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Dual Path

While conventional system designs can be used, they are not designed to produce the optimum humidity conditions needed in applications such as supermarkets and super-stores, as outdoor air is mixed with return air and then cooled and dehumidified. When forced to control humidity, their energy performance is usually poor, as they are typically run to cool all the air to a lower temperature to remove moisture. This supply air is then reheated, often using refrigeration waste heat reclaim, to avoid overcooling the store.

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To better handle these issues, special equipment is often considered, including:

  • Improved single path electric system, with a preconditioning coil,
  • Dual-path electric system which combines a separate ventilation and return air path in a single unit.
  • Advantages

    The dual path system:

    • Provides direct control of ventilation air quantity, regardless of VAV control on supply air to the conditioned space,
    • Provides excellent humidity control at all times, including part load, as moisture is removed at its source, regardless of building load,
    • Improves indoor air quality and mold and bacteria growth is reduced as there in no standing water in the drain pans. The intensive moisture condensation on the ventilation air coil creates abundant condensate flow, avoiding stagnant standing water.
    • Energy efficient while assuring an acceptable humidity level at all ventilation air volumes. Its use can also reduce demand and energy charges sufficiently to offset the higher first cost.
    • Can be combined with the refrigeration and water heating systems and the heat recovered from refrigeration used in water and space heating.

    Disadvantages

    There is a first cost premium since two coils and compressors are used.

    Applications

    Dual path systems can be installed separately or integrated with additional HVAC or refrigeration equipment. They are currently available in factory packaged units for indoor and outdoor installation. Heat pipe enhancements are available on some models.

    Best applications

    • Where the dual path system can be combined with the refrigeration and water heating systems; heat recovered from refrigeration can be used in water and space heating,
    • HVAC applications requiring good control of outdoor air quantity, and humidity control of supply air,
    • Supermarkets, hospital/health care areas requiring large ventilation air intake.

    Possible applications

    • Schools, commercial and office buildings where indoor air quantity must be increased and old CFC-using or inefficient equipment must be replaced.
    • Process or industrial applications requiring good control of outdoor air quantity, and humidity control of supply air.

    Technology types (resource)

    • Improved single path electric system uses a smaller volume of air is cooled to lower than conventional temperatures (about 40 to 45°F compared to 52 to 56°F), and the remaining air bypasses the cooling coil and is remixed before delivery to the space. This bypass design also permits a smaller compressor since the lower air flow compensates for the larger temperature drop through the coil. Using bypassed air also lowers reheat energy needs, while the smaller ducts and power wiring reduces first cost.
    • Dual-Path electric system uses two cooling coils to separately condition the incoming outdoor and return air. The hot and humid outdoor air and, in some cases, return air is directed to a primary coil for dehumidification, reducing moisture at the source by cooling air to 40 to 45°F. The secondary coil furnishes the sensible cooling of the relatively cool and dry return air as and if needed. These two air streams are then mixed, reducing the reheat energy needs, and supplied to the supermarket.

    Efficiency

    Dual path systems typically will reduce the HVAC installed tons by 15% to 23% over a conventional single path system. Dual path systems typically will reduce the annual energy use (kWh) between 14% and 27%. The reductions will depend on the location and the design parameters.

    Contact us to receive a copy of an EPRI report on Dual-Path Supermarket HVAC Systems or for more information.