Plant Services - November 2007 - (Page 54)

Recovering waste heat from your compressor is now smarter than ever before By Ben J. Sliwinski G as compression is a common industrial and commercial process. Among its most typical applications are providing compressed air in industrial plants and air conditioning and refrigeration in numerous industrial, commercial and residential systems. An interesting aspect of the gas compression process is that while the objective of the process is to increase the pressure of a gas, a significant fraction of the input energy (estimated to be between 80% and 90%) is consumed in raising the temperature of the gas. When compressing air, about 8 hp of electricity are required to produce 1 hp of compressed air. This provides an opportunity for heat recovery, which in this era of increasing energy costs, can be financially attractive. In the case of air compression, an estimated 50,000 Btu/hr of thermal energy is available to be recovered for every 100 cfm of air provided. Here a P there a T Without going into the thermodynamic details, don’t equate the temper54 ature rise in the gas being compressed to process inefficiency. A perfectly efficient compression process done on a perfect gas would still exhibit a temperature rise – it’s the nature of the universe we live in. One way to think of it is to imagine that when we add energy to a gas, it gets stored in two separate compartments. One compartment is temperature and the other is pressure. For a given amount of energy added, the fraction that goes into each compartment is a fixed property of the gas being compressed. Inefficiencies in the compression process result in the temperature increasing more than it would for the perfect process. Also, while the increase in gas temperature with increased pressure can be a detriment, in refrigeration systems, it’s the key element that makes the systems work at all. Low-temperature vapor from the refrigeration system evaporator is compressed and its temperature increases to higher than ambient levels. This increase allows the system to reject heat to the outside – which is the whole raison d’etre of the system. www.PLANTSERVICES.com How to cash in With all that excess energy lying around, you’d think it would be easy to recover some. Amazingly, about 80% to 90% of the input energy is up for grabs, with typical recovery rates between 30% to 90% of what’s available. The determining factors for compressor heat recovery are the same as for any heat recovery process. At a minimum, we need to determine three things: at what temperature is the energy available; at what temperature do we need the energy; and how much of it is there? One common way to recover the heat of compression is to duct the air leaving your air-cooled aftercooler or lubricant cooler to provide heated process or space heating air. A booster fan might be necessary to ensure that the pressure drop in the duct doesn’t reduce the air flow across the aftercooler. Figure 1 shows the ducting system used in a manufacturing plant. The arrangement ducts heated air to the plant during the winter and out the roof during the summer. Sometimes, simplicity is the best way to go. In the plant shown in FigNovember 2007 http://www.PLANTSERVICES.com

Table of Contents Feed for the Digital Edition of Plant Services - November 2007

Plant Services - November 2007 Contents New Tools Letters Let Us Know Where to Put the Gas Up and Running How are you Fighting Crisis? What Works Why is the Best Practice Transfer so Hard? Who are you Going to Call? Chained to Power Mobile Mindset Making Money in the Lube Lab Grabbing a Moving Target Bagging the BTUs Keeping Mellow Preserving Health and Safety The Power of Reliability Excellence

Plant Services - November 2007

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