Additional Math Pages & Resources

Wednesday, September 2, 2009

Hot and bothered

September in Southern California. I'm thinking energy, as I hang clothes out in the hot sun then duck back in the house.

Question 1 How do we compare efficiency of clothes dryers?

Question 2 What is the unit of measure for drying clothes?

    Answer 1 We don't.

    The Energy Star program was started by the US EPA in 1992. It compares and labels energy-consuming products, but doesn't include dryers. Apparently dryers of all sorts, as long as they are the same size, use virtually the same amount of energy.

    The European Union already rates dryers on energy consumption, capacity, and noise. Soon similar REGULATIONS will be coming to the US!

    All dryers use a motor to spin a drum filled with the wet clothing. The motor drives a fan which blows air through the clothes. The air is heated by a gas flame or an electric element. The features that vary between models are mostly fancy controls. (A few rare non-vented dryers use heat pump or condensation systems.)

    In most places in the US, gas dryers are cheaper to operate than electric ones. That doesn't necessarily mean more efficient, but reflects the price of natural gas versus electricity.

    Answer 2 - The unit of measure for clothes drying is kilograms of clothing per kilowatt/hour.

    In an earlier post called  How Bright Are We?, we learned a kilowatt is 1000 watts of electrical energy consumed for an hour. For example,  twenty 50-watt light bulbs glowing for 60 minutes = 1 kwh.

    Dryer Testing is done at 68 degrees F, 65% humidity with a mixed load at 70-90% moisture level. Drying stops when fabrics are at 3-6% moisture level. State-of-the-art dryer efficiency is about .7 kwh/kg, which translates to 1 kwh/3 lbs.

    In plain language, that means our US dryers take 3 pounds of clothing from wet to dry = 1 kwh.

    US average = (7 lbs/ load @ 3 lbs/kwh) = 2.33 kwh per load.
    Assuming 416 loads a year x 2.33 kwh = 967 kwh/yr
    With electricity at $.15-.30 per kwh, that's $145-300.

    Translated to real world numbers, that means 5-10¢ a pound.

    What about a gas clothes dryer? The same unit of energy is used. Somehow. 

    Here's a random number - the average US household runs the dryer 140 hrs/yr, or 22 min/day.


    Choose the dryer that matches your washer or your room (size, features, color, etc.). You can do a few things to keep it running well:
    • Clean the lint screen before every load
    • Clean lint out of the air vent tube
    • Fill to the proper level with clothes spun til most of the moisture is out
    • Make sure the laundry room is well-ventilated and dry
    • Run your dryer when rates are lower, if you have the chance
    Hot stuff, eh?

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