Compact, high-effectiveness, gas-to-gas compound recuperator with liquid intermediary

Abstract

A liquid-loop compound recuperator is disclosed for high-ε heat exchange between a first shell-side fluid stream and a second shell-side fluid stream of similar thermal capacity rates (W/K). The compound recuperator is comprised of at least two fluid-to-liquid (FL) recuperator modules for transfer of heat from a shell-side fluid, usually a gas, to an intermediary tube-side heat transfer liquid (HTL). Each FL module includes a plurality of thermally isolated, serially connected, adjacent exchanger cores inside a pressure vessel. The cores are rows of finned tubes for cross-flow transfer of heat, and they are arranged in series to effectively achieve counterflow exchange between the HTL and the shell-side stream. The HTL may be water, an organic liquid, a molten alloy, or a molten salt.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application claims the benefit of U.S. application No. 61 / 016,247 filed Dec. 21, 2007, and the benefit of U.S. application No. 61 / 034,148 filed Mar. 5, 2008, each of which is incorporated herein by reference for all purposes.FIELD OF THE INVENTION[0002]The field of this invention is heat exchangers, and more particularly, compact, gas-to-gas recuperation at high effectiveness for clean gases of similar heat capacity rates using compound recuperators with liquid intermediary.BACKGROUND OF THE INVENTION[0003]Gas-to-gas recuperation with both high thermal effectiveness and order-of-magnitude improvement in cost effectiveness is critical to addressing global energy needs, as shown in at least two co-pending patent applications. From a manufacturing perspective, the challenges arise from the fact that it is not practical to produce heat exchangers with closely-spaced fins on both the inside and outside of tubes, and alternative approaches...

AI Technical Summary

Benefits of technology

[0041]A liquid-loop compound recuperator is disclosed for high-s heat exchange between a first shell-side fluid stream and a second shell-side fluid stream of similar thermal capacity rates (W / K). The compound recuperator is comprised of at least two fluid-to-liquid (FL) recuperator modules for transfer of heat from a shell-side fluid, usually a gas, to an intermediary tube-side heat transfer liquid (HTL). Each FL module includes a plurality of thermally isolated, serially connected, adjacent exchanger cores inside a pressure vessel. The cores are rows of finned tubes for cross-flow transfer of heat, and they are arranged in series to effectively achieve counterflow exchange between the HTL and the shell-side stream. The HTL may be water, an organic liquid, a molten alloy, or a molten salt. Alumina-dispersion-strengthened-metal fins, superalloy tubes, and a lead-bismuth-tin alloy HTL may be used for high temperatures. Cumene may be used as the HTL in cryogenic applications.

Problems solved by technology

From a manufacturing perspective, the challenges arise from the fact that it is not practical to produce heat exchangers with closely-spaced fins on both the inside and outside of tubes, and alternative approaches thus far have had limited success.

However, most have not been directed at high thermal effectiveness ε for cases where the heat capacity rates in the two streams are similar.

Moreover, high ε there is not an objective, as the flue gas will be used subsequently for boiling.

Achieving high ε in gas-to-gas exchange with low pumping power has been challenging because volumetric specific heats are much lower than seen in liquids and thermal conductivities are usually low.

However, this microtube recuperator has not yet been shown to be commercially competitive with the brazed plate-fin type, in wide usage in recuperated open Brayton cycles in the 30-250 kW range and occasionally up to 25 MW.

These too have limited cost effectiveness and limitations in accommodating applications where there are large pressure differences (greater than ˜0.7 MPa) between the two streams at high temperatures (above ˜750 K).

Regenerators have seen very little usage largely because of the difficulties in obtaining adequate isolation between the high-pressure and low-pressure streams and because of the shedding of ceramic particles, leading to turbine abrasion.

Rotating ceramic honeycomb regenerators have demonstrated ε above 98% while the brazed plate-fin recuperators seldom achieve more than 87% ε, primarily because of cost and mass optimization constraints.

However, the rotating honeycomb regenerator still has substantial limitations, either where there are substantial pressure differences between the two streams, or where the size is small (below ˜100 kW), or where the lower-pressure stream is above ˜0.4 MPa.

This last condition leads to greater difficulties in limiting leakage and carry over, and it leads to unreasonably low porosity requirements (or high solidity) in the honeycomb (for sufficient thermal storage).

High solidity exacerbates axial thermal conduction losses and makes the regenerator more massive and perhaps more prone to stress-related failure.

When two or more of the above conditions are present simultaneously, the honeycomb suffers markedly.

Both step-down and interleaved tube-side flow would generally be disadvantageous for single-phase tube-side liquid flow in high-ε exchange, as addressed herein; but with such patterns avoided, common AC condenser cores may be utilized for high-ε recuperation.

However, the above differences are of enormous importance with respect to manufacturing, compactness, and cost effectiveness.

But apparently the value of thermally isolated serially connected cores inside a pressure vessel has not previously been appreciated as optimum to achieve high-ε.

Perhaps series arrangements of thermally isolated cores similar to those used in AC condensers have not been considered for high-ε gas-liquid exchange because most large applications also require dealing with moisture, acids, and particulates in the gas stream.

However, the heat pipe uses a self-pumped two-phase fluid tube-side, and it is poorly suited to gas-gas recuperation.

There, however, the large majority of the heat transfer in each exchanger includes phase change, and a very energy-intensive vapor pump is required.

It is possible that some air-to-air recuperators for heat recovery in buildings have utilized proprietary concepts somewhat related to those presented herein, but apparently all such have relied upon phase change in the fluid intermediary for most of the heat transfer, and there is no evidence that they have achieved high ε.

However, phase change is not desired in the instant invention, as it makes minimization of irreversibilities impractical (because it requires a very large number of intermediary loops).

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