Heat Recovery from Waste Water

COMMERCIAL LAUNDRY

WASTE WATER HEAT RECOVERY

NOTE:

Although this case study is based on a commercial laundry operation using gas fired boilers, the principles are applicable to all processes (Power Generation, Abattoirs, Dairy, Food, Beverage, Plastics etc.) in which waste heat is being produced by either electricity or gas.

In our continuous efforts to assist our clients, Fluid Dynamics has developed a unique proprietary computer programme to provide accurate estimates of the savings to be achieved by using heat exchangers to:

capture valuable heat energy in waste fluids (liquids or gasses)
return that heat energy back into the process and
optimise the most cost-efficient processing temperatures,

C ontact us to discuss further.

Figure 1: Warm water promotes algal growth, causing eutrophication of downstream surface waters.

Fig. 1: Warm water promotes algal growth causing eutrophication of downstream surface waters

The Issue:

A local council had directed a large commercial laundry to reduce the temperature of its waste water stream before discharge into the sewer.

After processing, the waste water was being released into the sewer at a temperature too high to be safe, promoting the growth of harmful bacteria in the sewer and posing a public health hazard.

The laundry approached Fluid Dynamics for assistance to cool the waste water stream before discharge.

The Brief:

To cool the waste water before discharge
Capture heat from the waste water stream and
Return the captured heat to the laundry process.

Conditions:

The feed water stream entered the process system via boilers at a variable ambient temperature (from 15 °C in winter to 25 °C in summer) .

After the laundry process the hot waste water was then discharged to the sewer.

The waste water consisted of typical laundry waste including lint and other organic matter which was prone to clogging.

As well as the excess heat in the waste water, the waste matter also promoted algal growth and it was also desirable to remove this waste matter before discharge to the sewer.

Algal Growth

As well as the excess heat in the waste water, the waste matter that was present in the water also promoted algal growth in the sewer and it was desirable to also remove this before discharge.

Fig. 2: S/S Tubestack showing welded and smooth tube ends

Design:

Fluid Dynamics designed an all stainless-steel corrugated tubular heat exchanger to take the excess heat from the waste water stream and apply that captured heat to the ambient feed water to provide cost savings.

Features:

The Fluid Dynamics heat exchanger was fully stainless steel to provide exceptional unit life and low maintenance
The heat exchanger was designed in sections to enable easy accessibility and serviceability
The tube-stack consisted of multiple tubes to provide enhanced surface area for more effective heat exchange
Corrugated tubes were used as the corrugations create turbulence in the waste water stream providing two major advantages - (1) the particulate matter in the water (lint & organic matter etc.) remains suspended reducing clogging and (2) the turbulent flow provides enhanced heat exchange performance
The tube ends were welded in place and the faces of the tubes were machined smooth and rounded, removing sharp edges and snag points to prevent clogging
Valves were incorporated to enable sections of the heat exchanger to be isolated permitting uninterrupted operation during servicing in summer
Removable bends were provided to enable easy cleaning in place (CIP)
No moving parts in the heat exchanger results in extremely low maintenance
A water separator was also installed in the process to separate lint and other particulates from the waste water before discharge to sewer and to reduce the particulate to easily handled solids (this unit was not included in the ROI calculations)

Fig. 3: Cutaway section showing multiple corrugated tubes for enhanced performance

Impact:

The heat exchanger significantly reduced the temperature of the waste water from 60^oC to 35^oC allowing it to safely discharge to the sewer. This drop in temperature prevented propagation of harmful levels of bacteria in the sewer system and satisfied the local council requirements.

The heat exchanger simultaneously raised the ambient boiler feed water to a constant 40^oC and by doing so significantly reduced the monthly heating bill of the laundry facility.

The water separator reduced the water-borne particulate to solids and significantly cleaned the waste water before its disposal to the sewer.

Fig. 4: Graph showing constant temperature using the heat exchanger compared to the fluctuating temperature before the heat exchanger

The seasonal variation in ambient water temperature accounted for an average 8% fluctuation in the monthly gas heating bill. By introducing the heat exchanger, this fluctuation was effectively neutralized with the feed water temperature at the outlet being kept constant.

Fig. 5: Drawing of a corrugated heat exchanger showing removable ends and frame

Fig. 6: Graph showing expected monthly savings achieved over a year post heat exchanger calculated on an annual heating bill of $1m

Summary:

The council mandated corrective action be taken by an industrial laundry facility to reduce the temperature of its waste water stream. Fluid Dynamics designed a heat exchanger which reduced the temperature of the waste water and simultaneously increased the temperature of the feed water stream. This corrective measure reduced operating costs associated with gas heating, reduced overall heating and eliminated seasonal cost fluctuations.

The heating costs for the laundry facility were reduced by 13% in the first month by capturing the excess heat from the waste water stream. This reduction provided an immediate return on investment (ROI) for the heat exchanger in less than 3 months. An ongoing annual saving of 17% in the annual gas heating costs was achieved.

RESULTS ACHIEVED

Industry leading waste-water treatment system installed by Fluid Dynamics that exceeded all relevant standards, reducing outflow temperatures from 60°C to 35°C
Significant reduction in energy costs of between 13% and 21% per month, with an annual overall reduction of 17%.
Removal of seasonal variations in the cost of heating ambient water is now a steady 40^oC. In the winter months, starting ambient temp was 15°C and, in the summer months, it rose to 25°C. This fluctuation accounted for an 8% increase in the energy bill each winter.
Constant feed water temperature (40^oC) now means that heating costs remain steady throughout the year allowing cash flow to be assessed accurately in advance.
Through the initial savings alone, the cost of the Fluid Dynamics system was recovered in less than 3 months.
Based on an annual energy spend of $1m p.a., ongoing annual savings of $170,000 can be expected in terms of reduced payments to the company’s energy provider.

Please Note:

For privacy reasons the costs shown are based on a notional annual gas bill of $1m for the facility. Those costs are not actual however the relative percentage savings are.
Costs, savings and temperatures have been rounded off.
The ambient water temperatures of 15^oC to 25^oC were provided by the Bureau of Meteorology and relate to parts of Victoria.
While cost savings will be achieved, results for other processing facilities may differ due to variables such as facility location, energy bills, temperatures and flow rates of fluids, the nature of any particulate matter in the waste water and the condition of the facility’s equipment.
Stainless steel may not be suitable for some fluids, particularly if they contain chlorides.