Innovative Heat Recovery from Air Cooled Compressor

When the topic of discussion is making ice cream, the first thing that comes to mind isn’t heat, but at Nestlé’s Ice Cream factory in Tulare, California, heat is recovered from air-cooled air compressors to heat process water. Nestlé USA falls under the global umbrella of Nestlé S.A., the world’s largest food company. The Tulare plant makes Nestlé, Dreyer’s Grand and Häagen-Dazs ice cream products on eight production lines and compressed air is used in a wide range of applications.

Right  out  of  the  gate,  everything  is  pneumatic,” explains Tom Finn, Project Engineer with Nestlé Ice Cream Division. “Air  cylinders  and  air  driven  motors,  the  process  piping  valves  which  divert,  route,  stop/start,  and mix  process  fluids,  our  packaging  machinery  including  rejection,  cleaning  and  vapor  removal  processes,  all  of these  rely  on  compressed  air.  If  we  don’t  have  compressed  air,  we’re  done.  As  a  food  processor  we  chose  oil-­free compressors  with  filtration  and  drying  systems  to  ensure  that  our  compressed  air  stream  is  free  of  moisture and  particulate.  What  is  required  is  clean  dry  air  to  prevent  product  contamination.”

Nestle Kobelco KNW Project

According to Finn, the Tulare plant has four 125-horsepower Kobelco KNW Series oil-free air-cooled screw compressors. Two compressors are typically running 24/7. When demand rises and pressure drops, a third compressor comes on automatically. The fourth compressor allows Nestlé to rotate compressors in service and provide redundancy to ensure a continuous air supply even if one compressor is off-line for maintenance.

“As a food processor we chose oil-free compressors with filtration and drying systems to ensure that our compressed air stream is free of moisture and particulate.”

Tom FinnProject Engineer, Nestlé Ice Cream Division

Air leaving each compressor passes through a moisture separator, then a regenerative-type dual tower dryer, and then to a dry air receiver tank that serves as a pressure buffer to help minimize compressor cycling. Clean, dry air exits the receiver into the piping network for use as part of the process throughout the plant.

On its own, Nestlé’s compressors are fairly typical of oil-free systems used in food processing environments. What makes this compressed air system unusual, however, is its innovative method for recovering and reusing the heat of compression.

Background

The story begins in the mid-1990s when Nestlé began a phased replacement of its existing oil-free reciprocating compressors, which were experiencing unacceptably high maintenance costs and down time.

The  recips  were  old  technology  in  a  hot  room,  which  was  a  bad  combination,” according to Lane Hawkinson, National Sales Manager Kobelco KNW Series Oil-Free Air Compressors for Rogers Machinery Company, Inc., of Portland, Oregon. “Nestlé  bought  their  first  Kobelco  in  1996,  then  a  second  in  1998,  and  two  more  in  2001.  Each Kobelco  rejects  11,500  cfm  of  air  at  160°F  with  high  potential  for  heat  recovery.”

An initial cooling plan for the air compressors involved simply ducting the cooling air out the roof. That would dramatically reduce the heat load inside the utilities area, but Nestlé’s global commitment to energy efficiency presented Finn with a challenge and an opportunity: instead of just rejecting all that heat, could it be put to work?

A common method to recover heat rejected from air-cooled compressors is to direct the warmed air via ductwork to a large space, such as a warehouse or garage, to provide interior heating. The need for low quality heating like this is seasonal, however, and because the heating season in California’s Central Valley is short, most of the time the heat still would be vented outdoors. Rather than settle for low quality heat used seasonally, Finn conceived of a system to recover BTUs rejected from the air-cooled compressors to heat water.

Nestlé  is  highly  tuned  into  resource  conservation,” Finn explains. “In  Tulare  we’re  already  at  the  stage  of  the game  where  we’ve  pulled  the  low  hanging  fruit,  so  we  have  to  be  more  creative  in  finding  ways  to  capture energy  savings.  With  heat  recovery  from  the  compressors,  I  was  looking  at  energy  we  can  use  year  round,  so  I had  to  think  about  what  we  use  year  round  in  a  food  production  facility.  The  answer  was  hot  water.  We  have many  hot  water  stations  around  the  plant  for  washing,  sanitary  systems,  waste  systems  and  a  lot  more,  and demand  for  hot  water  is  ongoing.”

Nestle Heat Recovery

Concept

It’s not unusual to recover heat from water-cooled compressors to produce hot water, but Nestlé’s Kobelco compressors are air-cooled. What makes this heat recovery project unusual and innovative is the use of air-to-water heat exchangers.

To  capture  energy  for  heating  water,  we  needed  a  more  specialized  approach,” Finn recalls. “I  believed  we could  accomplish  this  by  employing  air-­to-­water  heat  exchangers.  When  I  first  discussed  the  idea,  people  said, ‘You  want  to  do  what?  How  is  that  going  to  work?’  It  was  one  of  those  times  when  you  turn  your  engineering cap  around  and  make  it  a  sales  cap  because  you  need  to  sell  an  idea  to  management.”

As Finn explained to plant managers, heat rejected from each operating compressor, amounting to more than 350,000 BTUs per compressor per hour, would be scavenged in an 11,500 cfm air flow and fed via ductwork through a finned coil much like a car radiator. Water from the plant’s hot water make-up supply would flow through the coil and pick up heat rejected by compressor. Water exiting the coil would be warmer than when it went in, so less natural gas would be required to raise the temperature of water for the plant’s hot water applications. Each compressor would have its own heat exchanger and the heat recovery process would operate, in series, whenever a compressor was running.

“Just because you have an older system, doesn’t mean you’ve missed the boat — It can be retrofitted.”

Lane HawkinsonNational Sales Manager, Rogers Machinery Company

Finn developed the make-up water recirculation system and a colleague designed the air-to-water heat exchanger coils, but some additional system engineering was required. “We  determined  that  the  compressor’s  heat  rejection fan,  which  pushes  air  across  the  compressor  to  cool  it,  was  going  to  need  help  moving  sufficient  air  through  the coil  to  maintain  the  efficiency  of  the  rejection  system,” Finn explains. “Our  solution  was  to  add  an  assist  fan downstream  of  each  coil  to  pull  the  air  through.  We  got  in  touch  with  Lane  Hawkinson  from  Rogers  Machinery and  he  and  their  engineering  department  helped  us  by  providing  some  numbers  regarding  air  flow  and pressure  drop.  I  estimated  how  much  water  runs  through  each  heat  exchanger  and  made  adjustments  to  the flow  rate  until  I  reached  an  optimal  setting.  A  variable  frequency  drive  on  the  pump  made  this  precise adjustment  possible.  It  runs  at  46  Hz  to  pick  up  maximum  heat.”

Hawkinson points out that he has been involved with many heat recovery projects involving air compressors, but until he talked with Finn he never had heard of an air-to-water heat exchanger positioned inside a duct. “Nestlé needed  to  get  heat  out  of  a  room,  and  now  on  the  way  out  it  pays  a  visit  to  a  heat  exchanger,” says Hawkinson. “It’s  a  great  idea  that  Tom  Finn  and  Nestlé  executed  very  well.  Nestlé  has  been  a  customer  since  1996  and  the machines  have  been  running  trouble  free  for  years.  We  are  always  glad  to  provide  technical  information  to help  maximize  the  efficiency  of  customer  installations.”

Finn points out that actual heat gain varies, although on a June day when he checked the water temperature was 80°F going into the heat exchangers and 107°F coming out. “Our  target  for  temperature  gain  was  20  degrees,” says Finn,  “so  anything  we  get  above  that  is  icing  on  the  cake.”

Nestle Project System Layout

Takeaway

Finn calculates the total cost for the heat recovery system at $150,000 including touchscreen controls for the pumps and diverter valves. The reduction in natural gas consumption because the intake water is preheated amounts to 309 therms per 24 hours. The project’s payback period was initially estimated at 3.8 years, but that turned out to be too conservative as payback occurred in just over 3 years.

Hawkinson points out that practically any existing installation of air-cooled compressors has the potential to benefit from heat recovery. “Just  because  you  have  an  older  system,  doesn’t  mean  you’ve  missed  the  boat,”  he says.  “It  can  be  retrofitted.  Every  situation  is  unique  and  some  solutions  will  be  more  complicated  than  others. The  challenge  of  heat  recovery  is  that  sources  and  uses  can  be  inconsistent.  A  lot  of  air-­cooled  compressors  are installed  outdoors  where  heat  recovery  is  harder  and  makes  less  sense.  But  for  air-­cooled  compressors  installed indoors,  heat  recovery  via  simple  ducting  or  more  sophisticated  means  like  Nestlé  used  are  always  an  option and  should  be  studied.”

Finn attributes his insight into energy savings to his previous work in ship building. “On  a  ship  you  have  to  use every  watt,”  he says.  “I  just  started  thinking  that  way  and  never  stopped.  This  heat  recovery  project  was another  application  of  that  kind  of  thinking.  Another  project  I’m  looking  at  involves  the  heat  recovery  system’s assist  fans,  which  are  not  variable  frequency  drive.  Over  time  we  have  found  that  they  don’t  need  to  be  running full  speed  all  the  time.  We  have  plans  to  upgrade  our  evaporator  fans  with  VFD  motors  elsewhere  in  the  plant, and  when  we  do  we  will  upgrade  the  assist  fans  to  VFD.”

One of Nestlé’s global goals is to become the most efficient energy user among the food manufacturers and this project is a good step on that journey.

Learn more about the product line that made this project successful by visiting the Kobelco KNW Series website!

Related Posts

Kobelco KNW Series Skid
International Wire Kobelco KNW Project
KNWOO cutaway

Product Line Websites

Kobelco KNW Series website
Rogers K Series website

Follow Us