Environmental Heat Transfer

Environmental Heat Transfer

Contents: Heat Recovery Environemntal Benefits of Closed Loop Cooling Systems Energy Savings Leak detection Heat Recovery:   Although many traditional heat exchanger applications can be considered heat recovery, we are seeing a significant increase in new heat recovery opportunities. Most of the applications we focus on involve exhaust gas heat recovery and wastewater heat recovery. Flue gas heat recovery systems are a simple and cost-effective way to capture the energy that is lost in flue gas exhaust and to use that energy elsewhere. Installed in exhaust stacks for steam boilers, hot water boilers or high temperature ovens for process work, heat recovery coils can capture this waste heat to generate low, medium or high temperature fluids (air, thermal fluid, glycol, or water) for use in process fluid pre-heat, facility heat, combustion air preheat, etc. Our flue gas heat exchangers can be built for either condensing or non-condensing applications to match your requirements. Combined with Thermal Storage Systems or Heat Pumps, these systems can reduce flue gas temperatures to nearly room temperature - increasing combustion efficiency to greater than 95% - and store energy at a variety of temperatures. The heat exchangers can be designed for gas, oil or even chemical vapor and depending on the application may be coated to suit the exhaust gas conditions. Stainless steel, carbon steel, and aluminum heat exchangers are available with fin spacing and fin-tube construction designed to match temperature, fouling potential and maintenance requirements. Hot or cold water going to waste is an another opportunity for heat recovery. Similar to exhaust gas applications, the exhaust water can be used to heat or cool air, thermal fluid, glycol, water or other fluids. Even in circumstances where the water may be high fouling an appropriate heat exchanger can be selected. I cases of poor water quality, energy recovery systems can be designed to recover heat or cold from process grey water. Specifically designed for fibre-contaminated water, these systems can recovery 90% of the energy in hot or cold water systems: Commercial laundry services Hospital laundry wash water Hotel laundry wash water Institutional laundry wash water Textile manufacturers Vegetable wash water  Environmental Benefits of Closed-Loop, Evaporative Cooling:  Closed-Loop: We are fortunate in Canada that we have an abundance of cold water naturally available for our use. A common use of this water has often be to cool industrial equipment often through an intermediate heat exchanger. This practice has become less practical with the environmental concerns of leaks and also the changing water chemistry of river water can require some exotic and expensive metallurgy in the heat exchangers. Many “once -through” cooling systems are being replaced by Closed-Loop Systems. The alternative to “once-through” type cooling has typically been to deliver water that has been cooled by a Cooling Tower, however the cold temperature of the river water often cannot be matched by a Cooling Tower. Using Closed-Loop, Evaporative Cooling also know at Wet Surface Air Coolers (or WSAC) can typically gain a few degrees more cooling than a Cooling Tower/Heat Exchanger system. This can often be achieved with lower power demand, less water consumptions and a significant reduction in chemical treatments to the make-up water which is an added environmental benefit.  Power Consumption: Closed Loop Evaporative Coolers consume less parasite energy than a traditional cooling tower for the same heat rejection. Typical fan horsepower is reduced by 10-25% and a ypical pumping horsepower reduction of 10-40%. Water Conservation: Spray Water Source One of the key advantages to a Wet Surface Air Cooler over a traditional cooling tower is its capability to use poor quality water as the spray water. The fact that the spray water is not being sent through a heat exchanger, fouling is not a concern. Water that cannot be used for other processes can often be used as spray water in Wet Surface Air Cooler. Common spray water sources are; Blowdown from cooling towers or boilers Waste streams from demineralizers, HRSG and Reverse Osmosis processes Plant effluent wastewater FGD water Agricultural runoff Brackish water and seawater   Cycles of Concentration As discussed previously, the spray water is not being sent through a heat exchanger and fouling is not a concern, therfore higher cycles of concentration can be run. Cycles of concentration represents the accumulation of dissolved minerals in the recirculating cooling water. In a cooling tower or Wet Surface Air Cooler, the evaporated water leaves its dissolved salts behind in the bulk of the water that has not been evaporated, thus raising the salt concentration in the water. To prevent the salt concentration of the water from becoming too high, a portion of the water is drawn off for disposal (called blowdown). Fresh water makeup is supplied to compensate for the loss of evaporated water, drift and the blowdown. The chemistry of the makeup water including the amount of dissolved minerals can vary widely. Makeup waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Makeup waters from ground water supplies (wells) are usually higher in minerals and tend to be scaling (deposit minerals). Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping however excessive levels of minerals can cause scaling problems. As the cycles of concentration increase the water may not be able to hold the minerals in solution. In a typical cooling tower / heat exchanger system, When the solubility of these minerals have been exceeded they can precipitate out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the heat exchangers. In a Wet surface Air Cooler the water is sprayed on the outside of the tubes. The air and water are cocurrent, therfore the outside of the tube is completely covered by water, creating an even temperature and reducing the formation of deposits and scaling. Concentration cycles in the majority of cooling towers usually range from 3 to 7. Wet Surface Air Coolers can run cycles of up to 50 in some cases, reducing annual water consumption up to 70%. Auxilary Load The fact that a Wet Surface Air Cooler can operate under higher cycles of concentration that a cooling tower and the fact that cooling tower blowdown can be used as a spray water source, the opportunity exists to cool an additional cooling load using the cooling tower blowdown as a spray water source. This principle can be applied to an upgrade, new process or just to reduce the demand and water consumption of an existing cooling tower. The schematic below shows an example where 57,000,000 gallons of water is saved annually as well as a 57,000,000 gallon annual reduction of blowdown. Dry/Wet Cooler combination An alternative to Wet Surface Air Coolers and cooling towers are air cooled heat exchangers or dry coolers. They have the advantage of not requiring water as part of the cooling process, however they have the limitation that they cannot cooler to the same temperature as either WSAC's or cooling towers. Typically, at best, a dry cooler can cool a fluid to a temperature of 10°F above the ambient dry bulb temperature. Considering that typical ambient temperatures are 86°F (30°c) to 104°F (40°c), the lowest temperature that can be expected from dry coolers would be between 96°F (36°c) and 114°F (46°c). A wet surface air cooler can be used in combination with a dry cooler to cool a process below the temperature that a dry cooler can achieve on its own and provide significant water savings over a cooling tower or WSAC handling the total cooling load. The example to the right shows an application where a process stream is cooled from 170°F to 90°F in a 95°F ambient dry bulb temperature. Energy Savings: Industry strives to increase productivity, reduce costs and become more efficient and at the same time meet environmental regulations in a world where energy costs are increasing. Where shell and tube heat exchangers are the bottleneck, Koch Heat Transfer Group (KHT) has two solutions that can increase production and reduce energy costs and can often be the most economical solution to achieve both of these goals.  Twisted Tube® heat exchangers use a unique tube and distinctive design and construction that often allows increased performance in the same size as a conventional bundle at lower pressure drops on both the shell side and the tube side.  Often reduced energy costs provide the justification for installing Twisted Tube® heat  exchangers. Leak Protection:  A leak in a  heat exchanger, at a minimum, can cause maintenance headaches but also leaks can cause operational problems and can be a serious cause for environmental concern.  Heat exchangers commonly contain environmentally sensitive fluid that, if exposed to air, ground, water, or river water, an environmental risk can result.  In the case of the water-cooled transformers, the risk of a leak is two-fold.  If transformer oil leaks into the water side, a significant environmental problem has occurred.  If water leaks into the transformer oil, serious damage can occur to the transformer. Unifin International, LP has developed a design to deal with this risk.  Unifin's leak detector design involves two tubes bonded together with the “leak-off” channels to capture drain and isolate either fluid in the event of a leak.  This transformer oil cooler can be fitted with an alarm to signal a leak has occurred and the isolation chamber will hold the leaked fluid until repair or replacement can occur during the next outage.