Hot Tip #18 – One More Degree

Frequently when I’m talking to laundry operators or their engineers, I ask them what it would be worth to them if they could recover one more degree with their wastewater heat recovery system. The most common response is “Gee, I don’t know”. When encouraged to make a guess, their response is something like “I bet it’s a lot, maybe several hundred dollars a year”.

Since effort will be needed to recover this one more degree, I think it is important to calculate its’ value, to justify the recovery effort.

The calculation is:

(Pounds of water discharged down the drain yearly)  Divided by (1,000,000) Multiplied by  

(fuel cost, per 1,000,000 BTU’s) Divided by (overall thermodynamic efficiency)

Note:

• Water discharged is about 3% less than water purchased.
• Multiply gallons by 8.34 to get pounds.
• If you do not know your overall thermodynamic efficiency and you think you are doing a really good job at controlling fuel usage, use 80%. (read chapter 12 of TRSA’s Textile laundering Handbook)

Example:

(30,000,000 gallons per year) X (8.34#/gal) / (1,000,000) X ($11.00 MM BTU) / (80% Eff.) =

$3,440.25 per year. This number will be higher if you are not doing a real good job at controlling fuel usage.

As you can see, there is ample justification to expend effort to recover one more degree.

How many degrees are available for recovery?

If your discharge temperature is constantly within 10 degrees Fahrenheit of your incoming water temperature, you are doing very well! Many plants have reached this level. The best I have seen is a six-degree “cold end approach”.  That plant had 60 F incoming water with a 66 F effluent temperature. It becomes easier as your incoming water temperature increases.