Read this article now - Try Goliath Business News - FREE!   
You can view this article PLUS...

• Over 5 million business articles
• Hundreds of the most trusted magazines, newswires, and journals (see list)
• Premium business information that is timely and relevant
• Unlimited Access

Tell Me More   Terms and Conditions

Purchase this article for $4.95

...Forced-Circulation Air-Cooling and Air-Heating Coils (ARI 2001) present the same model for predicting the total and sensible energy transfer rates for wetted surface cooling coils. For the air side, the total (sensible and latent) energy transfer rate for wetted surfaces is proportional to the enthalpy difference between the airstream and saturated air at the temperature of the coil surface, while the sensible (convective) heat transfer is proportional to the temperature difference between the airstream and coil surface. A log mean temperature difference approach is utilized for estimating heat transfer between the coolant in the tubes (e.g., water) and the tube surface. Elmahdy and Mitalas (1977) and Braun et al. (1989) developed nearly equivalent and simpler models that transform the water stream to an equivalent airstream saturated with water vapor. With this transformation, the driving potential for the total energy transfer across the entire wetted section of the heat exchanger is written in terms of the enthalpy difference between the airstream and saturated air at the water temperature. The sensible heat transfer is determined using the model presented in the 2000 ASHRAE Handbook--HVAC Systems and Equipment (ASHRAE 2000) and ARI Standard 410 (2001).

Each of these models uses fin efficiency to simplify calculation of steady-state heat and mass transfer for the air side. They typically employ separate fin efficiencies for convective (sensible) heat transfer and combined heat and mass transfer. For convective heat transfer, fin efficiency accounts for the effect of the fin temperature distribution on total convective heat transfer to the fins. For combined heat and mass transfer, fin efficiency characterizes the impact of the distribution of wetted surface conditions on total energy transfer to the fins. Analytical expressions and correlations for heat transfer fin efficiencies have been developed for a wide variety of fin geometries based on a dry analysis, and these same expressions are typically applied for combined heat and mass transfer using modified properties (Kuehn et al. 1998; Xia and Jacobi 2005).

All of the existing cooling coil models that employ the fin efficiency concept utilize relationships developed for dry fins (i.e., no condensation) in determining sensible (convective) heat transfer. However, the fin temperature distribution is different for wet and dry fins and, therefore, the use of a dry fin efficiency relationship for convective heat transfer is not strictly correct under wet conditions. This paper develops a correction factor for existing fin efficiency relationships that allows a better estimate of convective heat transfer fin efficiencies under wet conditions. The improved method is validated using results obtained from a two-dimensional numerical analysis and experiments performed on an eight-row cooling coil.

DEVELOPMENT

This section develops a method for calculating sensible (convective) heat transfer fin efficiency under wet conditions for a classical straight fin geometry with uniform cross section. However, the method can be applied to any fin geometry where fin efficiency equations exist. Figure 1 illustrates the fin geometry and boundary conditions. In order to provide the proper background, classical fin efficiency relations for heat transfer under dry conditions and heat and mass transfer under wet conditions are first presented.

[FIGURE 1 OMITTED]

Fin efficiency for a dry fin is developed from the solution to a one-dimensional heat conduction problem along the fin height direction. Assuming the fin tip is adiabatic, uniform heat conductivity for the fin, [k.sub.f], and uniform convection coefficient across the fin, [h.sub.a-f], the fin efficiency for heat transfer only, [[eta].sub.f], is expressed as

[[eta].sub.f] = [[tan h(m*[H.sub.f])]/[m*[H.sub.f]]], (1)

where

m = [square root of [[h.sub.[a - f]]/[[k.sub.f]*t]]] (2)

and where [H.sub.f] is the fin height and t is half of the fin thickness (Kuehn et al. 1998).

Fin efficiency for a wet fin is developed using an analogous approach by adding the additional assumptions that the air specific heat, [C.sub.p,a], is constant and Lewis number is unity (Kuehn et al. 1998; Xia and Jacobi 2005). The fin efficiency for combined heat and mass transfer, [[eta]*.sub.f] is

[[eta].sub.f.sup.*] = [[tan h([m.sup.*]*[H.sub.f])]/[[m.sup.*]*[H.sub.f]]], (3)

where

[m.sup.*] = [square root of [1/[[[[C.sub.p,a]/[[h.sub.[a - f].sup.*]*[C.sub.s]]] + [[t.sub.w]/[k.sub.w]]]/[[k.sub.f]*t]]]] (4)

and where [k.sub.w] and [t.sub.w] are the thermal conductivity and thickness of the condensate water film; [h*.sub.a - f] is the convection coefficient for a wet fin, which is somewhat higher than for a dry fin; and [C.sub.s] is the air saturation specific heat (Braun et al. 1989) defined as the derivative of the saturation air enthalpy with respect to temperature. In practice, is evaluated at the base temperature of the fin so that

[C.sub.s] = [[[d[h.sub.s]]/dT]|.sub.T = [T.sub.b]], (5)

where is [h.sub.s] saturated air enthalpy and [T.sub.b] is the fin base temperature.

Equation 4 was developed assuming film condensate and uniform film thickness along the fin. However, the film thickness is not uniform in practice, and drop-wise condensate occurs as well. It is common to exclude the condensate conduction term and utilize an air-to-fin convection heat transfer coefficient that...

NOTE: All illustrations and photos have been removed from this article.



More articles from HVAC & R Research
A simplified dynamic model for chilled-water cooling and dehumidifying..., September 01, 2007
A simplified dynamic model for chilled-water cooling and dehumidifying..., September 01, 2007
Experimental energetic analysis of the liquid injection effect in a tw..., September 01, 2007
Numerical simulation of oil supply system of a reciprocating compresso..., September 01, 2007

Looking for additional articles?
Search our database of over 3 million articles.

Looking for more in-depth information on this industry?
Search our complete database of Industry & Market reports by text, subject, publication name or publication date.

About Goliath
Whether you're looking for sales prospects, competitive information, company analysis or best practices in managing your organization, Goliath can help you meet your business needs.

Our extensive business information databases empower business professionals with both the breadth and depth of credible, authoritative information they need to support their business goals. Whether it be strategic planning, sales prospecting, company research or defining management best practices - Goliath is your leading source for accurate information.