Presentation on theme: "Enthalpy Wheel Introduction Enthalpy wheels are starting to become a common device in Heating, Ventilating and Air Conditioning (HVAC) Systems. They transfer."— Presentation transcript:
Enthalpy Wheel Introduction Enthalpy wheels are starting to become a common device in Heating, Ventilating and Air Conditioning (HVAC) Systems. They transfer heat and humidity between the exhaust and supply air. Their net effect is to bring the supply (incoming) air closer in temperature and humidity to the exhaust air and reduce the load on the heating and cooling systems. A well-designed enthalpy wheel will recover 60% to 80% of the energy that would otherwise be needed to heat or cool outside air. It lowers building operating costs and the capital cost of cooling and heating equipment as smaller devices can be installed. The typical payback period of an enthalpy wheel in Southern Ontario is five years.
Description An enthalpy wheel is positioned so that the exhaust air and the supply air travel through it in opposite directions through separate ducts (see diagram). The incoming air travels through the wheel before it enters the rest of the HVAC system. The wheel rotates at a rate of one revolution every one to six seconds. As it rotates, it absorbs heat and moisture from the warmer air stream and loses heat and moisture to the cooler air stream. Usually this means the supply air is warmed and humidified in the winter and cooled and dehumidified in the summer. The same device is effective all year around at pre-conditioning the outdoor air. Enthalpy Wheels range from one to ten metres in diameter depending on the volume of air that must travel through them. Their rotation speed is variable and dependent on outdoor temperature and humidity. For example, in the Integrated Learning Centre, when the temperature outside in near room temperature, the wheel will shut down and air will travel through a separate duct. When the wheel will not save energy for the heating and cooling systems, energy is not wasted turning the wheel. Most wheels consist of an aluminum honeycomb structure within a supporting frame. The honeycomb structure provides maximum surface area, least air pressure drop, light weight and high structural strength. The aluminum is coated with a desiccant such as silica gel that can continually absorb and release moisture.
Rotary Air-to-Air Energy Exchanger Enthalpy wheel
Constant Volume Dual Duct System Fresh air Rotary Energy Exchangers Heating coil Cooling coil Cool and fresh air to greenhouse Return air form Greenhouse Exhaust Preheater Filters Hot and fresh air to Greenhouse
What Does It Do? The energy recovery wheel performs opposite processes in the cooling season and the heating season. When the outside air is hot and humid, the wheel transfers both heat and humidity from the incoming air to the exhaust air, thus decreasing the amount of cooling needed by the air handler. On the opposite end of the spectrum, when the outside air is frigid and dry, the wheel transfers heat and humidity to the incoming air from the exhaust air, thus decreasing the amount of heating and humidification needed from the air handlers.
How Does It Work? The wheel is designed to spin between the outdoor and the return air streams. It is constructed entirely of aluminum, with large spokes to support the desiccant-coated aluminum filter. As the transfer core rotates between outdoor and return air streams, the higher temperature air stream gives up its sensible energy to the aluminum. This energy is then transferred to the cooler air stream during the second half of the revolution. Just as the temperature is captured and released, so is the latent energy. This is accomplished by the desiccant coating of the wheel. The 3Å molecular sieve desiccant has a very strong affinity for water and an enormous internal surface area to bind the water on its surface. Since the opposing air streams have different temperatures and moisture contents, the vapor pressures on their surfaces differ. This vapor pressure differential is the driving force necessary for the transfer of water vapor. Additionally, the wheel is designed for high heat recovery effectiveness with low pressure loss. This makes the wheel a viable option for energy recovery because it does not cause a significant increase in fan power to reach the desired supply air pressure.
How Is It Controlled? The energy recovery wheel is monitored by a rotation detector which determines the angular velocity of the core. This sensor, along with air temperature sensors installed on either side of the wheel in both air streams, feed back to a solid-state controller which instructs a variable frequency drive connected to the wheel motor. This VFD commands the motor to speeds between ¼ and 20 rpm to most efficiently transfer total energy between the air streams.air temperature sensorssolid-state controllervariable frequency drive
How Is The Wheel Effectiveness Calculated? The effectiveness of the energy recovery wheel gives us a scale of how well the wheel is conserving building energy. This quantity is technically defined as the amount of energy recovered divided by the maximum energy that could theoretically be recovered. While effectiveness can be calculated for both sides of the wheel, the return air effectiveness is typically the determinate of efficiency. The effectiveness is calculated as: where: vr is the return air volume rate; vmin is the lesser of the air flow rates; and X is the temperature,