Numerical study of heat pump contact drying

Abstract

Drying is one of the most energy intensive industrial operations, and it is well established that heat pump driers (HPDs), by recycling waste heat, may provide significantly higher drying energy efficiencies and lower net greenhouse gas emissions than conventional driers. In addition, however, in the design of the HPDs themselves, there remains significant further scope for energy-efficiency improvements. Second-law analyses of HPDs have shown previously that losses associated with the convective transfer of heat to the drying process are a significant limiting factor for energy efficiency. This thesis uses numerical simulation to explore the possibility of improving on the energy performance of HPD systems by employing conductive heat transfer from the refrigerant condenser, through a heating plate and through the product itself, to drive the drying process in an "isothermal contact" HPD (ICHPD). The duct model that is developed combines a detailed air-flow model, which solves the mass, momentum and energy balances within the drier ducts, with a detailed internal drying process model, incorporating a description of the transport phenomena occurring within the porous product medium. The whole-system dynamical HPD model, which results when the drier-duct model is integrated with a pre-existing heat pump model, is capable of describing the evolution of non-steady batch drying.

It is established that for applicable products the ICHPD configuration may increase the energy efficiency of heat pump drying by as much as a factor of three compared with conventional adiabatic HPDs. This ICHPD energy efficiency gain (relative to the adiabatic mode) is, however, demonstrated to be highly sensitive to the product thickness (d). The energy efficiency gain of ICHPD is also shown to be sensitive to any constraint on the temperature and the maximum allowable relative humidity above the product. Isothermal HPD is thus likely to be most applicable in the drying of those products, such as sludges and pastes, that can be spread into thin layers, in particular those that also are least vulnerable to quality deterioration at high temperature and humidity. Product throughput is shown to be simultaneously maximised at low d, implying that ICHPD provides an opportunity to avoid the adiabatic mode’s trade-off between drying rate and energy efficiency, by using a thin product layer. A case-study is presented of the economics of ICHPD in an industrial sludge-drying application, showing that isothermal HPD provides an opportunity to lessen exposure to risk associated with electricity-price uncertainty. System performance is found to be quite sensitive to variation in the surface area available for drying and also to the dimensioning of the evaporator. A second-law analysis of the whole system is employed to examine the reasons underlying the energy performance gain associated with the isothermal mode; ICHPD is found to reduce irreversibility equally within the refrigerant cycle and in heat transfer from the condenser to the product plus the drying process itself – a demonstration of the synergy enabled by the ICHPD design.