Carey J. Simonson

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.

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.

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 Desiccant Coated Rotary Energy Exchangers: Part II. Validation and Sensitivity Studies

In this paper, results from the numerical model for coupled heat and moisture transfer in rotary energy exchangers, presented in Part I, are compared with experimental data. The agreement between the measured and simulated effectivenesses (sensible, latent, and total energy) is close and within the experimental uncertainty. In addition, the effects of certain assumptions were investigated for sensitivity on the predicted performance of regenerative energy exchangers The sensitivity results are calculated with a 0.9 m diameter by 0.1 m long aluminum matrix wheel that is coated with a molecular sieve desiccant (20% by mass). The desiccant has a maximum moisture capacity of 20% by mass. The porosity of the wheel is 85% and the normal rotational speed is 20 rpm. The design mass flow rate of air is 0.5 kg/s [1.6 kg/(s·m2)] giving a face velocity of 1.3 m/s. Neglecting thermal and moisture storage in the air is shown to result in errors as large as 2.4% in the predicted effectiveness of a rotary energy exchange...

Steady-State Performance of a Small-Scale Liquid-to-Air Membrane Energy Exchanger for Different Heat and Mass Transfer Directions, and Liquid Desiccant Types and Concentrations: Experimental and Numerical Data

A liquid-to-air membrane energy exchanger (LAMEE) is an energy exchanger that allows heat and moisture transfer between air and salt solution flows through a semipermeable membrane. For the first time, a novel small-scale single-panel LAMEE test facility is used to experimentally investigate the effect of the direction of heal and mass transfers for the air and salt solution flows, and the effect of different salt solution types and concentrations on the LAMEE effectiveness. The data for steady-state effectiveness of the LAMEE are compared to the simulation results of a numerical model. Two studies are conducted; first a study based on different heat and mass transfer directions (four test cases), and second a study focused on the influence of solution types and concentration on LAMEE performance. For the first study, NTU = 3 and four different heat capacity ratios (i.e., Cr* = 1, 3, 5, 7) are used, with a LiCl salt solution in the exchanger. Mass and energy balances for all the test cases and the repeatability of the experimental data for the air cooling and dehumidifying test case show that the experimental data are repeatable and within an acceptable uncertainty range. The results show increasing effectiveness with increasing Cr*, and good agreement between the numerical and experimental results for both air cooling and dehumidifying and air heating and humidifying test cases. In the second study, two different salt solutions (i.e., LiCl and MgCl 2 ), and three different concentrations for the LiCl solution (i.e., 25%, 30%, and 35%) are selected to investigate the effect of different salt solution types and concentrations on the performance of the LAMEE. A maximum difference of 10% is obtained for the LAMEE total effectiveness data with the different salt solution types and concentrations. The results show that both the salt solution type and concentration affect the LAMEE effectiveness, and changing the concentration is one way to control the supply air outlet humidity ratio.

Transient Humidity Measurements—Part I: Sensor Calibration and Characteristics

In this paper, a transient test facility is used to investigate the transient response of a polymer film electronic humidity sensor (capacitance type) following a step change in humidity and temperature. The response of the humidity sensor is measured for a step change in the humidity with and without a change in temperature, and the data are correlated with exponential functions with two time constants. The major time constant of the sensor is about 3 s for the isothermal test but increases by an order of magnitude when the sensor is exposed to a simultaneous change in temperature and humidity

The Effect of Membrane Deflections on Flow Rate in Crossflow Air-to-Air Exchangers

Manufacturing of air-to-air heat and moisture crossflow exchangers can result in membrane surfaces with significant deflections due to moderate pressure differences across the membrane under normal operating conditions. This paper presents a methodology for the determination of membrane deflections for two types of support systems. Both initial slack and pre-stressed membranes are considered. Experimental data for two types of membrane are presented, which confirms the theoretical model of membrane elastic deflections. It is shown for fully developed laminar flow in crossflow exchangers that significant flow differences will occur in the supply and exhaust flow channels for the same pressure drop across the exchanger. It is concluded that these crossflow exchangers should be manufactured with pre-stressed membranes, and a good support scheme should be chosen to minimize the effect of membrane deflections on the exchanger performance.

Effect of Axial Radiation on Heat Transfer in a Thermally and Hydrodynamically Developing Flow between Parallel Plates

The present work investigates coupled convection-radiation heat transfer through parallel plates with both heated and cooled walls. The flow is both hydrodynamically and thermally developing. The momentum and energy equations are solved with the computational fluid dynamics (CFD) code FASTEST3D, based on the finite-volume method (FVM). The medium is considered to be radiatively participating and the radiative heat transfer is also treated by the finite-volume method. Both axial and transverse heat transfer are considered. The effect of axial radiation is shown for different optical parameters by comparing results with and without axial radiation. The effect of hydrodynamically developing flows is also considered.

Transient Humidity Measurements: Part II—Determination of the Characteristics of an Interactive Device

In this paper, a humidity/temperature transmitter is used to determine the transient response characteristics of a passive interactive moisture transfer device (i.e., a desiccant coated energy wheel) following a step change in humidity and temperature. The output of the transmitter, which is placed downstream of the energy wheel, follows exponential functions with two time constants and includes the combined effects of the device and transmitter. The response of the unknown device needs to be deconvoluted from the measured data to determine the transient characteristics of the device alone. The results in this paper show that the transient moisture characteristics of the interactive device are strongly influenced by the temperature testing conditions. Isothermal test conditions are recommended when determining the moisture characteristics of devices that have a combined heat and moisture transfer interaction