A commercial laundry runs a dryer that has an exit flow of 0.5 kg / s moist air at 48^∘ C, 70 %

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A commercial laundry runs a dryer that has an exit flow of...

A commercial laundry runs a dryer that has an exit flow of $0.5 \mathrm{~kg} / \mathrm{s}$ moist air at $48^{\circ} \mathrm{C}, 70 \%$ relative humidity. To reduce the heating cost, a counterflow stack heat exchanger is used to heat the incoming fresh air at $20^{\circ} \mathrm{C}$ with the exit flow, as shown in Fig. $\mathrm{P} 11.105 .$ Assume the outgoing flow can be cooled to $25^{\circ} \mathrm{C}$ and the two flows have the same mass flow rate of dry air. Is there a missing flow in the figure? Find the rate of energy recovered by this heat exchanger.

Key Concepts

Psychrometrics

Psychrometrics is the study of the thermodynamic properties of moist air, including temperature, humidity, and the proportion of water vapor to dry air. This concept is fundamental for analyzing processes involving humid air, such as drying operations and the design of air-handling systems, because it allows engineers to determine properties like specific humidity and enthalpy, which are essential for energy balance calculations.

Sensible and Latent Heat Transfer

This concept differentiates between sensible heat, which changes the temperature of the air, and latent heat, which involves energy absorbed or released during phase changes (such as evaporation or condensation). Understanding both is crucial when analyzing systems where moist air is involved, as it impacts how energy is transferred and recovered in processes like drying and heating using heat exchangers.

Energy Balance Calculations

Energy balance calculations involve applying the principle of conservation of energy to determine the amount of energy transferred, stored, or lost in a system. In the context of heat exchangers, these calculations are used to compute the rate at which energy is recovered from one flow and transferred to another, by accounting for the energy changes associated with both sensible and latent heat.

Counterflow Heat Exchanger Principles

A counterflow heat exchanger is designed such that two fluid streams move in opposite directions, which maximizes the temperature difference between them over the length of the exchanger. This design improves the efficiency of heat transfer, making it especially effective for recovering energy from exhaust or waste heat sources.

Mass Balance of Dry Air

In processes involving humid air, it is often useful to perform the mass balance on the dry air component, which remains constant even as moisture levels vary. This concept simplifies the calculations by focusing on the invariant mass of the dry air, allowing for more straightforward analyses of energy and mass transfer in systems like heat exchangers.

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