What is a Cycling Dryer and Why Would You Want One?

Most plants currently have a dryer that continuously runs and uses a component called a hot gas bypass valve to keep the unit above 38° F and prevent it from freezing up. But, there are many different types of cycling dryers that accomplish the same result without the dryer constantly running.

• Cycling type with a thermal mass is cooled down to the required temperature and the compressor shuts off – this works much like your refrigerator – if no load, the unit does not come back on until required – very effective and the most reliable
  
• VFD cycling uses a variable speed drive on the refrigeration compressor that varies the amount of power/refrigeration used based on the temperature of the air – operates much like a variable speed air compressor – most have a range of 100% to 30% – not good in low range applications – fairly complicated electronically
  
• Load/unload cycling compressors – works much like larger refrigeration systems and can be effective when they are fairly steady medium loads – can short cycle with low loads – sometimes not as power cost effective as a true cycling dryer

With that being said, if you purchase a 500 CFM dryer ($8,000) for your plant and left it on while your compressor is running (or maybe you forget to shut it off at the end of the second shift), it would use 2.5 HP continuously. If you purchase the same 500 CFM thermal mass cycling dryer ($10,200) for the same application, it would require 2.7 HP. But, in an average plant it would be on only 20-25% of the time. The cost difference between the two is $2,200 and the savings is $1,490 for $6,000 a year at $.15 a KW. Using simple math, the ROI is 1.5 years.

Other advantages of using a cycling dryer are:

• You do not have to remember to turn the dryer off when you turn the compressor off as it shuts down
  
• Potential freeze up at low load conditions almost eliminated
  
• It can be used with lower CFM compressors while maintaining the same dew point
  
• It will tolerate higher ambient/higher inlet temperature conditions more readily over standard dryer.

Remember all dryers are rated at 100 PSI/100° F inlet air/100° F ambient temperature. If you increase the air pressure dryer performance gets better. If the ambient air temperature or the inlet air temperature gets above 115° F your dryer could be in big trouble and moisture will soon be invading your system.

If you would like to know more about how a cycling dryer would be a better fit for you than what you have we be happy to help or if you have any moisture issues let us know. Contact Frank Lederer: flederer@hopeair.com.