Demand Controlled Kitchen Ventilation System Different Sensors

Demand Controlled Kitchen Ventilation Systems Sensors, how do they differ, and how are they the same?

An update on the technology used in Demand Controlled Kitchen Ventilation Systems. The introduction of thermal imaging sensors.

The number and type of demand control systems for commercial kitchen ventilation systems 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 the 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.

  1. At least 50% of all replacement air is transfer air that would otherwise be exhausted.
  2. Demand Controlled Kitchen Ventilation system(s) on at least 75% of exhaust air. Such systems shall be capable of at least a 50% reduction in exhaust and replacement air flow 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.
  3. 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 Controlled Kitchen Ventilation systems (DCKV) fall into one of three basic categories. These categories are derived from the systems’ method of detecting 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 several factors. These may include the complexity of the ventilation system, the need for expandability, capital investment, and ROI.

The three primary types of DCKV systems are:

  1. Temperature Only
  2. Temperature  and opacity sensor
  3. Infrared Cooking activity sensors have been replaced by Thermal Imaging Sensors.
    1. What are the differences between an Infrared cooking activity sensor and Thermal imaging sensors, and what is the impact on DCKV performance?

Demand Controlled Kitchen Ventilation 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 RTDs 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 depends on the system set points for idle (the non-cooking period when appliances are turned on) and actual cooking.

Example of an RTD Sensor

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 idle or 48Hz on a 60Hz motor (80%), then a 48Hz 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 be used, then that must be considered 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 set points 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 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.

Demand Controlled Kitchen Ventilation Comparison Griddle Temperature

Demand Controlled Kitchen Ventilation Temp-only systems with opacity sensor

In an acknowledgment that it is difficult to gather accurate information about cooking activity and that there is a lag in response to heat generation by the appliances, some manufacturers of DCKV systems have put an opacity sensor in their system. This is a reflective beam in the canopy. Steam and or smoke can be generated before the temperature set point is reached. In this way, if the smoke and/or steam block the “beam,” it will automatically ramp the exhaust system to design airflow.

Opacity Sensor
Demand Controlled Kitchen Ventilation Temp-only System with Opacity Sensor

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 sensors’ signals 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.

DCV Comparison Griddle Temperature and Cooking Activity

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.

Some systems 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.

New Technology for Demand Controlled Kitchen Ventilation

Thermal Imaging Sensors:

reading points for thermal imaging sensor
Graphic of temperature reading points on a group of appliances

The Thermal Imaging Sensor is a development that advances DCKV technology with greater sensitivity and opportunity for energy savings. The sensor employs a grid-style thermopile for increased resolution and thermal sensitivity.  With a 110° viewing angle, a single sensor can be used in hoods up to 14’ long (canopy installations, not low proximity).  The Thermal Imaging Sensor provides 192 individual temperature measurements of the cooking area, allowing the Demand Controlled Kitchen Ventilation System to accurately determine what each appliance under the hood is doing. Air volumes are adjusted more precisely based on these readings rather than averaging temperature. Response time is instantaneous and based on the ramp speed of the VFD.

Thermal Imaging Sensor used with Demand Controlled Kitchen Ventilation.

Should you require help selecting the right demand controlled kitchen ventilation system, the experts that manufacture these systems can guide you on the proper selection needed to meet your requirements.

Read another recommended article.

System Neurology of a Kitchen Ventilation System

Several components make up a commercial kitchen ventilation CKV system.  There are hoods, the exhaust fan, the make-up air fan, fire suppression system, and often a demand control system. 

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