Dehydrator Exhaust Recirculation for Energy Savings

Dehydration is an important process that can be extremely profitable for food processors. Large-volume processors typically use continuous dehydration systems, while smaller volumes are processed batch-wise. Energy costs are frequently an issue in food processing operations, but particularly for those operating dehydrators. Over the years, many researchers have reported significant energy savings by recirculating dehydrator exhaust air. Recirculation is the practice of mixing dehydrator exhaust air with incoming fresh air. Five main advantages of exhaust recirculation were cited by Weigand ():

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1. Heat and fuel savings.
2. Moisture addition to air.
3. Decreased drying time.
4. Lower drying cost.
5. Increased product quality.

These advantages remain true today, but within limits that vary according to products and equipment. The purpose of this fact sheet is to discuss exhaust recirculation for batch dehydration processes and how it may be used for energy savings and to improve product quality.

Advantages of Exhaust Recirculation

Each of the five advantages of dryer exhaust air recirculation (listed above) are discussed in this section.

Heat and Fuel Savings

Substantial heat savings can be gained from recirculating exhaust air. This seems counter-intuitive, since exhaust air is laden with moisture, which can slow dehydration. Temperature of the exhaust air is the key factor to consider. When warm, moist exhaust air is mixed with fresh air, the temperature of the mixture is higher than the temperature of fresh air alone. This means that less energy will be needed to heat the mixture to the dryer’s set point. The evaporative capacity of the air mixture is less after exhaust air is added, but the decrease is only moderate.

One study (Walker and Wilhelm, ) suggested recirculation rates of up to 75% that resulted in energy savings ranging from 45 to 50%. Another study (Van Leersum, ) recommended recirculation rates from 60 to 85% with energy savings from 30 to 60%.

Both studies considered fruit in their drying processes. Walker and Wilhelm dried fruit between 24 to 27% moisture content and found that recirculation did not affect drying time.

Achieving lower moisture content in the final product (e.g. 10 to 15% range) may require reduced rates of exhaust recirculation. Reported studies experimented with fixed recirculation rates; variable rates may be required to achieve optimum results.

Moisture Addition to the Air

For most dehydration processes, low humidity air will dry foods rapidly and completely. Some foods however, exhibit “case hardening” from low-humidity drying, normally at the beginning of the process. Case hardening results from a too-rapid drying of the food, which forms a hard layer at the surface. The hard layer slows moisture loss and the tension between the hard outer layer and moist interior may distort the shape of the food and cause cracks to open (United States Bureau of Agricultural and Industrial Chemistry, ). Case hardening can be avoided by recirculating exhaust air to moderate the drying rate.

Decreased Drying Time

Case hardening (described in the previous paragraph) can cause increased drying time as the hard outer layer, or “case,” formed around the food product slows moisture loss. The addition of moisture to the air in the dehydrator (recycling exhaust air) helps to prevent the occurrence of case hardening. For products with case hardening issues, exhaust gas recirculation reduces overall drying time.

Lower Drying Cost

Researchers have reported energy savings from 30 to 60% for dehydration processes with exhaust recirculation. The savings will be offset by a partial reduction in drying capacity.

Increased Product Quality

If products are subject to case hardening, then exhaust recirculation will resolve the problem. When case hardening is not an issue, researchers have found little difference in the quality of products processed in the same dehydrator with and without exhaust recirculation.

Practical Application

Figure 1 shows a schematic of a dehydrator equipped to recirculate exhaust gases. The fresh air inlet is located on the suction side of the fan, and the exhaust outlet is located on the pressure side of the fan. Two flow control (damper) valves are used to adjust air flow rates. Butterfly-style dampers tend to be difficult to control, since airflow is not linear with respect to valve position. Louver or knife-style dampers are a good alternative with a more linear airflow response to valve position.

Figure 1. Schematic showing of a dehydrator with exhaust gas recirculation. 

Flow control valves can be hand-operated or automatic. Automatic and manual control may be based on properties measured in the dehydrator. Examples of control variables include humidity, wet bulb temperature and dry bulb temperature. Additional studies need to be completed to determine the best control scheme for exhaust recirculation. According to Walker and Wilhelm (), the exit air temperature of their test dehydrator increased with increasing amounts of air recirculation, but may have reached a maximum temperature at equilibrium conditions. Based on this information, one possible control strategy might be to regulate damper position (exhaust gas recirculation percentage) to achieve maximum exhaust air temperature. Practically, a maximum initial recirculation rate could be set, then reduced over time until a reduction in exhaust temperature was observed.

Dehydrators can be retrofitted for exhaust gas recirculation. All dehydrators have one or more fresh air inlet and exhaust air locations. Ductwork and valves, as shown in figure 1, can be added to enable recirculation. Figure 2 shows an enclosure built around an existing dehydrator to facilitate exhaust recirculation.

Figure 2. Drawing of a retrofit exhaust gas recirculation system designed for existing cabinet dehydrators.

Food Safety 

Dehydrated foods are generally considered safe, but studies have shown that pathogens can survive the moderate drying conditions of some processes (Allen et al., ). Processors can improve food quality and safety by using good manufacturing practices (GMPs) and effective cleaning and sanitation procedures. Understanding and minimizing temperature variation in a dehydrator and establishing a lethality process to minimize the likelihood of under-processed products is another important food safety measure. See Fact Sheets FAPC-121 and 165 for more information on sanitation and lethality treatment, respectively.

Conclusion

Dehydrated products are popular in today’s food market and are likely to remain so indefinitely. Operation and maintenance of dehydrators to save energy is an essential procedure that can increase profits and have a positive impact on the environment. Dehydrator exhaust recirculation is a simple and important tool for energy savings.

References

Allen, K.D., D. Cornforth, D. Whittier, M. Vasavada, and B. Nummer. . Evaluation of high humidity and wet marinade methods for pasteurization of jerky. Journal of Food Science; (72)7:  351-355.

Bowser, T.J. . Construction and operation manual for:  Low-cost, safe dehydrator for small and very small meat processors. Oklahoma State University, Robert M. Kerr Food & Agricultural Products Center. Internet: http://www.fapc.biz/files/DehydratorManualV1.pdf (accessed May 23, ).

United States Bureau of Agricultural and Industrial Chemistry. Vegetable and Fruit Dehydration: a Manual for Plant Operators. . Washington, D.C.: U.S. Dept. of Agriculture.

Van Leersum, J.G. . The use of rotary heat regenerators in fruit dehydration systems. Journal of Agricultural Engineering Research; 37:  117-128.

Walker, T.H. and L.R. Wilhelm. . Drying fruit with recirculated air for energy savings. Applied Engineering in Agriculture; (11)6:  861-867.

Weigand, E.H. . Recirculation driers. Circular 40. Corvallis:  Oregon Agricultural Experiment Station.

Tim Bowser
FAPC Food Process Engineer

Ensuring high indoor air quality in commercial and public spaces is crucial for the well-being, health, and comfort of occupants and visitors. According to the Environmental Protection Agency, Americans typically spend around 90% of their time indoors, including prolonged periods at work. 

The EPA notes that indoor air can often contain more pollutants than outdoor air. Consequently, subpar indoor air quality can lead to significant health issues. The resulting loss in productivity and increased sick days can lead to national revenue losses amounting to tens of billions of dollars each year.

What causes poor indoor air quality?

Indoor air quality can be diminished by a range of pollutants and chemicals originating from both outside and inside the building. Building occupants may be exposed to these harmful elements for long and frequent durations. The EPA cites the following:  

• Biological contaminants like bacteria, viruses, pollen, mold, dust mites, and water spills can enter enclosed spaces, triggering allergic reactions, shortness of breath, dizziness, and infectious illnesses such as influenza. 
• Chemical pollutants from sources like tobacco smoke, cleaning products, gases expelled by building materials and furnishings can irritate the nose, throat, lungs, and cause headaches. 
• Suspended air particles and pathogens can remain suspended in the air or travel on air currents until they settle on surfaces or individuals. These contaminants are more likely to spread among people in indoor environments due to higher concentrations, raising the risk of diseases such as influenza. 

The Role of Air Handling Units in Building Environments

Today’s modern buildings feature complex designs with diverse spaces dedicated to different functions, making it challenging to ensure proper ventilation and air quality throughout. HVAC systems are the first line of defense in removing and diluting airborne pollutants. 

Central to the systems are air handling units (AHUs), which process and circulate clean air back into the building through a network of ducts. Commonly positioned on rooftops, AHUs draw in outdoor air, filtering contaminants before redistribution. They have a direct impact on indoor air quality and the HVAC system’s lifespan by keeping equipment clean.

Central to the systems are the air handling units (AHU) which contain equipment such as cooling coils or heating elements that condition the air for temperature and humidity. A certain percentage of outdoor air is mixed with the recirculated indoor air to maintain a regular supply of fresh air. Powerful fans then direct the air mixture through filters for contaminant removal and then throughout the building within a network of ducts. 

MERV (minimum efficiency reporting values) indicates a filter’s effectiveness at trapping particles between 0.3 to 10 microns in size. This rating, established by ASHRAE Standard 52.2, aids in  selecting the appropriate filter for specific filtration need. ISO has introduced a filter standard as well, known as  ISO. This standard relies on  particle sizes classified into  PM1, PM2.5, and PM10. The table below is helpful when determining the appropriate filter efficiency required and the relationship between MERV and ISO.

During the pandemic, ASHRAE’s Epidemic Task Force  recommended  using filters with a minimum  MERV 13 rating for HVAC systems and many still follow the recommendation today. Filter selection should also consider the specific HVAC system’s capabilities to properly hold the filter in place and move air through the filter.  While HEPA filters might seem like a universal solution due to their high filtration levels, HEPAs require specialized holding frames and fans that can overcome the higher resistance to airflow which  could render them an impractical choice.

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Air Filter Types for AHUs in Commercial Settings

AHUs in commercial buildings are generally larger and more powerful which means they are often capable of holding a wide variety of air filter styles.  Here are various filters with the respective MERV ratings:

• HEPA or high-efficiency particulate air filters have very high efficiency, capturing a minimum of 99.97% of particles  as small as 0.3-microns which include respiratory aerosols that could contain harmful viruses.  The particles are much smaller than the eye can see.  
• V-bank style mini-pleats are the high-performance air filters on the market today for general ventilation. They have a very low resistance to airflow and coupled with the high dirt-holding capacity,  can remain in service for long periods. In some applications, V-bank-style filters can remain operating in AHUs for up to three years. 

• Bag filters are prevalent in commercial and public structure HVAC systems  for both primary and secondary filtering stages. Bag filters can be standalone for general air quality or act as pre-filters for specialized environments. The filters are also installed in the exhaust air or recirculation systems to protect air handling units.  With MERV ratings from 11 – 15, bag filters offer higher dust-holding capacity and longer lifetimes than most other filters. 

• Pleated panel filters are used as the primary filter in AHUs with limited space for air filters.  In AHUs with multiple filtration stages, panel filters are used as prefilters to keep the heating and cooling coils clean and to protect and extend the life of more expensive final filters by capturing larger particles in ventilation intake or recirculation air. The filters have a MERV rating of 8 to 13.

• Molecular filters are designed to eliminate gases, vapors, and molecular-scale pollutants. These substances are up to 1,000  times smaller than the particles that HEPA filters can capture. Molecular filters often use activated carbon or alumina to trap these tiny molecules using the adsorption filtration principle.

Air Filtration Solutions for Different Facilities

No single filter type is universally ideal for all commercial and public buildings. For instance, the filters needed in airports differ from those required in restaurants due to the various sizes and types of airborne particles that must be removed. The filter choice also hinges on several factors, including the capacity of the existing AHUs, the building’s function, cost considerations, desired efficiency levels, air quality objectives, and compatibility with the existing HVAC system. The overarching objective is to improve indoor air quality.

Below is a snapshot of the requirements and types of filters used in different spaces.

Airports 

Every day, millions of passengers pass through airport terminals, inadvertently shedding particles from their luggage, skin, and clothing that pollute the air. This is compounded by diesel fumes from passenger buses and baggage wagons, and the vapors from cleaning chemicals used within the terminal.

Airports combat these hazards by circulating a combination of outdoor and recirculated filtered indoor air,   throughout the terminals, thus protecting travelers and staff from dangerous substances. Typically, the central station HVAC systems or rooftop AHUs are equipped with air filters rated MERV 13 to 16, providing the greatest level of protection against fine particle contaminants while maintaining economic operation.

 Camfil, a leading manufacturer of premium clean air solutions, recommends these filters to strike an ideal balance between operational efficiency, long service life, and energy conservation. Additionally, carbon air filtration systems are employed to absorb diesel exhaust and odors from airport vehicles.

Retail Space

Maintaining fresh air in the confined spaces of shopping malls and retail stores is essential for a pleasant and safe shopping environment. Research indicates that indoor air quality in malls can be up to 10x times worse than air outdoors. A variety of factors contribute to this:  particles shed from customers’ skin and clothing, carbon dioxide from the breath of shoppers and staff, dust from restocking activities, intense cleaning processes, and emissions from transport vehicles all play a part in degrading air quality.

Short-term exposure to these elements can cause headaches and irritate the eyes and respiratory system. Airborne viruses shared by sick individuals can increase the risk of illness. Choosing an air filter depends on space, number of visitors, and type of airborne contaminants.  While there are no standards to follow, the Occupational Safety and Health Administration and ASHRAE recommends using filters with a minimum MERV rating of 13 to minimize the potential  spread of viruses such as COVID-19. 

 Hotels

Hotel guests expect easy check-ins, clean rooms, and fresh air.  In the hospitality industry, clean indoor air, along with climate control, ranks high on guests’ requests. Hotels face the challenge of filteringexternal pollution that seeps indoors, neutralizing cleaning agent odors, kitchen smoke, renovation dust, ductwork mold and bacteria, and carpet emissions. Different hotel zones like kitchens, lobbies, bars, and rooms each have unique air quality needs.

Energy-efficient filters with varying MERV ratings can effectively trap and remove a multitude of contaminants, from dust and bacteria to gases and volatile organic compounds (VOCs), to maintain high indoor air quality. These filters also contribute to energy savings and a lower carbon footprint.

Ensuring clean air is key to providing comfort and safeguarding the health of visitors, which in turn fosters guest loyalty and creates a wholesome environment for both work and leisure. While MERV 8-12 filters are commonly used for commercial buildings, higher MERV-rated filters such as 13-16,  control finer particles such as bacteria, tobacco smoke,  and some droplets from sneezes and coughs. The use of energy-efficient filters to improve indoor air quality can help save on energy costs that represent 15 – 20% of the total cost of hotel operations. 

Offices

Many offices have poor air circulation with outdated ventilation systems that just move unclean air from one part of the office to another. The wrong filters won’t offer the right results in removing small particles and odors in the air.  Studies indicate that good indoor air quality positively affects worker productivity.  

In addition to upgrading HVAC systems, office buildings should place the right filters throughout their space to purify air for occupants and increase the efficiency of HVAC systems. Filters with a MERV rating between 8 and 16 are recommended for the HVAC systems.  However, the higher the rating, the more contaminants a filter will block to protect equipment, operations, and building occupants.

Restaurants

Dining out is as much about the ambiance as it is about the food and service. Customers expect delicious meals, top-notch service, and a dining environment that’s comfortable and clean. Ensuring high-quality indoor air contributes to this experience by eliminating odors that can interfere with the enjoyment of the food and by safeguarding the health of both patrons and staff.

Managing the various spaces within a restaurant comes with distinct air quality challenges. Kitchens require robust ventilation to expel smoke, steam, and heat, while dining areas need to maintain an odor-free and comfortable environment. The solution is a proper combination of air filtration that removes oil and grease, collects particles, and eliminates gases responsible for odor.  

As outdoor air is brought in to replace the air extracted from kitchens and dining areas, it must be filtered to achieve high levels of indoor air quality. Fine airborne particles are particularly concerning and need to be efficiently captured with a minimum of  MERV 13 to 16 rated air filters,  ensuring an environment that meets the expectations for clean air in a dining setting.

Maintaining Filter Efficiency

After installing the appropriate air filters, it’s crucial to properly maintain them to ensure optimal performance and extend the life of the filters and the HVAC system. Buildings should have strict protocols for routine maintenance, checks, and filter replacements.  Ideally, air filters should be replaced when the resistance to airflow, aka pressure drop, increases to the point where clean airflow into the space is so restricted, proper conditions within the space are not being maintained.  Air filters should not be automatically replaced such as every one to three months. Changing air filters is an expensive undertaking and should be done when the filter’s usable service life is exhausted.  

Likewise, neglecting to replace filters when resistance has built up too high, can cause the filter to collapse in on itself and create serious bypass where no air is being filtered. Regularly monitoring pressure drop readings can be used to determine when filters need changing.  

Investing in high-quality air filtration to improve indoor air quality is directly associated with the good health, satisfaction, and productivity of building occupants and visitors and extended equipment life.

A specialist in air filtration, with expertise in servicing commercial and public buildings, can conduct a thorough evaluation and develop a tailored air filtration plan to meet the unique needs of the facility for the most effective results.

¹ https://www.epa.gov/indoor-air-quality-iaq/office-building-occupants-guide-indoor-air-quality#why-indoor

² https://www.epa.gov/indoor-air-quality-iaq/office-building-occupants-guide-indoor-air-quality#why-indoor

³ https://cleanair.camfil.us//12/18/is-the-air-quality-at-the-malls-making-you-sick/

⁴ https://www.osha.gov/sites/default/files/COVID-19-Guidance-Mall-Operators.pdf

⁵ https://www.camfil.com/en-us/industries/commercial-and-public-buildings/hotels

⁶ https://www.camfil.com/en-us/industries/commercial-and-public-buildings/offices

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