Overview
Heat recovery options are an important element of RTO systems installations. With rising energy costs and the state of the economy, building operators and managers are realizing, more than ever, the need to reduce operating costs and increase efficiencies within their facilities. RTO thermal recovery systems offer an excellent opportunity to accomplish both. There are three major options to RTO recovery systems:
- Retrofits that reduce overall airflow to the oxidizer system and /or increase the concentration of solvents to be treated.
- The internal thermal energy recovery of the oxidizer system itself.
- Energy recovery in the exhaust stack of the oxidizer.
This article will concentrate and discuss three important aspects of energy reclamation from hot oxidizer stacks and some of the possible implementations. There are many reasons why the first two choices for energy recovery in the above mentioned choices are not attractive or even economically feasible, making energy recovery in the oxidizer exhaust stack the best overall choice for energy conservation efforts.
Oxidizer Stack Energy Recovery
There are distinct challenges that must be addressed in the process of evaluating potential energy savings options with stack recovery systems, as well as, multiple equipment options for this application, each with it’s own benefits and limitations. The three (30) major challenges are:
- Capturing the Energy from the Stack itself.
- Delivering the Energy back into the Plant cost-effectively.
- Employing the Recovered Energy effectively inside the Plant.
Capturing the Energy
Evaluating and estimating how much energy is available for capture is the easiest of all three challenges. The appropriate devices for capture, along with their size and related costs, can be determined from the following performance and operating data:
- Expected airflow and average temperature in the oxidizer stack.
- Expected hours of operation per year.
- Current energy rates for the plant (gas, oil or electric).
The first two items are often monitored already on a continuous basis in oxidizer data recorders. If that is not the case for a particular installation, the most recent EPA (Environmental Protection Agency) stack testing data can be an excellent source for this information.
There are two (2) additional important issues to be taken into consideration during this phase of the evaluation process that can compromise the recovery equipment implementation on the oxidizer stack:
- Dew point temperatures of exhaust gases.
- System back-pressure penalties.
Any effort to reclaim energy from exhaust gases will lower dry bulb as well as dew point temperatures and create the potential for condensation of acids in the exhaust stack. However, given the typical solvent laden exhaust from printing presses, this is rarely an issue of concern for oxidizer systems related to the flexographic printing industry.
The existing oxidizer fan will usually be penalized in performance with the pulling and pushing of air through the hot side of heat recovery component. Therefore, the solution is to choose energy recovery equipment that will limit system back-pressures to a minimum to avoid fan modifications.
Delivering the Energy
Sizing energy recovery equipment and estimating the overall savings opportunities with oxidizer with oxidizer stacks is not a difficult task. But, to take an opportunity analysis and turn it into an actual payback period that makes business sense however, one has to determine the costs of installing equipment and providing infracstructure for delivering captured energy back to the plant facility.
A preliminary walk-through and analysis/evaluation of the different options are necessary to be performed by experienced professionals to determine the end-use applications and identify requirements going forward. This is often the challenge where the overall project feasibility hangs in the balance.
Employing the Recovered Energy
This final challenge is the key and extremely important for optimizing energy recovery project payback. Possible solutions are as follows:
- Using the recovered energy in the same process the oxidizer is connected to.
- Utilizing recovered energy to cool or heat a facilty.
- Taking stack air directly back for use in production processes.
In the South Florida area printing industry however, the most attractive is to utilize recovered heat for cooling the plant with specific downstream implementations to significantly reduce cooling demands within the plant.
End – Use Applications
For oxidizer system recovery to be economically feasible, proper end-use applications are a must. Each facility has it’s own unique finger print of energy systems applications and utilization. In the South Florida area printing industry, the most feasible option is Air-to-Fluid Heat Recovery to fire an absorption chilling machine to aid existing chilled water air conditioning systems. However, this implementation alone would not produce attractive pay-backs on their own. Modifications and techniques must be implemented downstream to systems in order to diminish or lower the air conditioning demands for cooling substantially. The following are the most impacting possibilities for demand reduction are as follows:
- Spot –Cooling and distribution techniques within the occupied zones, only. Not cooling the entire plant facility.
- Utilizing over-head ventilators (properly placed) to increase supply air temperatures to the occupied zones.
- Control Demand Ventilation implementations.
- Monitoring and verification procedures to insure system performances.
All of the above implementations hereinabove mentioned should yield a very attractive business and economically feasible plan (with or without government assistance). The goal always should be to yield a plant facility that would be at a minimum 50% more efficient than it’s original version, and at the same time save the building owners and/or operators 30% to 40% on their electrical bills.
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