ASHRAE Journal - November 2011 - 58

EMERGING TECHNOLOGIES

Drain Water Heat Recovery
By Alissa Cooperman; John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE

E

nergy consumption for water heating accounts for 17% of residential and 7% of commercial site energy consumed (13% of residential

and 4% of commercial primary energy consumed).1,2 A study conducted

by the U.S. Department of Energy (DOE) found that approximately 350 billion kWh of energy is sent down the drain in the form of used hot water annually.3 The use of drain water heat recovery (DWHR), also known as graywater heat exchanger (GWHE), technologies can reduce the energy consumed for water heating by recovering some of this energy, and recycling it to preheat incoming water.
At the time it is sent down the drain, used hot water on average still contains 80% to 90% of the thermal energy it contained (relative to the cold supply water) when it left the water heater. By using a heat recovery heat exchanger between drain (graywater) and incoming fresh water, up to 40% of this wasted energy can be saved, reducing wasted energy to 49% to 54% of energy used to initially heat the hot water demanded.4,5 DWHR systems use drain water to heat a building’s incoming cold water supply. This is accomplished by a copper heat exchanger, which takes advantage of the film flow of drain water through vertical pipes and the unused heat energy in the drain water. DWHR units are available in two types: storage and on-demand. Storage units (Figure 1) consist of a submerged copper heat exchanger in a tank of fresh water. Warm drain water flows through the heat exchanger heating
58 ASHRAE Journal

the tank water for later use. This enables the technology to be used with devices that do not continuously demand, or simultaneously demand and drain water, such as clothes washing machines and dishwashers. The warmed water in the
Drain Water In

storage tank unit provides preheated inlet water for the water heater.6 An on-demand DWHR unit (Figure 2) replaces a vertical segment of drain pipe. An inner copper pipe 3 in. to 4 in. (76 mm to 102 mm) in diameter wrapped in external 0.5 in. (13 mm) copper pipe(s) comprises the system.Warm drain water flows down the inner pipe while incoming cold supply water flows up the external copper piping. The outer pipes are slightly flattened where they abut the inner pipe, and are soldered to the inner large diameter pipe to improve the thermal connection between the two conduits. On-demand units take advantage of the drain water film flow down the inner walls of the vertical pipe. Unlike horizontal drain pipes, in which the water flow touches half of the inner surface of the pipe, the water flow in vertical drain pipes coats all of the inner surface area in a thin film. The thin film results in a reasonably high heat transfer coefficient

Warm Water Out

Water Storage Tank

Cold Water In Drain Water Heat Exchanger

Drain Water Out

Figure 1: Drain water heat recovery storage unit.
ashrae.org November 2011



ASHRAE Journal - November 2011

Table of Contents for the Digital Edition of ASHRAE Journal - November 2011

ASHRAE Journal - November 2011
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Commissioning High-Tech Facilities
Streamlining Energy Simulation to Identify Building Retrofits
Technology Award Case Studies:
Laundry Upgrade
Creating a Healing Environment: Efficient Hospital
Standing Columns
Emerging Technologies
Products
Special Supplement: BACnet® Today and the Smart Grid
Commentary
BACnet: Operator Workstations
BACnet Alarming Revised
Information Model Standard for Integrating Facilities with Smart Grid
Demand Response and Standards: New Role for Buildings in the Smart Grid
Getting Smart on the Electrical Grid
Washington Report
Special Section
InfoCenter
IAQ Applications
Classified Advertising
Advertisers Index
ASHRAE Journal - November 2011 - ASHRAE Journal - November 2011
ASHRAE Journal - November 2011 - Cover2
ASHRAE Journal - November 2011 - 1
ASHRAE Journal - November 2011 - 2
ASHRAE Journal - November 2011 - Contents
ASHRAE Journal - November 2011 - Commentary
ASHRAE Journal - November 2011 - 5
ASHRAE Journal - November 2011 - Industry News
ASHRAE Journal - November 2011 - 7
ASHRAE Journal - November 2011 - Letters
ASHRAE Journal - November 2011 - 9
ASHRAE Journal - November 2011 - 10
ASHRAE Journal - November 2011 - 11
ASHRAE Journal - November 2011 - 12
ASHRAE Journal - November 2011 - 13
ASHRAE Journal - November 2011 - 14
ASHRAE Journal - November 2011 - 15
ASHRAE Journal - November 2011 - Meetings and Shows
ASHRAE Journal - November 2011 - 17
ASHRAE Journal - November 2011 - Commissioning High-Tech Facilities
ASHRAE Journal - November 2011 - 19
ASHRAE Journal - November 2011 - 20
ASHRAE Journal - November 2011 - 21
ASHRAE Journal - November 2011 - 22
ASHRAE Journal - November 2011 - 23
ASHRAE Journal - November 2011 - 24
ASHRAE Journal - November 2011 - 25
ASHRAE Journal - November 2011 - 26
ASHRAE Journal - November 2011 - 27
ASHRAE Journal - November 2011 - 28
ASHRAE Journal - November 2011 - 29
ASHRAE Journal - November 2011 - 30
ASHRAE Journal - November 2011 - 31
ASHRAE Journal - November 2011 - Streamlining Energy Simulation to Identify Building Retrofits
ASHRAE Journal - November 2011 - 33
ASHRAE Journal - November 2011 - 34
ASHRAE Journal - November 2011 - 35
ASHRAE Journal - November 2011 - 36
ASHRAE Journal - November 2011 - 37
ASHRAE Journal - November 2011 - 38
ASHRAE Journal - November 2011 - 39
ASHRAE Journal - November 2011 - 40
ASHRAE Journal - November 2011 - 41
ASHRAE Journal - November 2011 - 42
ASHRAE Journal - November 2011 - 43
ASHRAE Journal - November 2011 - Laundry Upgrade
ASHRAE Journal - November 2011 - 45
ASHRAE Journal - November 2011 - 46
ASHRAE Journal - November 2011 - 47
ASHRAE Journal - November 2011 - 48
ASHRAE Journal - November 2011 - 49
ASHRAE Journal - November 2011 - 50
ASHRAE Journal - November 2011 - 51
ASHRAE Journal - November 2011 - Creating a Healing Environment: Efficient Hospital
ASHRAE Journal - November 2011 - 53
ASHRAE Journal - November 2011 - 54
ASHRAE Journal - November 2011 - 55
ASHRAE Journal - November 2011 - 56
ASHRAE Journal - November 2011 - 57
ASHRAE Journal - November 2011 - Emerging Technologies
ASHRAE Journal - November 2011 - 59
ASHRAE Journal - November 2011 - 60
ASHRAE Journal - November 2011 - 61
ASHRAE Journal - November 2011 - 62
ASHRAE Journal - November 2011 - 63
ASHRAE Journal - November 2011 - Products
ASHRAE Journal - November 2011 - Special Supplement: BACnet® Today and the Smart Grid
ASHRAE Journal - November 2011 - B2
ASHRAE Journal - November 2011 - B3
ASHRAE Journal - November 2011 - Commentary
ASHRAE Journal - November 2011 - B5
ASHRAE Journal - November 2011 - BACnet: Operator Workstations
ASHRAE Journal - November 2011 - B7
ASHRAE Journal - November 2011 - B8
ASHRAE Journal - November 2011 - B9
ASHRAE Journal - November 2011 - B10
ASHRAE Journal - November 2011 - B11
ASHRAE Journal - November 2011 - BACnet Alarming Revised
ASHRAE Journal - November 2011 - B13
ASHRAE Journal - November 2011 - B14
ASHRAE Journal - November 2011 - B15
ASHRAE Journal - November 2011 - B16
ASHRAE Journal - November 2011 - B17
ASHRAE Journal - November 2011 - Information Model Standard for Integrating Facilities with Smart Grid
ASHRAE Journal - November 2011 - B19
ASHRAE Journal - November 2011 - B20
ASHRAE Journal - November 2011 - B21
ASHRAE Journal - November 2011 - B22
ASHRAE Journal - November 2011 - Demand Response and Standards: New Role for Buildings in the Smart Grid
ASHRAE Journal - November 2011 - B24
ASHRAE Journal - November 2011 - B25
ASHRAE Journal - November 2011 - B26
ASHRAE Journal - November 2011 - B27
ASHRAE Journal - November 2011 - B28
ASHRAE Journal - November 2011 - Getting Smart on the Electrical Grid
ASHRAE Journal - November 2011 - B30
ASHRAE Journal - November 2011 - B31
ASHRAE Journal - November 2011 - B32
ASHRAE Journal - November 2011 - B33
ASHRAE Journal - November 2011 - B34
ASHRAE Journal - November 2011 - B35
ASHRAE Journal - November 2011 - B36
ASHRAE Journal - November 2011 - Washington Report
ASHRAE Journal - November 2011 - InfoCenter
ASHRAE Journal - November 2011 - 67
ASHRAE Journal - November 2011 - 68
ASHRAE Journal - November 2011 - 69
ASHRAE Journal - November 2011 - 70
ASHRAE Journal - November 2011 - 71
ASHRAE Journal - November 2011 - 72
ASHRAE Journal - November 2011 - IAQ Applications
ASHRAE Journal - November 2011 - 74
ASHRAE Journal - November 2011 - 75
ASHRAE Journal - November 2011 - 76
ASHRAE Journal - November 2011 - 77
ASHRAE Journal - November 2011 - Classified Advertising
ASHRAE Journal - November 2011 - 79
ASHRAE Journal - November 2011 - Advertisers Index
ASHRAE Journal - November 2011 - Cover3
ASHRAE Journal - November 2011 - Cover4
ASHRAE Journal - November 2011 - 121
ASHRAE Journal - November 2011 - 122
ASHRAE Journal - November 2011 - 123
ASHRAE Journal - November 2011 - 124
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