system instead of using the actual heat exchanger. This alternative is studied also in the calculations but finally is rejected for the problems that it carries. The method to find a proper solution is exposed in the third part. There is also explained the simplifications that have to be assumed to get a result as similar as possible to a real case. The most important simplifications are: don’t consider the pressure drop in thepipes, don’t consider the heat loses through the pipes and consider constant enthalpy through the expansion valve. The results are divided in two main parts: the reductions on the energy usage and the economic study of each design. The bigger the reduction on the energy usage, the higher the economic saves. This fact is important, but the economic study is as important as this one. The economic study indicates the risk of the investment. For the different alternatives, the reduction of the annual energy usage goes from 185713 kWh in the case where the actual heat exchanger is changed to 352101kWh in the most complex alternative (optimized heat pump with a new heat exchanger). In economic terms, it can be translated as savings from 114436,35 to 216964,64 SEK per year in the electricity bill. In the other hand, the economic study of the payback time gives values from 1,86 to 3 years. The results obtained show that it is not that easy to recover the wasted heat for different reasons as the risk of the inversion, the size of the new designs, etc. The engineer only makes the study and presents the different alternatives, but the final decision is going to be taken by the company who invest the money. They should evaluate the risk of the investment and if they have the capacity to build up the different options.