Heat-pump clothes dryer

At the Nyle Corp. in Bangor, Maine, there are a half-dozen prototype electric clothes dryers that claim some impressive attributes:

  • They are not vented, so they can be put nearly anywhere.
  • They run on 110-volt circuits; most electric dryers require 220 volts.
  • They circulate a larger volume of more humid air; thus they are gentler to fabrics and reduce static cling.
  • The cabinets are insulated, which makes the machines very quiet.
  • Best of all, they use one-third the energy of a standard electric dryer.

What are these machines? “Basically they are dehumidifiers,” explains Donald Lewis, president of Nyle and holder of the relevant patents. He explains: “Using a refrigeration cycle similar to what is used in a heat pump, we cool the air and condense out the water. The latent heat removed in condensing the water we immediately put back into the air. In addition, we constantly add heat from the compressor. And while most clothes dryers vent about five thousand cubic feet of air per drying cycle, we don’t vent any air, so the temperature keeps rising.”

Just as a heat pump in most climates will heat a house more efficiently than will electric-resistance elements, a heat-pump (or dehumidifying) clothes dryer will burn less energy to dry clothes. The diagram and caption give the details.

Nyle based the clothes-dryer design on much bigger machines.

“We make commercial lumber- and leather-drying systems and food dehydrators,” says Lewis. Looking for ways to expand the business, he decided the technology could be used in a residential clothes dryer. So he applied for and got a $90,000 U.S. Department of Energy grant (from the Inventions Program in the Office of Energy Utilization) to build the prototypes.

“There have been heat-pump dryers on the market in Europe,” Lewis points out, “but they’ve never been popular because they take a long time to dry a load of clothes.” That’s because they don’t get very hot.

Like most refrigeration systems, they are engineered to work within a narrow temperature range. Lewis explains: If the system is optimized to work at relatively low air temperatures, then as the temperature gets higher all the refrigerant inside the evaporator coil will boil and turn to gas without absorbing enough heat from the passing air to drop it to its dew point and condense out some water vapor. For this reason the European dryers have a top temperature of about 125 degrees F.

On the other hand, if the system is optimized to work at a relatively high air temperature, then at the beginning of the cycle when the temperature is low, the evaporator would chill the air so much that the coil would freeze. Lewis’s solution is to send only part of the air over the evaporator and to vary the percentage in response to changing temperature. So when the air contains fewer Btu, a greater volume of air is delivered. Conversely, when it contains more Btu, the quantity of air is reduced.

The dryer can use either automatic dampers (as shown) or a variable-speed fan at the evaporator coil to alter the airflow pattern. It operates efficiently within a temperature range of 60 to 160 degrees F, Lewis claims.

David Mello, the DOE invention coordinator who supervised the agency’s grant to Nyle, speaks highly of the heat-pump dryer. “It’s a fine product and makes a lot of sense for a lot of good reasons,” he notes. “Refrigeration systems are among the most trouble-free machines we have: You plug them in and they work. Except for the fact that it uses a chlorofluorocarbon,” Mello goes on, “the Nyle dryer really doesn’t seem to have a down side.” (Chlorofluorocarbons are implicated in the destruction of Earth’s protective ozone, but the dryers can be engineered to use whatever replacement fluids are developed, according to Lewis.)

Nyle had arranged for an offshore manufacturer to make the dryers, but economic problems in the country sank the deal. Now new negotiations are in progress.

A heat-pump dryer will sell for around $800, Lewis predicts, about twice the price of a conventional dryer. “People would have to pay more for them at first,” Mello concurs. “But if they ever caught on, the guys making the conventional kind would be hard pressed to compete.”

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