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Controlling Indoor Humidity Using Variable-Speed Compressors and Blowers

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Title: Controlling Indoor Humidity Using Variable-Speed Compressors and Blowers
Author(s): Andrade, M.A.; Bullard, C.W.
Subject(s): stationary air conditioning system analysis
Abstract: Residential-scale ale systems control indoor temperature but not humidity. The advent of variable-speed compressors and blowers make such control possible. This paper uses a detailed simulation model to explore the tradeoff between energy efficiency and comfort, as measured by indoor humidity. Simultaneous solution of heat and mass transfer equations for the building, along with simulation of the air conditioner, makes it possible to calculate indoor humidity as a function of outdoor dew point and infiltration rate. The results presented in Chapter 4 show how indoor humidity can be lowered by decreasing the evaporator air flow rate, because the fin temperatures are reduced and remove more condensate. In a moderate climate condition (75°P indoor temperature, 80°P outdoor temperature and 700P outdoor dew-point temperature), indoor relative humidity can be reduced from 61 to 57% by reducing the blower speed from 500 cfm/ton to 200 cfm/ton. In addition to the extra comfort, system power input, averaged over the on/off cycles, decreases for most climate conditions with this blower speed reduction. In a moderate climate (75/S0/70), average power drops from 0.S3 to 0.77 kW, a 7% decrease in power usage in addition to the increase in comfort. Reducing the compressor speed, and therefore refrigerant mass flow rate, increases EER by utilizing the heat exchangers for longer runtimes at lower Ll T's. However the refrigerant-side LlT becomes a larger fraction of the total LlT, so the effect of reducing blower speed is diluted. Overall, the system runs more efficiently but for a longer time, removing the same sensible load and latent load, and preventing shortcycling that would increase indoor humidity. In the baseline climate (75/S0/70) halving the compressor speed and reducing the blower speed from 500 to 200 cfm/ton has a negligible effect in indoor humidity, and reduces average power 33% from 0.S3 to 0.56 kW.
Issue Date: 1999-07
Publisher: Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report: Air Conditioning and Refrigeration Center TR-151
Genre: Technical Report
Type: Text
Language: English
URI: http://hdl.handle.net/2142/11894
Sponsor: Air Conditioning and Refrigeration Project 69
Date Available in IDEALS: 2009-05-22
Identifier in Online Catalog: 4259499
 

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