Are You Wasting Heat Energy?
Exploring the pros and cons of wastewater heat recovery systems

By John Pabalan

Author’s Note: This article was written in memory of Lee “Skip” Kemberling, my friend and mentor; a man who dedicated his life to helping the laundry industry.

When evaluating the overall efficiency of a plant, one of your key questions should be: How much of the energy expended contributes to wasted heat? If the heat energy in your plant can be recovered and returned to the system, the energy is not wasted. Is your plant wasting precious heat energy? Do you have: wastewater discharge to the sewer, boiler flue gas, condensate tank vent lines, reuse water heat, or gas-fired dryer exhaust?

In these instances, with the exception of gas-fired dryer exhaust, wasted heat can be used to preheat or maintain temperatures in a laundry’s process water system. One area that has substantial recoverable energy is the wastewater discharged from industrial laundering machines. Every plant has the potential for recovery. However, not every plant has installed the means to recover the heat.

Wastewater Heat Recovery Systems
The most common means for industrial wastewater heat recovery are shell and tube heat exchangers and plate and frame heat exchangers. In both instances, wastewater is pumped from a surge and flow control pit (a reservoir for balancing the wastewater flow to sewer), preheating the incoming fresh water supplied for the cleaning process.

Shell and tube heat exchangers and plate and frame heat exchangers are classified as single pass, counter-flow, and indirect type heat exchangers. The wastewater moves through the tubes (or plates) of the heat exchanger in a flow direction that is opposite the water on the outside of the tubes in the shell, which yields the maximum potential for heat recovery. How much heat is recovered utilizing these types of systems depends on the following:

Under normal design conditions, the average attainable temperature (fresh water temperature vs. the temperature of the wastewater) is 10°F for shell and tube systems and 5°F for plate and frame systems. The square footage for heat transfer is determined to satisfy certain design criteria including waste and fresh water flows, approach temperature, and is used to determine design performance. Water velocities in the heat exchangers of either type affect the efficiency of the transfer of heat. The propensity for fouling of each system is the indeterminate variable when selecting the type of heat exchanger best suited to your operation.

Pitfalls
In both systems pit screens are employed with no more than one-half-inch spacing or a hole-size protecting sump pumps from the occasional rag or piece of linen moving into the pump impeller. Shell and tube manufacturers utilize three-quarter-inch inside diameter tubes that will pass solids up to this size. These are proven for reliability without pre-screening. However, plate and frame heat exchangers require a minimum 200-micron filter for prescreening to prevent passage blocking. This is the design weakness of plate and frame systems.

Shell and tube heat exchanger designs employ a valve for reversing the flow of the wastewater through the heat exchanger. This is an inherent weakness in the system because the loss of recoverable heat is about 30% to 40% when reorienting the flow from counter flow to parallel flow. This system flaw can be avoided by using a similar valve on the fresh water side to ensure that the system always remains in counter-flow orientation. When comparing different manufacturers and different designs, pay special attention to design criteria and performance. Additional focus should be given to the differences in projected operating and maintenance costs for each of the systems.

While plate and frame systems have a potentially more desirable approach to temperature, it is necessary to operate and maintain two additional motors to chip away at the 5°F difference. The two motors are used to operate the screening device and the additional pump. These additional motors require maintenance, which is not an integral part of shell and tube designed systems.
If incoming city water is 40°F and the outgoing wastewater is 120°F, there is the potential to capture the heat and preheat the process water to 110°F. Accounting for heat losses inherent in boiler operations and heating water with steam, average water heating efficiency is 65% to 70%. For example, in an average-sized healthcare plant producing 500,000 lbs. to 700,000 lbs. per week, the water-heating load at cold temperature would require approximately 92 therms of gas per hour, vs. 33 therms per hour if the heat was recovered. The equivalent savings is approximately 64% of the process water-heating load. This example yields an approximate 6-month installed return on investment based on fuel priced at 81 cents per therm.

Considerations
There are additional factors that impact the design and necessity of wastewater heat recovery systems. Some local authorities have restrictions on the temperature at which wastewater can be discharged; where these are present, 120°F is a common maximum. Some POTWs have restrictions on Total Suspended Solids. In this case, it makes sense to pre-screen the water prior to recovering the heat. If this is a possibility, careful consideration should be made when selecting screening devices.

At what threshold does wastewater heat recovery make sense? Some causes for consideration should include:

• Efficiency
• Temperature
• Fuel pricing
• The two-year installed payback rule
• If your laundry plant is producing
  1,000 lbs. per hour
• Processing 65 hours or more per week

Wastewater heat recovery has been around for decades with many proven designs that should be evaluated.

Editor’s Note: This story is the first in a series exploring improvements in plant efficiency. TR

John Pabalan is vice president of operations for American Laundry Systems, a division of E&O Mechanical, Haverhill, MA. Contact him at his New York office at 646/734-4323 or e-mail jpabalan@eomech.com.