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. Lower in-store humidity (about 40%) made possible by specialized air conditioning systems means lower refrigeration energy costs. Reduced humidity directly reduces this energy use by reducing frosting and thus enhances coil heat transfer. To better handle these issues, special equipment is often considered, including:
- Improved single path electric system,
- Dual path electric system (discussed in a separate segment) which combines a separate ventilation and return air path in a single unit.
The improved single path system:
- Offers reduced supply air volume, bypass, and high efficiency air handler motors that provide reduced air handler demand and energy charges throughout the year,
- Substantially increases dehumidification performance,
- Reduces or eliminates reheat requirements
- Has maintenance costs proportionately lower than conventional systems due to the equipment capacity reduction,
- Achieves both low first cost and simplicity,
The dehumidification characteristics may not match modern supermarket loads exactly.
Single path systems can be used in both retrofit and new installations, separately or integrated with additional HVAC or refrigeration equipment. They are currently available in factory packaged units for indoor and outdoor installation.
- Their excellent operating performance and design simplicity make them a natural choice for supermarket designers and operators looking for a proven and reliable technology with no special maintenance requirements,
- HVAC applications requiring good control of outdoor air quantity and humidity control of supply air,
- They are a good choice for replacing conventional rooftop units being retired.
- Schools, stores, restaurants, commercial office and other buildings where humidity is a concern and old CFC-using or inefficient equipment must be replaced.
- Process or industrial applications requiring good humidity control of supply air.
Applications to Avoid
Where good control of outdoor air quantity, and humidity control of supply air is not required, the added cost of a single path system over the lowest cost system may not be offset by the benefits and the user will not be able to capitalize on the system's benefits.
The features that distinguish improved single path systems from conventional systems include:
- A mixture of fresh and return air is cooled by lower evaporator temperatures for better dehumidification; coil leaving air temperature of about 40 to 45°F compared to 52 to 56°F in conventional systems,
- Reduced total supply air volume, as low as 0.5 to 0.7 cfm per sq ft of sales area, compared to about 1.0 cfm per sq ft for conventional systems,
- Partial evaporator air bypass allows the air treated by the evaporator to be cooled to a lower temperature, increasing moisture removal and reducing or eliminating reheat requirements,
- The cooled and bypassed air is remixed before the heat recovery and heating coil and before delivery to the space. This bypass design also permits a smaller compressor since the lower airflow 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,
- Use of high efficiency air handler motors can reduce air handling energy by 10% or more,
- Careful sizing to avoid excess capacity,
- Improved part load controls with face-split cooling coils that permit a portion of the coil to be operated at a lower temperature while air is bypassed around the rest of the coil.
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