|
Article Excerpt
INTRODUCTION
Traditionally, dehumidification equipment was used in some spaces with special requirements, such as in some electronic component manufacturing shops and some ICUs in hospitals. Dehumidification can be accomplished using either active desiccants (solid or liquid) or cooling coils (Mumma 2001). As a general HVAC design rule for commercial and institutional buildings, cooling coils have been a better choice when the required dew-point temperature is above 40[degrees]F (4[degrees]C). On the other hand, active desiccants are a better choice when the dew-point temperature is below 40[degrees]F (4[degrees]C). Sensitized by litigation regarding indoor air quality problems that are often related to mold and moisture problems, building owners have been more willing to invest in better HVAC designs with improved dehumidification capabilities. Their interest is also prompted by comfort problems caused by the high internal relative humidity and moisture load caused by higher ventilation rates and building envelope leakage rates (Harriman et al. 2001 and Harriman and Judge 2002).
Dehumidifier wheels, or desiccant dryer wheels, must have their desiccant-coated surfaces periodically regenerated to a dry condition once every rotation cycle, using a hot regenerative air. Although the literature for regenerative heat wheels has been developed over 85 years, the literature for regenerative dehumidifier and energy wheels goes back only 35 years. During this time, rigorous test standards have evolved for heat and energy wheels using effectiveness as the most important wheel performance factor (ASHRAE 2008). ANSI/ASHRAE Standard 139-1998, Method of Testing for Rating Desiccant Dehumidification Utilizing Heat for the Regeneration Process (ASHRAE 1998), for testing dehumidification wheels appears to be less well known because nonstandard methods of testing and definitions of performance factors are still common in the literature.
Zhang and Scott (1993) presented a unified approach for discretization of the analysis of thermal regenerator heat exchangers. Laplace transform and numerical techniques were compared for two models of regeneration operation. Changes of time scale and parameters were shown and compared. Overall wheel performance factors or comparisons with measured data were not presented; however, typical graphical time-dependent outlet gas temperatures were shown for several cycles after a cold start and at steady state for counter-current flow.
Collier et al. (1990), Worek et al. (1991), Belding et al. (1991), Zheng and Worek (1993), and Zheng et al. (1993) used mostly numerical methods to investigate the performance of dehumidifier wheels using various desiccant types, a defined nondimensional time or wheel speed factor, number of transfer units, etc. as independent variables where the COP of the system and cooling capacity were investigated as dependent performance factors. They concluded that type 1A desiccants were best for dehumidifier wheels. Other findings from these papers are difficult to use by designers because the functional relationships between wheel design variables and performance factors remains hidden in the numerical code. There are no comparisons between measured data and numerical predictions; rather, their predictions are compared to other simulations and show good agreement. Example designs, defining all of the dimensional variables, are not presented.
Using a small portion of a dehumidifier wheel, Czachorski et al. (1997) used a transient test method similar to the single-blow test method in a stationary dehumidifier wheel of Collier et al. (1992) where the airflow into the two-wheel test sections is reversed at a selected time and all of the inlet and outlet temperatures, humidities, and flow rates are measured. Using the numerical model of Zheng and Worek (1993) and the results of Zheng et al. (1993), they predicted an optimum wheel speed of 16 rph for their particular wheel.
Zhang and Niu (2002) developed a two-dimensional, transient numerical model to study the effects of rotary speed, NTU, and exchanger surface area on rotary wheel performance. They found that the heat and mass transfer response for rotary desiccant-coated wheels depends on the speed of the wheel, as well as the inlet air conditions. Typical cyclic internal air temperature and humidity simulations were presented for energy and dehumidifier wheels that are similar to those of Zhang and Scott (1993) for heat transfer. For energy wheels, they presented simulated sensible and latent effectiveness versus NTU, specific area, and wheel speed. Their effectiveness results for the effect of wheel speed are not consistent with the data and simulations of Simonson et al. (2000) and the theoretical model of Shang and Besant (2008, 2009a, 2009b).
Gao et al. (2005) used a numerical control volume method for the one-dimensional Navier-Stokes equations to predict the transient and steady state of the outlet air temperature and humidity of a dehumidifier wheel moisture transport in each half of this wheel. Assuming fully developed turbulent flow in each flow channel of a dehumidifier wheel, they compared their simulations with measured data, but the agreement was not good. This disagreement may be due to their assumption of turbulent flow for flows that were laminar. Also, their sensitivity investigation and conclusion on the effect of the shape of the flow channels in the wheel matrix does not appear to be consistent with the simulation models of Simonson and Besant (1998) and other researchers or the theoretical models of Shang and Besant (2008, 2009a, 2009b) where flow channel total surface area per unit face area of the wheel was shown to be the dominant flow channel geometry factor. For dehumidifier wheels, they made similar comparisons using a nonstandard definition of dehumidification effectiveness and dehumidification power, so direct comparisons are not possible. Nonetheless, their prediction of the effect of wheel speed does not appear to be consistent with the predictions of Shang and Besant (2009a, 2009b).
Jia et al. (2006) presented property data and parametric data for two nonstandard performance indices for two similar dehumidifier wheels: one coated with silica gel (approximate particle size range 30 < dp < 60 nm) and another coated with composite silica gel and LiCl particles (approximate size range 10 < dp < 30 nm). They concluded that there was a 50% improvement in the moisture removal capacity for the wheel coated with their new composite coating compared to the silica gel-coated wheel. It is not clear from this paper just what mass fractions of each particle species were used in their composite coating, nor what was the average mass density and thickness of the two coatings. Shang and Besant (2009a and 2009b) discuss how particle size differences may be very important for water vapor sorption, because the specific surface area in a particle bed will vary inversely with the particle diameter. So, the performance difference in Jia's measured test data may have been strongly influenced by this particle size difference and the composition. In addition, since LiCl has a very low deliquenscence humidity, the performance of any dehumidifier wheel that uses LiCl in its coating can be expected to deteriorate with the number of cycles of exposure at high humidities (Belding et al. 1991).
Wang et al. (2005) and Abe et al. (2006a, 2006b) used a different transient test to determine the time response of small sections of stationary energy wheels in order to predict their effectiveness using a theoretical model that employed well known parallel and counterflow heat exchanger effectiveness equations where NTU and airflow capacitance ratio are the two independent parameters.
Using basic equations and wheel flow channel properties, Shang and Besant (2008) presented a mathematical model to predict the sensible effectiveness of a rotary regenerative wheel for equal flow areas and mass flow rates in the supply and exhaust streams. They presented an analytical equation for predicting the fully developed flow sensible effectiveness of an energy wheel that only depends on the wheel speed and time constant and includes corrections for entrance, axial conduction, carry-over, sorption phase change, and manufacturing effects. Comparisons of this model with data show agreement within the uncertainty bounds for energy wheels.
Shang and Besant (2009a and 2009b) also presented a mathematical model based on fundamental heat and mass transfer for predicting the latent effectiveness of energy wheels that have equal airflow areas and balanced flows. The equation for the transient humidity step response of an energy wheel is developed from physical principles using a similar model to that used for the sensible energy response. This fully developed flow model is used to derive a simple characteristic moisture transfer time constant in terms of the flow channel and its desiccant-coating properties and the airflow velocity. Scanning electron microscope (SEM) images and photos for one typical desiccant coating showed particles bonded on to one aluminum matrix. Isotherm data presented for the moisture content of these coatings and similar desiccant particles at several temperatures show significantly lower moisture content isotherms for the coatings than the for the particles. Calculated equilibrium isotherm moisture content changes, when compared to transient moisture content change correlations for a step change in humidity, showed agreement within the uncertainty limits for time periods of 1000 s...
|
