The process of heat transfer through fluid-to-fluid heat exchangers is based on basic properties of heat conduction. When two independent chambers or circuits carrying fluids with a temperature difference are touching, the heat always moves to cold, trying to find equilibrium. Two important factors in the effectiveness of heat transfer are the amount of surface area that comes in contact with the heat source and the materials used to conduct the heat.
Over the past decade, advancing heat exchanger technologies have been penetrating the U.S. market. Paralleling the advances in technology are an increasing interest in the cost benefits of efficient use of energy resources, as well as other concerns about resource management. The efficient management of heat resources already available within the plumbing and mechanical systems is an effort to turn a “waste stream” into a “resource stream” or commodity.
The tube and coil Gravity Film eXchanger (GFX)system is installed on the drain line of a plumbing system in order to extract heat from draining wastewater, no matter how dirty it is.
Gravity Film eXchanger (GFX)
Gravity Film eXchanger (GFX) is a tube and coil drain heat recovery (DHR) system that works as a simple plumbing device. This well-engineered plumbing device transfers heat from drainwater to preheat incoming cold water through a copper supply line mechanically wrapped in a coil around a copper DWV pipe/tube. The key to the effective heat transfer is the large amount of copper surface area in contact with the thin, falling film of drain water. When wastewater drains through a vertical pipe, it does not fall down the center. Instead the water clings to the inner surface of the DWV pipe as it displaces the air in the pipe, taking the path of least resistance and obeying natural properties of surface tension. The water travels down the inner pipe walls in a thin, turbulent film that allows efficient heat transfer through the pipe wall. This falling film system will not work in the horizontal position; it must be vertical to distribute the hot fluid over a large surface area of copper drainpipe.
Tube and Coil Drain Heat Recovery
Drain Heat Recovery (DHR) is the capture of energy contained in drainwater that would otherwise be lost into the sewer system. Common sources include showers, sinks, clothes washers, dishwashers, and many other locations. The U.S. Department of Energy’s Energy Information Administration (EIA) estimated that in 1995, residential water heaters consumed 740 billion kWh of energy, and commercial water heaters consumed 320 billion kWh. Approximately 80-90% of this energy goes down the drain, carrying with it billions of kWh and billions of dollars. Efforts to recover this energy, such as the tube and coil GFX system, can result in significant energy savings. As hot water goes down the drain, the GFX efficiently transfers the heat through the walls of the copper drain line to the cold water as it simultaneously flows up through the supply coil. Depending on the amount of use and the size of the system, 50-85% of the drain heat can be transferred to the cold supply water through the copper without contamination. The heat recovery significantly preheats the incoming water, thereby reducing the work of water heaters and greatly reducing the amount of energy/money spent to heat water.
Two standard plumbing components make up the Gravity Film eXchanger (GFX) units. They are type L copper water supply tubing and copper Drain Waste Vent (DWV) pipe. The supply tubing is mechanically attached by tightly coiling it around the DWV pipe. This construction maintains the UPC-listed parts in the product, making it double walled with leak detection, vented through the atmosphere. The double-wall vented construction in GFX drain heat recovery systems is necessary to ensure public safety by preventing cross-contamination, and it is mandatory in food processing applications and most residential plumbing codes. The copper material used is economical, as well as being an excellent conductor of heat. The unique, non-welded construction is easy to install and maintain. The GFX is constructed in a large variety of sizes with additional options for customization. Lengths of 2″, 3″, and 4″ are the most common sizes for the DWV pipe and 5/8″ and 7/8″ OD (Outside Diameter) are the common sizes for supply tubing. Unit lengths typically range from 20″ to 80″. Manifolded units are commonly used for commercial applications. The variety and versatility of GFX systems can fit the designer and engineer’s imagination.
GFX as a heat exchange system can be used wherever sufficient temperature difference exists between fluids. There are many drain heat recovery applications in residential, commercial and industrial situations. Some of these are:
- Single family homes, apartment buildings, and multi-family developments.
- Hotels, hospitals, dormitories, and senior living facilities.
- Prisons and military facilities.
- Gyms and public showers.
- Beauty salons and laundries.
- Commercial kitchens, restaurants, food processors, etc.
- Cooling-walls, manufacturing, and process fluids in industrial applications.
The plumbing systems of such locations have the perfect mix of waste heat and demand for hot water to justify using GFX systems for preheating water.
The GFX is a counter-flow heat exchanger. In its most common application it is installed vertically as a section plumbing drain line. The cold supply line goes into the bottom of the GFX outer coil, and then the top of the coil is connected to the incoming cold water line just before the water heater, directly to the cold side of the shower fixture, or to both. The GFX recovers heat if the warm wastewater is flowing at the same time as the cold supply water. This is easy for showers and sinks; however, to recover heat from batch processes such as bathtubs, clothes washers and dishwashers there must be a second demand for cold water supply occurring as the batch is draining or shortly after. This can easily be done with a little effort in residential situations, and with some synchronization in commercial applications.
The typical household percentage of hot water sent down the drain from showers and running faucets is about 57%, based upon a published study from Lawrence Berkeley Laboratory.(2) DHR efficiency of 60% is cost-effective in many homes; corresponding to a 34% reduction in domestic hot water energy. This figure was used in a comprehensive study entitled, “Electric Water Heating Situation Analysis,” by A.D. Little, Inc. for the Edison Electric Institute and the U.S. Department of Energy.(3)
The economics of the GFXimproved with the number of daily showers in the residence. In general, buildings that require large amounts of hot water for showers would be ideal candidates for the GFX and would lead to shorter paybacks.
Example:Figuring the potential energy in shower drain water per 12-minute shower, here is the calculation:
Q = F x (To-Ti) x 8.34 x 12
Q = power available from shower drain water in Btu per shower (Btu/Shr)
F = flow rate down the drain in gallons per minute (gpm)
To = temperature of the drain water out (degrees F)
Ti = temperature of cold supply water (degrees F)
8.34 = the heat capacity of water in Btu per gallon per degrees F temperature difference (Btu/gal-degrees F)
12 = the national average minutes per shower
For a 3.0 gpm shower with 95 degrees F drain water (a 105 degrees F shower cooling to 95 degrees F on the way to the drain) and 50 degrees F cold water supply:
Q = 3.0 x (95 – 50) x 8.34 x 12
= 13,500 Btu per shower or 3.96 kilowatts
With 60% recovery efficiency, the GFX can recover 8,106 Btu or 2.38 kW per shower and deliver that energy to the water heater or directly to the cold side of the shower fixture. That 2.38 kW is enough energy to run more than 200 11-watt compact florescent light bulbs for an hour.