Detailed Design and Testing of Low Humidity Air Conditioning Unit

Photo of Cole Carlson, Daniel McCalib, Derek Weber Students: Cole Carlson, Daniel McCalib, Derek Weber

Sponsor: Austin Management Group

Date: Fall 2009

Requirements:
The air handler unit the design team used for testing is a new custom built prototype. The unit featured the sponsor's patent pending idea of a cross counter-flow evaporator coil. A 2.0 ton 14.5 SEER Goodman condenser unit was used that operated with refrigerant R-22. For the performance of this unit to be considered adequate, the main requirement is that the system must deliver supply air between 40°F and 45°F and maintain 40% relative humidity in the room. Also, the unit must consume less energy than a standard air conditioning unit.

Problem:
A new air conditioner design reduces energy consumption by maintaining a comfortable living environment at higher temperatures and lower humidity. For this project, the team performed detailed design and testing of the low humidity air conditioner unit developed in previous semesters. The focus was to optimize the blower configuration and its operation. This required an economic analysis and performance testing to determine if a design change was warranted to replace the existing variable frequency drive with a multi-speed blower configuration. The other aspect of the project was to write a proposal to Austin Energy to award rebates to air conditioning units that save energy through improved dehumidification performance.

Solution:
A controller to cycle the blower between high and low speeds was installed. A thermal switch mounted just after the evaporator coil on the return line activated a relay to switch blower speeds. To obtain maximum dehumidification without freezing the evaporator, it was determined the low speed should achieve a flow rate of 200-250 cfm. This low air flow through the evaporator cools the return line to 35°F before cycling back to high speed to avoid sending liquid refrigerant into the compressor. Testing demonstrated that similar results were obtained in the ability of the unit to dehumidify with both the multi-speed blower control and the VFD configurations. This warranted a design change to replace the costly variable frequency drive with a less expensive multi-speed blower control.

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