ASHRAE Journal -November 2012 - 37

setpoint temperature, operating on a temperature curve to increase exhaust airflow proportional to the temperature difference between exhaust and space temperature and operating on a temperature curve in combination with a cooking activity sensor (CAS) to drive the system to design when cooking is detected. Additionally, an evaluation was done to determine energy savings for a DCV system with balancing dampers installed on a four exhaust hood, island configuration.

Appliance 600°F Charbroiler 400°F Griddle, Thermostatically Controlled 350°F Open-Vat Fryer, Single Vat

Fuel Source

Design/Capture And Containment Airflow 1,800 cfm

Natural Frozen Gas Hamburger Patties Natural Frozen Gas Hamburger Patties Natural Gas Frozen French Fries

1,000 cfm

Test Setup

The objective of the first round of tests was to compare performance of DCV systems that use temperature sensors only Table 1: Cooking appliance and associated food product. to those that incorporate cooking activity and temperature sensors. Currently, only two manufacturers offer the latter. One the timer expiration, if no new cooking activity is detected, the design uses optical opacity sensors to detect the presence of system returns to the “curve” control algorithm. cooking effluent in a hood cavity. Another design uses infrared The exhaust hood was installed at 80 in. (2 m) above fin(IR) temperature sensors to monitor the surface temperature ished floor with a temperature sensor mounted in the exhaust of cooking appliances. Data from these IR sensors along with collar. The infrared sensors were positioned in the front, inspace temperature and hood exhaust temperature sensors are terior face of the canopy to sense the cooking surface. The analyzed to interpret the status of cooking appliances (idle, temperature sensor was installed so that it was centered in the cooking or off) and adjust hood exhaust airflow accordingly. hood collar. A 72 in. (1.8 m) long wall canopy exhaust hood was configThe test protocol included a range of appliances that all ured to simulate various DCV control algorithms available on demonstrated similar trends, but due to space limitations only the market: exhaust temperature-based system that operates at data for appliances most commonly seen in kitchens is prea fixed setpoint, exhaust temperature-based system that oper- sented: a charbroiler, griddle and open-vat fryer. ates on a curve and exhaust temperature coupled with a cookAirflows in Table 1 represent hood C&C airflow, and whening activity sensor system (includes IR sensors). ever the hood operates below this value when cooking occurs, Fixed setpoint exhaust temperatures of 90°F, 100°F and the hood is spilling. Details of appliance fuel source and prod130°F (32°C, 38°C and 54˚C) were evaluated. For these con- uct cooked are shown as well. figurations, the minimum exhaust airflow rate was 80% of deDuring testing, each combination was evaluated at the idle sign airflow rate (a common value for temperature only based and cooking states. Exhaust airflow rate and temperature were systems due to the limited ability to detect when cooking plotted versus time. The onset of the cooking process was notstarts and ramp-up of exhaust airflow). Exhaust airflow was ed to determine system response time. varied by a VFD in an attempt to maintain the tested temperature setpoint. Results and Discussions For exhaust temperature systems that operated on a curve, Charbroiler the minimum exhaust airflow rate was again 80% of design. Figure 1 summarizes testing conducted with the charbroiler, When using the curve, exhaust airflow was incrementally in- which exhibited the highest exhaust temperatures of all tested creased as the temperature difference between exhaust and kitchen space in2,000 150 Constant Constant Constant Temperature Temperature creased. This algorithm ensures C&C 145 Temperature, Temperature, Temperature, + CAS Curve 1,900 SP = 90°F SP = 100°F SP = 130°F of convective heat from appliances in140 stalled under the hood. 135 1,800 Minimum exhaust airflow for the 130 1,700 system with cooking activity sensors 125 120 installed was limited to 40% of the de1,600 115 sign rate to ensure that the exhaust fans 110 were operated in their recommended 1,500 105 range. This system used the “curve” Patties On 1,400 100 temperature control as described previously when appliances are in idle mode and transitioned to design exhaust airflow for an adjustable period (which was Design Airflow Duct Temperature Airflow set to seven minutes for this test) upon detection of cooking activity. Following Figure 1: Charbroiler testing.
November 2012
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1,000 cfm

ASHRAE Journal

Temperature (°F)

Airflow (cfm)

ASHRAE Journal -November 2012

Table of Contents for the Digital Edition of ASHRAE Journal -November 2012