The number and type of Demand Control Ventilation (DCV) systems for commercial kitchens have grown significantly in recent years. This can be attributed to several factors, principally the adherence to ASHRAE 90.1 ventilation standard. This is the standard that affects design for commercial kitchens, and it states that on exhaust systems greater than 5000 cfm, (Washington State and other municipalities have adopted lower exhaust rate thresholds) you need to incorporate one of three energy conservation measures. Some States and Provinces have adopted ASHRAE 90.1 as building code.
- At least 50% of all replacement air is transferred air that would otherwise be exhausted
- Demand Control Ventilation system(s) on at least 75% of exhaust air. Such systems shall be capable of at least a 50% reduction in the exhaust and replacement airflow rates, including controls necessary to modulate airflow in response to appliance operation and to maintain full capture and containment of smoke, effluent, and combustion products during cooking and idle.
- Use of a listed energy recovery device with sensible heat recovery effectiveness of not less than 40% on at least 50% of the total exhaust airflow.
The types of Demand Control Ventilation systems fall into one of three basic categories. These categories are derived from the method the systems use to detect heat and/or cooking. Each type of system has its benefits and drawbacks. The correct choice of which system is of the most value depends on a number of factors. Those may include the complexity of the ventilation system, the need for expandability, capital investment, and ROI.
The three primary types of DCV systems are:
- Temperature Only
- Temperature and Opacity Sensor
- Infrared Cooking Activity Sensor
Temperature Only Systems
The simplest of systems are known as temp only. This type of system uses a Resistance Temperature Detector (RTD). It is a device with a significant temperature coefficient (that is, its resistance varies with temperature). It is used as a temperature measurement device, usually by passing a low-level current through it and measuring the voltage drop. A thermistor is a common type of RTD.
The RTD is typically located in the exhaust collar of the hood. Some manufacturers have multiple RTD’s within the canopy to detect heat over the entire length of the hood. The RTD has a temperature set point that once reached, signals the exhaust fan VFD (Variable Frequency Drive) to exhaust a percentage of air. The amount of air is dependent on the system setpoints for idle (a non-cooking period is when appliances are turned on) and actual cooking.
On systems that don’t measure the actual airflow in the hood, a signal that is proportional to the exhaust frequency is sent to the corresponding supply air unit. For example, if the exhaust is at idle or 48Hz on a 60Hz motor, then a 48Htz signal is sent to the supply air to keep the system balanced. Caution should be used in determining this output signal because it assumes a one to one relationship of exhaust to supply air. Should transfer air be used or other means of make-up air used, then that must be taken into account when estimating the supply air signal.
Temp only systems tend to have an initial lower first cost but may not capture all potential savings due to a limited turndown ratio. (this is the difference between full airflow and idle airflow). Reaction time tends to be slower since the signal for appliance start-up and/or the onset of cooking is measured in the exhaust collar or hood canopy, as well as the time to heat the thermal mass of the RTD. This delay may create conditions where heat and/or smoke may escape the hood. Care must be given to the temperature setpoint temperature for the initial start. Further complicating matters, these systems generally increase airflow with higher sensed temperatures and lower airflow with lower sensed temperatures. When cooking starts, however, the temperature often drops (think cold burgers covering a hot griddle), which would erroneously result in a reduced cfm when design cfm would actually be required.
Temp only systems should also be adjusted for seasonal changes to optimize the RTD set points. If this is not done, the system performance can be compromised when the system operates in the opposite season from when the start-up was completed.
Temperature Only with an Opacity Sensor
In an acknowledgment that it is difficult to gather accurate information about the cooking activity and that there is a lag in response to heat generation by the appliances, some manufacturers of Demand Control Ventilation systems have put an opacity sensor in their system. This is a reflective beam in the canopy/exhaust hood. Steam and or smoke can be generated before the temperature setpoint is reached. In this way, if the smoke and steam block the “beam” is broken, it will automatically ramp the exhaust system to design airflow.
Cooking Activity Sensors
More sophisticated systems use a variety of sensors, duct temperature, space temperature and infrared sensors directed at the appliance surface. These types of systems compare the signals from all the sensors to determine cooking status. If the temperature is at a steady-state and the infrared sensor detects a sudden drop in temperature at the cooking surface (frozen fries into hot oil for example), that is interpreted as a cooking signal and the system responds instantaneously. Only lag time is associated with the ramp speed of the VFD.
These systems measure the actual exhaust rate and rather than drive to a specific frequency on the VFD; they send a signal to achieve a given CFM at the hood. Based on this signal, a proportional low voltage signal is sent to the make-up air system or Building Management System to adjust the make-up air volume. In addition, customizable algorithms are available for project-specific requirements.
There are systems that can also provide zone control over the make-up air volume or output a split signal for multiple supply air units.
Further differentiation comes from the ability of the systems to modulate and control exhaust hoods independently of each other on a common fan and duct system.
Most manufacturers can provide a detailed analysis and report on the energy savings associated with their respective systems. Consideration should be given to budget, potential energy savings, expandability, remote monitoring capability, and algorithm changeability.
Marketing and Segment Development Director, Halton Group Americas
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