Robert W. Besant

Energy wheel effectiveness: part I—development of dimensionless groups

Abstract The fundamental dimensionless groups for air-to-air energy wheels that transfer both sensible heat and water vapor are derived from the governing non-linear and coupled heat and moisture transfer equations. These dimensionless groups for heat and moisture transfer are found to be functions of the operating temperature and humidity of the energy wheel. Unlike heat exchangers that transfer only sensible heat, the effectiveness of energy wheels is a function of the operating temperature and humidity as has been observed by several energy wheel manufacturers and researchers. The physical meaning of the dimensionless groups and the importance of the operating condition factor (H∗) are explained. The dimensionless groups are used in Part II to develop effectiveness correlations for energy wheels.

A mathematical model for predicting the densification and growth of frost on a flat plate

Abstract Frost deposition on a cold surface exposed to a warm moist air flow is simulated using a one-dimensional, transient formulation based on the local volume averaging technique. The spatial distribution of the temperature, ice-phase volume fraction (related to frost density), and rate of phase change within the frost layer are predicted. The time variation of the average frost density, frost thickness and heal flux at the cold surface shows a good agreement with the experimental data some distance downstream of the leading edge of a cold flat plate, providing that the proper transport properties are used. The results indicate that the local effective vapor mass diffusivity is up to seven times larger than the molecular diffusivity of water vapor in air as expressed by Ficks first law for frost temperatures between 264 and 272 K. This result is comparable with data measured for water vapor diffusion in snow.

Energy wheel effectiveness: part II—correlations

Effectiveness correlations are presented which allow the designer to predict the sensible, latent and total effectiveness of energy wheels when the operating conditions are known. The correlations agree with simulation data within ±2.5% for sensible, latent and total effectiveness when the desiccant coating on the energy wheel has a linear sorption curve. The sensitivity of the sorption curve and operating condition factor are studied and the use of the design correlations is shown with an example.

Heat and Moisture Transfer in Desiccant Coated Rotary Energy Exchangers: Part I. Numerical Model

A numerical model for coupled heat and moisture transfer in rotary energy exchangers is developed. The numerical model is one dimensional, transient, and is formulated using the finite volume method with an implicit time discretization. The model is developed from physical principles with a limited number of simplifying assumptions. This enables the study of several assumptions and their effect on the predicted performance of regenerative energy exchangers. In particular, the diffusion of the energy of phase change is treated in a unique manner which has a significant effect on the performance of rotary energy exchangers with thin desiccant coatings, as shown in Part II of this paper.

HVAC Duct System Design Using Genetic Algorithms

A genetic algorithm technique is used to design an HVAC air duct system with minimum life-cycle cost. The approach has the capability to incorporate standard (discrete) duct sizes, variable time-of-day operating conditions and variable time-of-day utility rates. An example is used to illustrate these capabilities and results are compared to those obtained using weighted average flow rates and utility rates to show the life-cycle cost savings possible using this genetic algorithm methodology. Life-cycle cost savings are minimal for some designs, but much larger savings are possible for complex designs and operating constraints.

Investigation of some large building energy conservation opportunities using the doe-2 model

Owing to their complexity, the design and efficient operation of HVAC systems in commercial buildings can be greatly enhanced by using accurate thermal simulation models. The DOE-2 model was validated using monitored data for a large (28,000 m2) commercial building. The model provided a means to evaluate some conservation measures which included window glazings, occupancy sensors, cold deck temperature set point and reduced ventilation air. Reductions in cold deck temperature and ventilation air were attractive as no-cost options which could be easily returned to original settings if problems arose. The potential energy savings for occupancy sensors was substantial and could prove economical.

Fan supplied heat exchanger fin performance under frosting conditions

In this paper, a validated numerical model for frost growth on heat exchange fins is modified to simulate a fan-supplied finned heat exchanger under refrigeration frosting conditions. It is found that frost growth on refrigeration heat exchangers causes a dramatic drop in the fin heat rate, airflow rate, and fin efficiency while the pressure drop increases. A sensitivity study shows the effects of changing several design parameters including the type of fan.

Performance of a Run-Around System for HVAC Heat and Moisture Transfer Applications Using Cross-Flow Plate Exchangers Coupled with Aqueous Lithium Bromide

A two-dimensional steady-state mathematical model is developed to study the heat and water vapor transport in a run-around heat and moisture exchanger coupled with a lithium bromide solution for air-to-air exchanger applications. A finite difference method is employed to solve the governing equations of the heat and moisture exchange, which gives the outlet air properties and effectiveness for selected operating conditions for each cross-flow exchanger. Using algorithms for the HVAC supply and exhaust exchangers coupled with a run-around liquid loop, the overall effectiveness of the run-around energy recovery system is shown to be dependent on the flow rate of both the pumped fluid and each airflow, the size and design of each exchanger, and the inlet operating conditions. It is shown that an overall effectiveness of 70% can be achieved when the run-around exchanger sizes and operating conditions are correctly chosen.

Heat and Moisture Transfer in Energy Wheels During Sorption, Condensation, and Frosting Conditions

A numerical model for coupled heat and moisture transfer with sorption, condensation, and frosting in rotary energy exchangers is presented and validated with experimental data. The model is used to study condensation and frosting in energy wheels. Condensation/frosting increases with humidity and at some humidity level, water/frost will continually accumulate in the wheel. The sensitivity of condensation and frosting to wheel speed and desiccant type are studied. The energy wheel performance is also presented during both sorption and saturation conditions for a desiccant coating with a type I sorption isotherm (e.g., molecular sieve) and a linear sorption isotherm (e.g., silica gel). Simulation results show that the desiccant with a linear sorption curve is favorable for energy recovery because it has better performance characteristics and smaller amounts of condensation/frosting for extreme operating conditions.

Unsteady heat and mass transfer with phase changes in an insulation slab: frosting effects

Abstract Moisture and frost accumulation in a glass-fiber slab are analyzed, using a one-dimensional, transient, vapor diffusion model with phase changes and variable properties. The cold sink boundary is impermeable and subject to a temperature below the triple point of water while the opposing warm boundary is open to a convective moist air. The numerical results are presented for moisture/frost accumulation under various conditions. It is shown that the motion of the frozen boundary is primarily governed by thermal diffusion. The effects of moisture/frost accumulation on thermal performance are also discussed.