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Energy-efficient Kitchen Ventilation
May 23, 2021
By Mrinmoy Dey
An energy-efficient commercial kitchen ventilation system will not increase the sustainability quotient of the kitchen but also will result in reduced operational cost. However, achieving this would require an all-inclusive strategy encompassing all the stages of the installation starting from the planning phase.
In view of fast-depleting natural resources and the threats of adverse climatic conditions looming large in the form of global warming and other such collateral disasters, there is a conscious effort to reduce energy consumption and adopt sustainable practices. Nations across the world are working in this direction.
As per a May 2016 report by U.S. Energy Information Administration titled ‘International Energy Outlook 2016’, energy consumed by the building sector – residential and commercial combined, accounts for 20.1% of the total delivered energy consumed worldwide. In India, it is as high as 35%. And, the kitchen accounts for a lion’s share in that segment.
Talking about commercial kitchens, the HVAC system accounts for about 29% of the energy consumed in restaurants. Naturally, while building a sustainable kitchen or the strategy to reduce energy uses in the kitchen invariably involves optimising the kitchen ventilation system.
Minimising Energy Usage
To design a commercial kitchen ventilation system while minimising energy usage, the designer has to be well aware of the codes governing that local area, the appliances that will be used, menu of food items and the cooking process etc.
The moot point is to decide on the appropriate exhaust rate which has a larger impact on the amount of energy usage. Reducing exhaust CFM between 10-50% results in 27-87% saving in energy, according to ASHRAE. However, while an effective ventilation system will have to take care of heat, humidity, aerosols emitted during cooking, indoor air quality and fire safety and prevention as well. An energy optimising strategy should ideally consider all these options.
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Selection of Appliances:
The most obvious strategy is to reduce the load on the ventilation system. American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has categorised the cooking appliances as light, medium, heavy, and extra heavy-duty, depending on the strength of the thermal plume and the quantity of grease, smoke, heat, water vapour, and combustion products produced. The strength of the thermal plume is a major factor in determining the exhaust rate.
The cooking process impacts the selection of hoods. For example, if it involves a lot of smoke and grease liquid-tight construction with a built-in fire suppression system (Type I hood) is required. However, if it is only the heat and moisture that has to be taken care of type II hood will do.
The size and type of hoods
The exhaust rate also depends on the hood style and construction features. Wall-mounted canopy hoods, island (single or double) canopy hoods, and proximity (backshelf, pass-over, or eyebrow) hoods all have different capture areas and are mounted at different heights and horizontal positions relative to the cooking equipment.
A design guide published by South California Edison noted, “Generally, for the identical (thermal plume) challenge, a single-island canopy hood requires more exhaust than a wall-mounted canopy hood, and a wall-mounted canopy hood requires more exhaust than a proximity (backshelf) hood. The performance of a double island canopy tends to emulate the performance of two back-to-back wall canopy hoods.”
Optimising exhaust rates
Building and/or health codes typically provide basic construction and materials requirements for exhaust hoods, as well as prescriptive exhaust rates. Standard 154 of ASHRAE adopted by International Mechanical Code (IMC) prescribed these minimum exhaust rates based on appliance duty and length of the hood.
Designation
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The unlisted hoods which are not tested against a recognized standard, such as Under-writers Laboratories (UL) Standard 710, need to maintain the upper limit in the above table as a minimum requirement and must also meet materials and design requirements of the local building codes. So using listed hoods provides more leeway as if everything is optimised, the code-specific exhaust rate might be greater than what is required and might place an extra energy cost burden.
During design optimisation, the exhaust rates may be adjusted to account for diversity of operations and the position of appliances under the hood. For instance, appliances with strong thermal plumes, located at the end of a hood, tend to spill effluent more easily than the same appliance located in the middle of the hood. Also, the number of appliances that will be used at the same time will decide on the amount of smoke, moisture, heat or grease generated that needs to be captured and exhausted.
Makeup air strategy
Reducing the volume flow also have a significant effect on energy-saving potential because it reduces not only the ventilator's power consumption, but also the amount of energy needed to heat fresh air. First of all, a reduced exhaust rate will result in a reduction in the Make Up Air (MUA) supply requirement. Secondly, instead of sourcing the MUA completely from the outside, a local MUA strategy would offer energy-saving potential. For example, outside air supplied by HVAC system to dining room to maintain that area as per code requirement can be used. Depending on the architectural layout it might be practical to transfer most of the MUA from dining to kitchen.
The ASHRAE guideline suggests that rather than relying on one MUA strategy to supply 80-90% of the requirement, the designers should make an effort to keep this below 60%. Other 40% should be derived using other strategies like transfer air, another local strategy or HVAC.
The unlisted hoods which are not tested against a recognized standard, such as Under-writers Laboratories (UL) Standard 710, need to maintain the upper limit in the above table as a minimum requirement and must also meet materials and design requirements of the local building codes. So using listed hoods provides more leeway as if everything is optimised, the code-specific exhaust rate might be greater than what is required and might place an extra energy cost burden.
During design optimisation, the exhaust rates may be adjusted to account for diversity of operations and the position of appliances under the hood. For instance, appliances with strong thermal plumes, located at the end of a hood, tend to spill effluent more easily than the same appliance located in the middle of the hood. Also, the number of appliances that will be used at the same time will decide on the amount of smoke, moisture, heat or grease generated that needs to be captured and exhausted.
Makeup air strategy
Reducing the volume flow also have a significant effect on energy-saving potential because it reduces not only the ventilator's power consumption, but also the amount of energy needed to heat fresh air. First of all, a reduced exhaust rate will result in a reduction in the Make Up Air (MUA) supply requirement. Secondly, instead of sourcing the MUA completely from the outside, a local MUA strategy would offer energy-saving potential. For example, outside air supplied by HVAC system to dining room to maintain that area as per code requirement can be used. Depending on the architectural layout it might be practical to transfer most of the MUA from dining to kitchen.
The ASHRAE guideline suggests that rather than relying on one MUA strategy to supply 80-90% of the requirement, the designers should make an effort to keep this below 60%. Other 40% should be derived using other strategies like transfer air, another local strategy or HVAC.
Avenir Light is a clean and stylish font favored by designers. It's easy on the eyes and a great go to font for titles, paragraphs & more. Avenir Light is a clean and stylish font favored by designers. It's easy on the eyes and a great go to font for titles, paragraphs & more.
Full Name
Designation
Generally, for the identical (thermal plume) challenge, a single-island canopy hood requires more exhaust than a wall-mounted canopy hood, and a wall-mounted canopy hood requires more exhaust than a proximity (backshelf) hood.
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Other factors
Other than these, the energy-saving potential can be harnessed by reducing pressure losses and also using heat recovery. As per the working group on affordable passive houses findings, Kitchen ventilation systems have to move relatively large amounts of air, so reductions in pressure losses have a significant impact on the system's power demand. However, despite the potential, there is a reluctance to use heat recovery in the kitchen ventilation systems. The group’s findings acknowledged that the concerns about hygiene and fire hazards are simply too great.
Conclusion
While all the individual steps have potential energy-saving options, to truly harness the full potential a coordinated and all-inclusive strategy is required. All the stakeholders including the architects, building service planners, kitchen planners and users, have to be involved in the planning phase to avoid oversized ventilation systems. The selection of cooking equipment and other appliances is important to reduce the load on the ventilation system. Above all, the commercial kitchen ventilation system has to be calibrated for the cooking process, workflows and kitchen usage.
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