Enthalpy wheel

Outdoor air is treated and pre-conditioned by a central water-to-air heat pump. The outdoor air is delivered into the return air plenum of the zone heat pump units. A total enthalpy wheel can reduce the energy consumed for treating outdoor air. 

In the majority of installations, a flapper damper or diverter valve is employed to vary the flow across the heat exchanger to maintain a specific design temperature of the hot water or a specific steam generation rate. In some CHP designs, the exhaust gases are used to activate a thermal enthalpy wheel or a desiccant dehumidifier. A thermal wheel uses the exhaust gas to heat a rotating wheel with a medium that absorbs the heat and then transfers the heat into the incoming airflow into which the wheel is rotated. 

Most common humidifier devices are used mainly for air hydration, but sometimes also for hydrogen humidification, and are based on bubblers, water evaporators, enthalpy wheels, membranes, or on a pump for liquid water injection inside a mixer or directly inside the first part of the cathode side collector. Membrane humidifiers or injection pumps have been prevalently proposed for hydrogen humidification. 

During start up, when the stack temperature and then performance (voltage) are low the passive BOP devices devoted to stream and stack humidification (enthalpy wheels membrane humidifiers) do not work as they are essentially based on the difference of temperature and then are practically excluded by the FCS management on the other hand, at low temperature the requirements for stack humidification are strongly reduced and membrane could self-hydrate if a sufficient water production (proportional to power requirement) occurs. 

With a lower temperature, the turbine is best used by allowing the baek-pressure to fall and thus obtain more power. In radial inflow turbines, the relative veloeity at the turbine inlet is small. Any ehanges are, therefore, far less signifieant than with high relative veloeity impulse wheels. Commonly, a turboexpander tolerates as mueh as a 30% ehange from its designed enthalpy. The effeet on effieieney was shown earlier in Figure 3-12. 

Fig.3 Schematic representation of a macroscopic ratchet, structure of the molecular ratchet 6 and calculated enthalpy changes for rotation of the triptycene wheel around the helicene pawl [21,22]...

Similar phenomena characterize the clathration behavior of the larger host tris(2,3-naphthalenedioxy)cyclotriphosphazene 13). Due to the boIki tack-bone, this host forms, in relation to the former structure type, a markedly expanded (withchaimelsof diameter 9-10 A at the narrowest point) and thus less stable structure. Evidently, the driving force for clathration of the substituted cyclotriphosphazene species is associated with their unusual paddle-wheel like shape. Van der Waals packing of these molecules in a high symmetry arrangement leaves substantial voids in the lattice which are fillta by guest components to lower the enthalpy of the total system. The potential use of this clathration behaviour for separation of hydrocarbons was envisioned already about fifteen years ago... 

When discussing efficiencies, the two most important design parameters are the isentropic enthalpy drop across the expander and the volumetric flow rate at the expander outlet. Stress limits the rotor tip speed to a certain extent, but more often the speed is determined by the enthalpy drop. The outlet volumetric flow rate likewise controls the expander wheel flow area. These parameters within mechanical limits determine the basic configuration of the hydraulic channel, which in turn directly affects the turboexpander efficiency. 

The dominating principle in turbine design involves expression of the efficiency of the energy conversion in nozzles and buckets or in reaction blades, usually referred to as stage efficiency, as a functon of the ratio u/C. The blade speed u, feet per second, is calculated fi em the pitch diameter of the nozzle and thus determines the size of the wheel at a given number of revolutions per minute and C, also in feet per second, is the theoretical velocity of the steam corresponding to the isentropic enthalpy drop in the stage, expressed by the formula... 

Let us assume, as an example, a blade speed of500 ft/s, corresponding to a turbine wheel with 32-in pitch diameter operating at a speed of 3600 rpm the optimum steam velocity would be 500/0.45 = 1100 ft/s. The kinetic energy of the steam may be expressed in Btu by the relation Btu = (C/223.8) = 1100 /50,000 = 24 thus the enthalpy drop utilized per stage at the point of maximiun efficiency is about 24 Btu for the above condition. 

The approximate size of the unit may be arrived at by the quality-factor method referred to in Fig. T-72. By applying an appropriate-size factor, the topping turbine may in this case be designed for an efficiency of, say, 67 percent, corresponding to a quality factor of about 4500. With an available enthalpy drop of 147 Btu the sum of the velocity squares is 660,000. Because of the comparatively small volume flow and the high density of the steam, small wheel diameters are used thus the bucket speed is rather low. If we assume, for instance, 350 ft/s, corresponding to about... 

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