Total Heat Recovery Research Articles

Effects of material properties on heat and mass transfer in honeycomb-type adsorbent wheels for total heat recovery

Desiccant wheels have been widely used for humidity treatment: air dehumidification and total heat recovery. Since the operating conditions are different, heat and mass transfer behaviors in the wheels are quite different. This study aims at investigating the performance of honeycomb type desiccant wheels for total heat recovery with various solid desiccant wall materials. A one-dimensional, transient heat and mass transfer model is used to predict the cyclic behaviors of the desiccant wheels. Effects of the working conditions, the rotary speed, the inlet velocities of process air (fresh air) and exhaust air on the performance of the wheel are investigated and compared with various desiccant wall materials. Totally ten most commonly used desiccant materials are considered, with different adsorption and thermophysical properties. It is found that of the 10 materials, the zeolites 5A and 13X perform better than other desiccants for total heat recovery under the high rotary speed (higher than 15rpm) under typical working conditions. The results are different from desiccant wheels for air dehumidification.

Fuel oil production through high-temperature coal tar catalytic hydrocracking in supercritical xylene

ABSTRACTThe paper aimed to build a model for high-temperature coal tar (HTCT) catalytic hydrocracking in supercritical xylene with the conceptual design of energy self-sustainability based on Aspen Plus. The following tasks were done to boost the reliability of the model: selecting reasonable model compounds for HTCT, using pseudo-components to describe the nature of products and choosing reliable property methods for blocks. Results showed products can be well separated by the block rectifying column (DIST); additionally, the total heat recovery was as high as 2,355 kW h, among which 1,434 kW h was from the heat carried in the outlet stream of REACTOR. The total energy consumed by the energy consumption equipments was 882.5 kW h.

Evaluation of a flue gas driven open absorption system for heat and water recovery from fossil fuel boilers

This paper presents an open absorption system for total heat recovery from fossil fuel boilers using the high temperature flue gas as the regeneration heat source. In this system, liquid desiccant serves as the recycling medium, which absorbs waste heat and moisture contained in the low temperature flue gas in the packed tower and then regenerates in the regenerator by the high temperature flue gas. Water vapor generated in the regenerator gets condensed after releasing heat to the heating water system and the condensing water also gets recycled. The return water collects heat from the solution water heat exchanger, the flue gas water heat exchanger and the condenser respectively and is then used for district heating. Driven by the vapor pressure difference between high humidity flue gas and the liquid desiccant, the heat recovery efficiency of the system is not limited by the dew point of the flue gas, enabling a warmer water to be heated up than the conventional condensing boiler. The performance of this system was analyzed theoretically and experimentally and the results showed that the system operated well for both district heat supply and domestic hot water supply. The system efficiency increased with the moisture content of flue gas and the total heat recovery was about 8.5%, 17.2%, 21.2%, and 9.2% higher than the conventional condensing system in the case of coal fired boiler, fuel oil boiler, natural gas boiler, and coke oven gas boiler, respectively.

급·배기 일체형 디퓨저의 환기 및 온열환경 유지성능에 대한 연구

Installing a ventilator for an apartment house composed of over 100 dwelling units was mandated in 2006 to improve indoor air quality. In the case of mechanical ventilators, the air change rate is kept stable, however 68% of dwellers do not operate their ventilator because of an increased electrical bill and noise. In the case of natural ventilators, the former problems are settled, but there are concerns about cold draught and an increase of heating/cooling cost. Authors are developing a heat recovery type natural ventilator which is a natural ventilator equipped with total heat recovery element, and it is expected that those problems of mechanical ventilator and natural ventilator are resolved by this. The combined type diffuser of this study is an under developed fit to the heat recovery type natural ventilator above. There are no standards to evaluate the performance for this type of diffuser. Due to this issue this study investigated the performance of the ventilation and maintained a thermal environment for the combined type diffuser by comparing it with existing diffusers. The results revealed that the performances of the combined type of diffusers showed different features from the existing ones, and was estimated to be high enough in the performances above.

Numerical Evaluation of Heat Recovery Performance of a Switchable Exhaust Air Window

Switchable exhaust air (SEA) window allows the exhaust air from HVAC system to be ventilated through the cavity of the window to ambient. It can be regarded as an exhaust air heat recovery device in buildings. The annual energy requirement of the window has been calculated in Wuhan, China. The energy requirements for conditioning fresh air with and without the total heat recovery ventilators (THRVs) are also investigated. Heat recovery performance of the SEA window and the THRVs is compared. Results show that the SEA window can potentially provide alternative solution for recovering the low-grade energy from air-conditioning exhaust air.

Molecular sieving effect of zeolites on the properties of PVA based composite membranes for total heat recovery in ventilation systems

The molecular sieving effect of a PVA/zeolite membrane accelerated the transfer of H2O and barred the transfer of CO2, which gave the membrane both an effective high total heat exchange and gas barrier property.

Performance investigation of an advanced multi-effect adsorption desalination (MEAD) cycle

This article presents the development of an advanced adsorption desalination system with quantum performance improvement. The proposed multi-effect adsorption desalination (MEAD) cycle utilizes a single heat source i.e., low-temperature hot water (as low as 55°C). Passive heating of the feed water (no direct heating) is adopted using total internal heat recovery from the kinetic energy of desorbed vapor and water vapor uptake potential of the adsorbent. Thus, the evaporation in the MEAD cycle ensues at low temperatures ranging from 35°C to 7°C yet providing significantly high performance ratio. The energy from the regenerated vapor is recovered for multiple evaporation/condensation of saline water by a water-run-around circuit between the top brine temperature (TBT) effect and the AD condenser. The adsorbent material is the hydrophilic mesoporous silica gel with high pore surface area. Numerical simulation for such a cycle is developed based on experimentally verified model extending to multi-effect cycle. The system is investigated under several operation conditions such as cycle time allocation, heat source temperature and the number of intermediate effects. It is observed that most of the evaporating–condensing effects operate at low temperature i.e., below 35°C as opposed to conventional multi-effect distillation (MED) cycle. For a MEAD cycle with 7 intermediate effects, the specific water production rate, the performance ratio and the gain output ratio are found to be 1.0m3/htonne of silica gel, 6.3 and 5.1, respectively. Low scaling and fouling potentials being evaporation at low temperatures yet high recovery ratio makes the cycle suitable for effectively and efficiently handling highly concentrated feed water such as produced water, brine rejected from other desalination plants and zero liquid discharge (ZLD) system.

Simulation of a heat pump system for total heat recovery from flue gas

This paper introduces an approach of using an open-cycle absorption heat pump (OAHP) for recovering waste heat from the flue gas of a gas boiler with a system model. And equivalent energy efficiency is used to evaluate two other heat recovery systems that integrate an electric compression heat pump (EHP) or an absorption heat pump (AHP) with a boiler. The key factors influencing the systems are evaluated. The OAHP system efficiency is improved by 11% compared to the base case. And the OAHP system is more efficient than the AHP or the EHP systems, especially when the solution mass flow rate is only a little less than the cold water mass flow rate. The energy efficiency comparison is supplemented with a simplified economic analysis. The results indicate that the OAHP system is the best choice for the current prices of electricity and natural gas in Beijing.

Cost Minimisation for Total Site Heat Recovery

In this paper minimisation of total cost for retrofit of Total Site heat recovery system is proposed. It based on analysis of balanced Total Site Profiles and includes procedure for calculation of heat transfer area. Heat transfer area is calculated for different regions with use of intermediate utility and direct heating and cooling. Minimal temperature difference between Total Site Profiles is analysed. Global minimum of heat transfer area for Total Site recovery is calculated for array of minimal temperature differences. It is analysed with utility consumption, numbers of units and material of equipment. Selection of minimum total cost for retrofit project of site recovery system is proposed.

Heat Integration Between Plants with Combined Integration Patterns

Total site heat integration provides more energy saving opportunities than conventional heat integration within single process. Some total site design methods including direct integration using process streams and indirect integration using intermediate-fluid circuit have been proposed during these years. In these works, it is found that direct heat integration can recover more heat, require less heat transfer area, but incur higher cost in pipeline compared with indirect heat integration. However, most research focused on total site heat integration with considering only indirect heat integration or direct heat integration, no methodology simultaneously considered both direct and indirect heat integration. Therefore, some optimal designs will be missed by using current total site integration methodologies for they do not fully utilize the features of indirect and direct heat integration. In this work, the situation that only applying direct heat integration, only applying indirect heat integration and applying both heat integration in total site heat integration are analyzed. The new idea allows the direct and indirect heat integration to be considered simultaneously. The application of the new methodology can bring a significant energy saving in total site. Heat integration across plants can bring large energy savings and has been studied for many years. The concept of total site heat integration, first introduced by Dhole and Linnhoff (1993), describes heat integration of multiple plants with a central utility system. The site sink-source profiles proposed by them can be used to determine the different levels of steam that can be generated in order to indirectly integrate heat through multiple processes. Hu and Ahmad (1994) developed a total site heat integration methodology that considers the utility system. In their work, different levels of steam were used to transfer heat between processes, and such integration using intermediaries was defined as indirect integration. Klemes et al. (1997) further developed the total site profile and the site utility grand composite curve to evaluate total site potential heat recovery. Rodera and Bagajewicz (2001) introduced a mathematical methodology to compare the difference between indirect integration and direct integration. Moreover, they analyzed factors such as number of intermediate-fluid circuits and type of intermediate-fluid in terms of cost. Because total site contains a number of processes and each process may require different minimum temperature difference, Varbanov et al.(2012) developed a modified total site targeting procedure that can obtain more realistic heat recovery targets for total sites by specifying the minimum temperature difference. More recently, Perry et al. (2008) extended the total site concept to a broader spectrum of processes in addition to the well-studied industrial processes in terms of carbon footprint. Varbanov and Klemes (2011) further developed the total site methodology of Perry et al. (2008) by incorporating the aspects of renewable energy sources and CO2 emissions. Kapil et al. (2012) proposed a methodology for estimating the cogeneration potential for a site utility system via bottom-up and top-down procedures. In their methodology, the low-grade heat is used through heat pumping, organic Rankine cycles, energy recovery from exhaust gases, absorption refrigeration and boiler feed water heating. Although many aspects of total site heat integration have been studied by various researchers, there are still some unexplored problems. In the work of Wang (2013)，they found that because of long distance between plants, pipeline cost becomes a major part of capital cost, and the number of heat exchange

Total heat recovery of gas boiler by absorption heat pump and direct-contact heat exchanger

This paper presents a new technology which combines the absorption heat pump and the direct-contact heat exchanger. Based on the analysis of natural gas features and thermal efficiency of the gas boilers, it analyzes existing and new techniques for heat recovery, focusing upon removing heat from the flue gas to a point where it can be below the dew point as significant to realizing the total heat recovery for the gas boiler. Through field test and experimental analysis of an actual case in a district heating system operating at various system load, this paper strives to identify challenges related to high return water temperature, achieving a low-temperature cold source, and enhancing the heat transfer effect; while observing if the system performance is satisfactory, and whether the system can achieve very stable performance under different working conditions and realize the total design heat capacity improvement of 12%. Data will be examined for boiler load and its influences on the condensing rate with suggestions for further improvements.

Minimum heat transfer area for Total Site heat recovery

In this paper a further development of methodology for decreasing the capital cost for Total Site heat recovery by use of different utility levels is proposed. The capital cost of heat recovery system is estimated for certain temperature level of intermediate utility applying Total Site Profiles. Heat transfer area is reduced by selection of appropriate temperature of intermediate utility. Minimum of heat transfer area depends on slopes of Total Site Profiles in each enthalpy interval. This approach allows estimating the minimum of heat transfer area for heat recovery on Total Site level. Case study is performed for fixed film heat transfer coefficients of process streams and intermediate utilities. It indicates that the total heat transfer area of heat recovery can be different up to 49.15% for different utility temperatures.

A retrofit framework for Total Site heat recovery systems

Heat Pinch Analysis retrofit projects are typically performed by evaluating and maximising the heat recovery potentials within the individual process units. Once the potential improvements from the individual units have been assessed, the Total Site (TS) Heat Integration analysis is performed. Such approach may steer designers away from the promising retrofit opportunities and to lead towards suboptimal heat exchanger networks (HEN). This paper presents an effective retrofit framework for a TS system to determine the most cost-effective retrofit options and maximise the potential savings. Instead of performing the typical unit-wise process retrofit, the strategy is to determine the baseline total site consumption and benchmark targets, and to identify retrofit options from the TS context. This TS retrofit framework has been tested on a case study involving a petrochemical plant comprising of multiple process sections. The results of the analysis show that significant energy savings can be realised when both direct and indirect heat recovery retrofit options are evaluated. Further energy savings can be achieved via the Plus–Minus Principle that helps pinpoint the correct locations of heat surpluses and deficits and lead to the appropriate TS retrofit solution. As a conclusion, energy retrofit projects should be approached from the TS context, followed by the unit-wise retrofit (i.e., retrofit of the individual process sections).

Research on a Refrigeration Dehumidification System with Membrane-Based Total Heat Recovery

An independent air dehumidification system is helpful to improve indoor air quality and decrease energy consumption by heating, ventilation, and air conditioning (HVAC). A refrigeration dehumidification system with membrane-based total heat recovery is the key equipment to realize this goal. The system comprises two subsystems: a membrane total heat recovery and a direct expansion refrigeration system. The total heat exchanger has a membrane core where the incoming fresh air exchanges moisture and temperature simultaneously with the exhaust air. In this manner, the total heat or enthalpy from the exhaust air is recovered. Then the fresh air flows through a cooling coil where it is dehumidified below the dewpoint. Finally, the cold and dry air is supplied to indoors. A prototype of practical application is designed and fabricated. Experiments are conducted under variable operating conditions in the psychrometric calorimeter chamber. The effects of varying operating conditions like temperature and air humidity on the air dehumidification rate, cooling power, coefficient of performance, and compressor power are evaluated with indoor exhaust air dry bulb 27°C, wet bulb 19°C, and fresh air flow rate 200 m3/h. In comparison with a conventional refrigeration dehumidification system, the coefficient of performance and air dehumidification rate of the prototype are 2.3 times and 3 times higher, respectively. The performance of the prototype is rather robust under a hot and humid environment.

Poly (vinyl chloride)/montmorillonite hybrid membranes for total-heat recovery ventilation

Poly (vinyl chloride)/sodium montmorillonite (Na+-MMT) hybrid membranes with varied Na+-MMT content were prepared using casting processes for potential use in total heat recovery ventilation systems. The impact of Na+-MMT content on the morphology, thermal stability, mechanical property, gas transmission, pore diameters and total heat exchange effectiveness of the membranes were studied. It was observed that hybrid membranes with high content of Na+-MMT tended to have large roughness, improved thermal stability, high water vapor transmission as well as good temperature and enthalpy exchange effectiveness. With increasing Na+-MMT content, although the water desorption activation energy increased steadily together with the increased hydrophilic nature of the membranes, the water transmission rates increased dramatically caused by the introduction of larger pores in the systems. Balancing the requirement of low gas permeation, good water vapor and heat exchange effectiveness and mechanical properties, hybrid membranes with 60% Na+-MMT content seemed to have the highest potential to be used as Total Heat Recovery membranes in energy saving ventilation systems.

Total Site Targeting with Stream Specific Minimum Temperature Difference

The paper focuses on extending traditional Total Site Integration methodology to produce more meaningful utility and heat recovery targets for the process design. The traditional methodology leads to inadequate results due to inaccurate estimation of the overall Total Site heat recovery targets. The new methodology is a further development of a recently extended traditional pinch methodology. The previous extension was on the introduction of using an individual minimum temperature difference (ΔTmin) for different processes so that the ΔTmin is more representative of the specific process. Further this paper deals with stream specific ΔTmin inside each process by setting different ΔT contribution (ΔTcont) and also using different ΔTcont between the process streams and the utility systems. The paper describes the further extended methodology called stream specific targeting methodology. A case study applying data from a real diary factory is used to show the differences between the traditional, process specific and stream specific total site targeting methodologies. The extended methodology gives more meaningful results at the end of the targeting with this avoiding the over or under estimated heat exchanger areas in the process design.

Capital Cost Targeting of Total Site Heat Recovery

Exploiting heat recovery on Total Site level offers additional potential for energy saving through the central utility system. In the original Total Site Methodology (Klemes et al., 1997) a single uniform ΔTmin specification was used. It is unrealistic to expect uniform ΔTmin for heat exchange for all site processes and also between processes and the utility system. The current work deals with the evaluation of the capital cost for the generation and use of site utilities (e.g. steam, hot water, cooling water), which enables the evaluation of the trade-off between heat recovery and capital cost targets for Total Sites, thus allowing to set optimal ΔTmin values for the various processes. The procedure involves the construction of Total Site Profiles and Site Utility Composite Curves and the further identification of the various utility generation and use regions at the profile-utility interfaces. This is followed by the identification of the relevant Enthalpy Intervals in the Balanced Composite Curves. A preliminary result for evaluation of heat recovery rate and capital cost can be obtained.

Alkanes as fluids in Rankine cycles in comparison to water, benzene and toluene

Waste heat recovery from the exhaust gases of gas turbines, geothermal resources, and from industrial plants offers a great opportunity for rational energy usage by productively converting the waste energy. In the present communication we report a systematic study starting with the hydrocarbons for the different temperature ranges at which the waste heat is available from high, medium, or low temperature heat sources (viz., 773.15, 623.15 and 523.15 K). In the present work hydrocarbons from n-pentane to n-dodecane were investigated in comparison to water, benzene and toluene. Model calculations have been performed at various boiler pressures. Higher efficiencies are achieved at higher boiler pressures. In the low temperature range (523.15 K) hydrocarbons such as n-hexane, n-pentane are promising. If the heat is available at higher temperature (773.15 K) n-dodecane and toluene are the suitable fluids. In the case studies for the intermediate temperature range (623.15 K), it turns out that for many conditions, octane, heptanes and water are well suited working fluid for cogeneration systems. The results for the total heat recovery efficiency and the surface area of the heat exchanger have been discussed. T–H Diagrams were used to select the best possible conditions.

Sorption, permeation and selective transport of moisture/VOCs through a CA membrane for total heat recovery

This article studies in detail the permeation properties of highly permeable and highly selective polymer membrane for water vapor/volatile organic compound (VOC). A dense cellulose acetate (CA) membrane is prepared by the dry method. The sorption and permeation properties of water vapor and VOCs (acetic acid, formaldehyde, acetaldehyde, toluene and ethane) are experimentally investigated. Besides, the mass transfer coefficient of self-membrane can be separated from the total mass transfer coefficient. Using sorption and permeation data of gases, diffusion coefficient is calculated based on a solution–diffusion mechanism. The selectivity for water vapor/VOC is discussed and analyzed. The permeability selectivities of water vapor/VOC are all over 100. The result shows that the novel CA sense membrane is suitable for air dehumidification and total heat recovery. Copyright , Oxford University Press.

Effects of radiant terminal and air supply terminal devices on energy consumption of cooling load sharing rate in residential buildings

The energy consumption of ceiling radiant cooling combined with air supply system of different cooling load sharing rates (CLSRs) in residential buildings in summer was analyzed. Two air supply strategies were considered, including the all outdoor air ventilation strategy and the primary return air ventilation strategy. The appropriate CLSR range within the energy saving standard was discussed, and the appropriate ventilation strategies which could reduce system energy consumption effectively in summer were also analyzed. The results show that the energy consumption of the composite system reduces when the sensible heat load sharing rate (SHLSR) of the radiant terminal device (RTD) increases. In the all outdoor air ventilation strategy, when ωc (the SHLSR of RTD) varies in the range 45–100%, the system energy consumption can be controlled within the energy saving standard, and taking the total heat recovery unit is an effective way to reduce energy consumption of outdoor air handling. In the primary return air ventilation strategy, ωc=0.5 should be taken as a critical point. When ωc<0.5, the primary return air ventilation strategy is recommended to reduce system energy consumption; when ωc>0.5, the all outdoor air ventilation strategy is suggested.