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Article Excerpt
INTRODUCTION
Twenty-five years ago air-to-air exchangers were rarely used to recover heat from exhaust air in buildings, but now that they can transfer both heat and water vapor, they are included in most HVAC designs for commercial buildings. When energy wheels are employed in HVAC designs, the capacity of auxiliary heating and cooling equipment is reduced (Asiedu et al. 2004, 2005). ANSI/ASHRAE Standard 84-2008, Method of Testing Air-to-Air Heat Exchangers (ASHRAE 2008), sets out a method of test to determine the performance of air-to-air exchangers for transferring heat and water vapor between supply and exhaust air streams, as shown in Figure 1. At any operating condition, this performance is characterized by the determination of the dimensional [delta][p.sub.s] and [delta][p.sub.e] for pressure drop in the supply and exhaust air streams and six dimensionless factors: [[epsilon].sub.s], [[epsilon].sub.l], [[epsilon].sub.t], EATR, OACF, and RER for the sensible, latent or moisture, and total energy effectiveness, exhaust air transfer ratio, outdoor air correction factor, and recovery efficiency ratio. These performance factors will, in general, vary for each operating condition, so ARI Standard 1060, Rating Air-to-Air Energy Recovery Equipment (ARI 2005) restricts its certification effectiveness tests to only one summer and winter test condition. To test large commercial air-to-air exchangers, these performance factors require a large test facility with expensive instrumentation and, most importantly, a sophisticated on-line data acquisition and analysis methodology. As a consequence, only research or special test laboratories can be expected to acquire and analyze accurate test data for a few operating conditions, and then only when testing small or medium-sized commercial exchangers.
[FIGURE 1 OMITTED]
The desiccant coated air-drying or dehumidifier wheel is closely related to the energy wheel because it too transfers heat and water vapor. The energy wheel requires no external power other than for wheel rotation while the dehumidification wheel requires a high regeneration air temperature and flows that will differ from the supply air. The construction of these drying wheels, with desiccant coatings on the wheel matrix surfaces, is nearly identical to that of energy wheels; however, the operating conditions differ with respect to the regeneration temperature, wheel speeds, airflow speeds, and the fraction of the wheel used for regeneration. For example, desiccant drying wheel speeds of less than 0.5 rad/s are typically 10 to 100 times lower than energy-wheel speeds, and inlet air temperature differences for drying wheels are typically 10 to 20 times larger than those for energy wheels at ARI certification operating conditions. The performance of desiccant drying wheels may be characterized by a similar set of factors as energy wheels; however, some factors, such as sensible and total energy effectiveness, EATR and OACF are much less significant and are usually not considered while the regenerator input energy needs to be characterized by a dimensionless coefficient of performance (COP) (Charoensupaya and Worek 1988; Van den Bulck et al. 1988). A question that may be asked here is What is physically occurring in these two different regenerative wheel applications--air drying and energy recovery--that accounts for the two different types of wheel performance?
Decades of research on desiccant coated energy wheels, which are now common in HVAC designs in North America and elsewhere, has revealed that great care must be taken to minimize laboratory testing errors and uncertainty in obtaining the new ASHRAE performance factors (Simonson and Besant 1998; Shang et al. 2001). By and large, field testing is impractical. Alternatively, validated simulation methods and correlations can be used to predict the difficult-to-measure effectiveness, but it still requires many flow channel property data (Simonson et al. 1999; Simonson and Besant 1999), so different tests and measurements are required to get these important flow channel properties. Recently, Abe et al. (2006a, 2006b) devised a simple transient test method in an attempt to determine the characteristic temperature and humidity step response of a stationary energy wheel and, using an analytic model, predict the sensible and latent energy effectiveness. Considering the uncertainty in their data, the agreement between these predictions using only transient test data and steady-state test data appeared to be satisfactory, but small bias differences with steady-state test data suggested that corrections should be applied to these transient test results, because there were small differences between the transient test predictions and the steady-state test data.
The literature on desiccant drying wheels or rotary dehumidifiers also includes extensive research investigations over the past few decades. Jurinak and Mitchell (1984) used a finite difference model to investigate the effect of matrix properties on the performance of a counterflow rotary dehumidifier. First assuming infinite transfer coefficients, Van den Bulck et al. (1985) developed correlations for the humidity and enthalpy effectiveness of rotary heat and water vapor transfer wheels. Zheng and Worek (1993) presented a numerical model similar to that of Jurinak and Mitchell to simulate the combined heat and mass transfer processes that occur in a rotary dehumidifier and investigate the effect of the rotational speed on the performance of the dehumidifier. Van den Bulck and Klein (1990) used a single-blow transient test procedure to determine the overall heat and mass transfer coefficients of dehumidifier matrices. Their analysis technique--based upon the transformation of the model partial differential equations into a set of ordinary differential equations--and the temperature and mass-fraction distributions are modeled by a system of nonstiff ordinary differential equations, which can be integrated numerically. More recently, Golubovic et al. (2006) presented sorption property data for different types of molecular sieves in equation form and investigated the influence of different assumptions for heat of sorption and equilibrium equation of molecular sieve on predicted optimum performance of a rotary dehumidifier.
Some authors have considered both energy wheels and rotary dehumidifiers. For example, Zhang and Niu (2002) assumed desiccant film equilibrium in their numerical models for both recovery energy wheels and rotary dehumidifying wheels and predicted moisture effectiveness for each type of wheel.
Most recently, Shang and Besant (2008) presented a theoretical analysis that can be used to correct transient test data for the sensible effectiveness of energy and heat wheels and showed that accurate sensible effectiveness values can be predicted using only the property data of the wheel matrix flow channels, wheel speed, and the inlet airflow properties. The questions that now need to be considered are: Can a similar theoretical transient model, combined with appropriate corrections, be developed for the latent or moisture transfer effectiveness in energy wheels? Can this model be applied to desiccant drying wheels, albeit with slightly modified flow channel properties?
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