Pneumatic System By Rs Mujumdar Pdf Book
Kenneth Calimlim
Maize is a staple food and source of income in Kenya. However, postharvest losses are estimated at 12% to 20% of the national total output primarily due to high moisture storage. Drying to safe level of 13.5% before storage is essential. One of the main factors which influence drying process is initial moisture content (MC) of the grain. Therefore, this paper presents the effect of initial MC of maize grain on moisture removal rate (MRR) and energy used in drying. The experiments were based on selected initial MC levels of 20%, 25% and 30%, wet basis (wb). The first experiment involved loading experimental vertical pneumatic dryer with 70.0 kg of wet maize grain with initial MC of 20%. The grain was then dried for 2 hours as MRR and energy used monitored at an interval of 15 minutes. The grain and drying air mass flow rate was controlled at 771 kg/h and 547 kg/h, respectively. The plenum chamber air temperature was maintained at 70C using proportional integral derivative controller. The maize grain variety used was hybrid 614 sourced from a farmer in Njoro sub-County, Nakuru County, Kenya. Similar experiments were repeated but using maize grain with initial MC of 25% and 30%, wb. The MC of 20%, 25% and 30% were obtained by rewetting maize grain with initial MC of 11.4% (wb) in tap water at a temperature of 18C for 0.75 hours, 1.75 hours and 5.75 hours, respectively. The MRR results ranged from 0.0914 kg/kg.h to 0.0357 kg/kg.h for maize grain with initial MC of 20%, 0.1043 kg/kg.h to 0.0556 kg/kg.h for 25% and 0.1185 kg/kg.h to 0.0705 kg/kg.h for 30%. The energy used for air heating (Ea) for each level of MC was 10.5 kWh. The energy used for grain transportation (Eg) was 4.6 kWh for MC of 20%, 4.8 kWh for 25% and 5.0 kWh for 30%. Data analysis results showed that the initial MC of the maize grain had significant effect (P < 0.05) on MRR. However, the effect of initial MC on Ea and Eg was not significant (P > 0.05).
A typical DTSFB system consists of four interconnected zones, as shown in Figure 7.1: a spout-fluid bed feeder, a moving bed annulus, a draft tube (pneumatic conveyor), and a freeboard fountain. For a given geometry and particles, five control quantities determine the bed hydrodynamics: the inlet jet fluid mass flowrate, Fj0; the inlet auxiliary fluid mass flowrate, Fax0; the internal annulus fluid mass flowrate, Fa; the inlet section length, Li; and the annulus height, Ha. The draft tube inlet length, Li, controls both the fluid leakage and solids crossflow. The auxiliary fluid mass flow, Fax0, controls the pressure drop across the bed and alters the internal solids flow. To describe the overall and local dynamics of such systems, appropriate fluid and particle models must be devised for each separate region of the DTSFB. The discussion in this chapter addresses operating characteristics, applications, and design concepts. Initially we focus on the basic hydrodynamic features of these systems, and then we provide an overview of applications involving the basic DTSFB and novel hybrid configurations.