Heat-Transfer Units

Cylinder Heat-Transfer Units Sometimes called can dryers or drying rolls, these devices are differentiated from drum diyers in that they are used for solids in flexible continuous-sheet form, whereas drum dryers are used for liquid or paste forms. The construction of the individual cyhnders, or drums, is similar in most respec ts to that of drum diyers. Special designs are used to obtain uniform distribution of steam within large drums when uniform heating across the drum surface is critical. 

Heat may be transferred between two insoluble liquids in countercurrent flowthrough an extractor, and the performance can be evaluated in the same general manner as in mass transfer (Fig. 15-20). For a differential contactor the number of overall heat-transfer units based on the hot phase can be derived from the same equations used for the number of mass-transfer units based on the feed (raffinate) phase [Eq. (15-36)]. 

LMTD = log mean temperature difference, °F M = mass flow rate, Ib/hr Ntu = number of heat transfer units, dimensionless N = number tubes/row in direction of air flow n = number tubes/row, per ft of exchanger width, 1 /ft Q = total exchanger heat load (duty), Btu/hr R = = heat capacity ratio, dimensionless... 

Heat transfer Unit cost low to moderate Labor cost low to moderate Investment low to moderate Unlimited Somewhat restricted... 

The supersaturation in condensers arises for two reasons. First, the condensable vapor is generally of higher molecular weight than the noncondensable gas. This means that the molecular diffusivity of the vapor will be much less than the thermal diffusivity of the gas. Restated, the ratio of NSc/Npr is greater than 1. The result is that a condenser yields more heat-transfer units dTg/(Tg — Tt) than mass-transfer units dYg/(Yg — Yt). Second, both transfer processes derive their driving force from the temperature difference between the gas Tg and the interface Tt. Each incremental decrease in interface temperature... 

An alternative concept is the so-called direct absorption concept, which applies solar-receiver reactors. According to this concept a solar reformer was developed by DLR in the SOLASYS project (Tamme, 2003) based on earlier experiences in the projects SCR and CAESAR (Bauer, 1994). It allows the concentrated radiation to penetrate through a transparent aperture into the reformer, where it is absorbed directly by the irradiated absorber. The reaction gases pass through the absorber which serves simultaneously as a heat transfer unit and as support for the catalyst. The quartz window, used as aperture closure, enables the reformer to be operated under pressure. A schematic of the solar receiver is shown in Figure 3 (right). The reformer was operated up to 0.9 MPa and 780°C. The pilot reformer was tested at a power level of about 300 kW(th) at the solar tower of the Weizmann Institute of Science in Rehovot, Israel. In a follow-up project SOLREF (Moller, 2006), the operation conditions will be about 1.5 MPa and 950°C. 

The Chemical Development Drug Evaluation branch of Johnson Johnson Pharmaceutical Research Development LLC in Raritan, USA, tested microreactors for processes at elevated temperatures above the limit of most multipurpose conventional reactors, which is above 140°C [34], Operation above this limit is only possible by means of special reactors equipped with heat transfer units. 

Vb /(1 — x )> to be taken at liquid feed temperature yB/yB = saturation of solvent in feed gas ff0G ffG q the height of a gas phase heat transfer unit. (The FfOGs are computed from equation 54 the individual heights of a transfer unit for the solvent and for heat may be estimated from HGA and fiLA using the Chilton-Colburn analogy. The m in equation 54 is to be evaluated at the temperature of the liquid feed.) Reprinted by permission (45). 

EOS fl/h" lV h Number of heat transfer units Dimensionless Dimensionless... 

The number of heat transfer units Na on any given side of the heat exchanger is... 

The ineffectiveness of a heat exchanger and the associated heat flow to the cold end as given by equation (12) are normally associated with the complete heat exchanger. The number of heat transfer units for the complete heat exchanger is given by... 

Equation (21) gives the specific area in terms of the relative heat loss, whieh is a better design parameter than the number of heat transfer units used in equation (11). However, for highly unbalaneed heat capacity flows the actual overall heat loss will deviate from the sum of the individual losses used in these calculations. The more accurate heat loss for the eomplete heat exehanger is given by equation (15) when the overall ineffectiveness is found from the Ntua evaluated from the individual Ntu by equation (17). In this expression the density p is a strong function of temperature. 

---