|
Article Excerpt
INTRODUCTION
Plate heat exchangers are designed to achieve high heat transfer capacity in a small volume. Due to their compact size, plate heat exchangers have clear advantages over shell-and-tube heat exchangers and are rapidly replacing conventional shell-and-tube evaporators. Several types of plate heat exchangers are currently used in industry, including conventional gasket plate-and-frame, compact brazed, semiwelded plate-and-frame, and shell-and-plate (Ayub 2003). The disadvantage of conventional gasket heat exchangers is leakage due to failure of gasket material. Brazed heat exchangers were initially designed for cooling oil and liquid-to-liquid applications. They are also used as evaporators and condensers in the refrigeration industry. When used as evaporators, brazed heat exchangers showed poor performance at high load capacities, and failures were reported for low temperature applications (Ayub 2003). Shell-and-plate is the newest design in the plate exchanger technology. It has high mechanical integrity and superior thermal characteristics (Ayub 2003).
Some main geometric features of a heat exchanger plate are discussed below and are shown in Figure 1.
[FIGURE 1 OMITTED]
Chevron Angle: [beta], typically varying from 20[degrees] to 65[degrees], is the measure of softness (large [beta], low thermal efficiency and low pressure drop) and hardness (small [beta], high thermal efficiency and high pressure drop) of thermal and hydraulic characteristics of plates. Some authors use ([pi] - [beta]) in their investigations to represent the chevron angle.
Surface Enlargement Factor: [phi] is the ratio of developed area, based on corrugation pitch [P.sub.c] and plate pitch p, to the projected area (i.e., [L.sub.w] x [L.sub.p]). [L.sub.w] and [L.sub.p] are estimated from port distances [L.sub.v] and [L.sub.h] and port diameter [D.sub.p]: [L.sub.w] = [L.sub.h] + [D.sub.p], and [L.sub.p] = [L.sub.v] - [D.sub.p].
Mean Flow Channel Spacing: b = p - t is the difference between plate pitch p and the plate thickness, t.
The boiling of refrigerants in shell-and-tube heat exchangers has been studied and reported by a number of researchers. Relatively fewer similar studies have been conducted for plate heat exchangers. Two-phase flow heat transfer in a plate heat exchanger is a function of parameters such as quality, heat flux, mass flux, incipient boiling, surface structure, local flow regimes, dry out, film thickness, oil concentration, etc. Panchal et al. (1983), Panchal and Hillis (1984), Jonsson (1985), Hesselgreaves (1990), Kumar (1993), Thonon (1995), and Thonon et al. (1995) briefly discussed two-phase flow in plate evaporators. Boccardi et al. (1999) reported performance of plate heat exchangers using R-134a, R-407C, R-410A and R-22. Panchal et al. (1983) and Panchal and Hillis (1984) performed experimental work on plate heat exchangers as ammonia evaporators. Palm and Claesson (2006) presented methods for predicting single and two-phase flows in plate heat exchangers by studying the effect of various geometric parameters on heat transfer and pressure drop in single and two phases. Contrary to previous work by others, their work indicated a predominant effect of heat flux over mass flux. The heat transfer performance was correlated by conventional pool boiling correlations.
The potential threat of ozone depletion and global warming has increased interest in the use of natural refrigerants in the air-conditioning and refrigeration industry. Ammonia, a low-cost, environmentally safe natural fluid with excellent thermophysical properties, has received increased attention in light of the phasing out of CFCs, HCFCs, and the potential/possible phase-out of HFCs. Ayub (2006) suggested ammonia as an attractive and viable replacement for HFCs. Ammonia as a refrigerant has played an important role in the industry, particularly in the fields of food processing, dairy, and marine refrigeration. Ammonia has four- to six-fold better heat transfer characteristics compared to halocarbon refrigerants (Stoecker 1998). Djordjevic and Kabelac (2008) studied flow boiling of R-134a and ammonia in a plate heat exchanger with two chevron plate configurations. Heat transfer coefficient was reported to be a strong function of vapor quality, which also increased with an increase in heat and mass flux. Experimental data were presented, but a correlation for heat transfer coefficient based on their study was not reported. However, ammonia was reported to have better heat transfer characteristics compared to R-134a.
A disadvantage of ammonia is its toxicity, which has impeded its wider use. However, taking advantage of the large area-to-volume ratio offered by compact heat transfer equipment, such as plate heat exchangers, and the superior heat transfer characteristics of ammonia, a significant reduction in the operating charge of refrigerants can be achieved. Plate heat exchangers are therefore particularly suitable for ammonia. However, for plate heat exchangers to cope with high-pressure applications for ammonia and carbon dioxide, economic brazing techniques need to be developed.
This paper provides a review of heat transfer and pressure drop correlations for fluid flow evaporating in plate heat exchangers. Emphasis is placed on the application of a plate heat exchanger as ammonia evaporator in a refrigeration system. Effects of compressor oil/lubricant on the evaporator heat transfer performance are also covered.
CORRELATIONS FOR AMMONIA
Ayub (2003) presented extensive literature review of available single-phase and two-phase correlations for plate heat exchangers. The single-phase correlations are geometry and plate specific. It was suggested that the augmented single-phase heat transfer performance of a plate heat exchanger is due to mechanisms such as disruption and reattachment of boundary layers, vortex and swirl flows, and secondary circulations. For evaporating two-phase flow in a plate heat exchanger, due to its complex narrow passages, it is possible that heat transfer mostly takes place by two-phase flow convection, except at the lower section of the evaporator plate where nucleate boiling may play an important role. In plate heat exchanger evaporators, flow is vertical and against gravity; therefore, the flow regime is relatively simple, and phase separation is not a severe issue even at low mass fluxes. Ayub (2003) proposed the following two-phase heat transfer coefficient correlation for the evaporation of ammonia and R-22 in direct expansion (DX) and flooded evaporators. The correlation was developed based on field data, and no detailed experimental work was performed in a laboratory environment:
[h.sub.tp] = C([k.sub.l]/[D.sub.e])[([[[Re.sub.l.sup.2][h.sub.fg]]/[L.sub.p]]).sup.0.4124][(p/[p.sub.cr]).sup.0.12][[(65/[beta])].sup.0.35] (1)
where C is a constant, [k.sub.l] is thermal conductivity of liquid, and [D.sub.e] is equivalent diameter. Re is the Reynolds number based on [D.sub.e], [h.sub.fg] is latent heat of vaporization, [L.sub.p] is effective plate length, p is pressure, [p.sub.cr] is critical pressure, and [beta] is the chevron angle in degrees.
C = 0.1121 for flooded and thermo-siphon
C = 0.0675 for DX
Ayub (2003) also proposed to evaluate evaporation pressure drop using the following correlation...
|
