fitting a textile finishing mill with RCR: an experience
This document describes the installation of Pozzi’s heat exchanger for polluted waters when used as a heat recovery unit in a printing & finishing mill in Turkey, where we have replaced a conventional heat recovery system made with plate heat exchangers. Energy savings well over 100000 EUR per year can be obtained when installing a single RCR where no previous heat recovery solution exists, while replacing an existing plate heat exchanger gives a performance boost corresponding to 40000 EUR additionally saved per year.
All hot effluents in the plant are produced by continuous machines, so that recovered heat energy can be immediately used avoiding complex set-ups to level flow-rates.
A tradeoff must be found between the two extremes:
a centralized system, with a battery of heat exchangers that treat all the effluents taking them from the main draindistributed system, where each machine that requires heat recovery is served by a local heat exchanger.
In our context, the second option was chosen because of different reasons:
the mixing of all effluents would decrease the average temperature of the effluents, making energy recovery more difficultand control systems costs would exceed the cost of the exchangers, thus making the option non cost effectiveinvestment can span a longer time and can be made is self-sustaining from a financial point of view: the savings from the first installation can be used to buy the second and so on.
The customer had already tried heat recovery on finishing machines, but was completely dissatisfied because of the poor performance of the system because of the lint and fibers that are naturally present in the effluents of textile industry.
The traditional plate exchangers that had been installed required additional filters and pumps to be operated and yet needed daily cleaning of the filters and weekly cleaning of the exchangers by disassembly because of excessive fouling.
This is a typical pitfall when talking about heat recovery in the textile industry: the costs hidden costs of accessories and downtimes can be extremely high compared to the cost of the heat exchanger itself.
Pozzi Leopoldo has designed and built a self-cleaning heat exchange unit, which has become the most successfull model in the textile sector over the last 25 years.
Made entirely of stainless steel, the heat exchanger has a central element rotated continuously by a small motor. This element comprises a series of hollow discs fitted with baffle plates connected by a hollow axle through which clean water flows. It rotates inside a trough, again equipped with baffles, through which the effluent flows in the opposite direction to the clean water.
This new design brings very definite advantages when this exchanger is used with polluted effluents.
Its rotation creates turbulence in both the primary and secondary flow which enhances heat transfer
Rotation also prevents the build up of deposits from the dirty effluent.
Turbulence due to the rotation creates a centrifugal separation effect which keeps the contaminants away from the exchanger surfaces.
Thus maintenance for cleaning is practically nonexistent and the system has benefits from a constant heat exchange efficiency.
The typical installation of the RCR is shown in Figure 2. Compared to Figure 1, we see that no filters (as the unit is self cleaning) nor pumps (as the unit as an extremely low pressure drop of 5 cm) are required.
RESULTS & performance data
The first installation of the RCR has been performed on a Benninger continuous bleaching machine and has been operating for about two months.
The following data have been collected in a typical working week of the machine:
Flowrate: 15.2 m3/h (for both hot effluent and fresh water)
Temperature of freshwater (tap water): 17°C
Temperature of effluents: 86°C
Temperature of cooled effluents: 33°C
Temperature of heated water: 66°C
Recovered energy: 860kW (approx. 738000 kilocalorie/hour)
Heat recovery efficiency: 70%
Taking into account the fact that the plant is working continuously 144 hours per week and 48 weeks per year, we easily calculate that the savings correspond roughly to 702000 cubic meters of Natural Gas which would have a cost of 119000 EUR (NG is approximately 0.17 EUR/m3 in Turkey).
Since the complete solution (RCR exchanger and installation) costs about 20000 EUR, the payback on the investment is about two months.
The theoretical efficiency of the heat exchanger that has been replaced was close to 95%, unfortunately the frequent maintenance stops reduced the real application efficiency to about 45%.
Comparing the constant efficiency of the RCR, gives a substitution payback of approximately 6 months, not taking into account production stops and the salary of the maintenance person in charge of cleaning the filters/exchangers.
The customer has already bought and installed another RCR for a similar substitution and has ordered 9 more for the current year for new installation where no energy recovery solution was in place. This will total an energy bill saving of approximately 1.3M EUR per year.