KIEAE Journal

Recently, studies on the pollutants generated during kitchen cooking in Korea ^(8~9) have reported that kitchen cooking accounts for a large proportion of indoor pollutants and emphasized the solution through ventilation. However, there are few studies on the performance verification of the range hood which should be operated during kitchen cooking and the performance is expected to deteriorate through consideration of previous studies. Thus, this actual condition survey measured the airflow of the hood installed in the home and compared the performance of the hood with the rated flow of the hood. In addition, we were to diagnose the range hood installed in the existing apartment rather than the simple survey, and then, following the survey, we interviewed the individuals in detail to identify occupants’ perception of the range hood.

The survey was conducted by randomly selecting 80 households in Seoul, Gyeonggi, Daejeon, and Chungcheong. The survey has of a total of 8 questions, consisting of basic questionnaires on residents and range hoods.

If there are many occupants, the survey and interviews were conducted for housewives who mainly use the range hood. The survey for each step was conducted in the order of Table 1, and the details of each step are as follows:

In Korea, KS C 9304 is used to measure the air flow rate to find out the performance of the range hood. Foreign countries also do not define the criteria for the unique performance measurement of a separate range hood. ASHRAE⁽¹⁰⁾ presents 100cfm (50L/s) as the minimum standard of range hood air flow and in the case of an enclosed kitchen, the minimum air flow for the normal function of the range hood was suggested by presenting more than 5ACH in consideration of the kitchen volume.

Singer et al⁽⁷⁾ measured airflow compared to the rated flow of a range hood certified under the same conditions as the Home Ventilating Institute (HVI) and demonstrated the capture efficiency of the range hood using tracer gas, and demonstrated the performance deterioration of the existing range hood using. However, there is currently no established definition of an internationally recognized range hood efficiency rating. This study measured the air flow rate of the range hood by using equation of KS C 9304 (1) and measuring instrument (Table 2).

Where Q_m：Measured air flow of the range hood (m³/s)
　　　　V¯ ：Measured air flow (m/s)
　　　　A ：Range hood exhaust area (m²)

As shown in Fig. 1(a), this study measured air flow in exhaust filter of the range hood. The wind speed can be adjusted depending on the type and the specification of the range hood and the shape of the exhaust vent is also very diverse. Therefore, this actual measurement measured the air flow rate by differentiating the measurement points according to the number of the exhaust filters divided. Fig. 1 (b) is an island-type large range hood with an exhaust port divided into four filters and the average wind speed was calculated by measuring 9 places and 36 places by filter compartment.

Fig. 1. 
Testing of range hood

For the accuracy of the measurement, the hood began to be measured at least 5 minutes after start-up and the wind speed was measured in the order of the highest wind speed to the lowest wind speed. When the number of steps of the range hood is changed, the process of stabilizing the air flow of the hood should be preceded. Therefore, it was measured after changing the exhaust airflow due to the regulation of the number of steps of the range hood and after operating for more than 5 minutes and wind speed was measured for more than 1 minute at each measurement point.

The rated flow of the existing range hood presents the maximum number of steps as a representative value in general, but this study measured the number of all steps. Based on this, the rated flow and the measured air flow for each step were compared to evaluate the performance deterioration of the range hood installed in the apartment.

This study measured the flow rate of the range hood and evaluated the performance of the range hood installed in the existing apartment and compared it with the rated flow (Fig. 5). Although the rated flow of the basic range hood is based on the maximum air flow rate, this study measured and investigated the number of all steps to evaluate the performance.

Fig. 5. 
Result of measurement for air flow rate. M/R ratio: Measured flow per rated flow

Of the 80 households to be measured, five households that do not use the range hood at all or have range hood that is so aged as to be impossible to measure were excluded from the measurement and this study was based on the results of 75 households.

The rated flow of the range hood was 299.9, 386.4, and 530.2 m³/ h, respectively, for each number of steps. On the other hand, the air flow rate was measured as 121.2, 138.2, and 167.9 m³/h on average, respectively, by the number of steps. This is 41.4, 38.0 and 32.0% compared to the rated flow of the product and the decrease in the measured flow rate compared to the rated flow was noticeable, and especially, the decrease rate was found to be higher in the hood with higher rated flow. In addition, even if a product with the same range hood or similar rated flow is installed at home, the performance was found to be very different.

As shown in the introduction, the specifications of the range hood are very diverse, so this study specifies a range approximate to the sample value and sample value within a certain range by the number of steps as shown in Table 5.

First, the sample value of the three-step rated flow is 608.1±12.0m³/h, whereas that of the measured air flow is 189.9±49.1m³/h and the standard deviations was found to be relatively great with distinct air volume reduction. In addition, sample values(with standard deviations) were measured as 140.4±53.6m³/h, 160.5±58.2m³/h in the other steps, showing that even with the range hoods with the similar rated flow. The performance variation is remarkable.

In addition to the ventilation of the apartment and building conditions, the variation of the performance is large depending on various conditions of the installed range hood. As indicated in 2.2, the measured air volume of the range hood herein is not the measurement under the same conditions as those of KS C 9304 due to the nature of field survey but even considering this, performance deterioration of the range hood installed in the general house is large. In addition, the ratio of the measured air flow to the rated air flow is 32.0% in the 3-step stage (the maximum air flow mode of a general range hood), showing the greatest performance deterioration. This figure is based on the maximum air flow rate and 64.0% out of the target households were found not to meet minimum air volume standard (100 CFM) of range hood recommended by the ASHRAE.

This study comprehensively conducted a survey, interviews, performance measurements, and found that the performance deteriorations of the hood were caused by the following four problems.

The most obvious problems in the survey of the range hood include the maintenance and aging of the hood. The hood in the building that is more than 10 years old was also found to be outdated and not to be used by users and to operate in a state where performance is severely degraded.

In the range hood, a filter is installed typically at the suction portion to play a primary role in protecting the exhaust fan. As shown in Fig. 4, this filter must be cleaned and in extreme cases, two households without filters were found. As a result, the range hood exhaust motor was broken, loud noise was generated during operation, resulting in a deterioration in performance. In addition to the range hood, just as maintenance issues, including filters, are important in all ventilation installations, the range hood must also be maintained.

Fig. 6 shows the performance of the hood by dividing the maintenance state into 3 steps of Good, Fair and Bad through the inspection of the filter, fan, and internal condition of the target hood. If the hood is in good condition, the performance on step 1(rated flow versus measured flow) is 56.5% whereas if the maintenance state is bad, the performance is found to be lowered to 33.2, so if the maintenance status of the hood is bad, the performance of the hood is found to be reduced to less than 1/3.

Fig. 6. 
Maintenance problem for Range hood. Measured flow per rated flow

Nevertheless, as indicated in the survey improvement section, users said it is necessary to provide education on the basic operation and maintenance of the range hood, showing that their awareness of maintenance and the actual results are very contradictory. Based on this result, it seems that guidelines for maintenance are needed.

Because of the wide variety of range hoods and building types, it is very difficult to type the ducts of the range hood. As shown in Fig. 7, however, if the duct is over- or under-refracted for no reason, the unintentional performance deterioration of the equipment occurs. In fact, the measured air flow of all hoods in which the duct was placed abnormally was measured to be less than 50% of the rated flow. Especially in the case of KS C 9302, the air flow is measured under the condition that the duct is basically straight, and the loss of pressure is relatively small, so it is considered that the difference between the measured air flow and the rated flow due to the duct is large.

Fig. 7. 
Problem of exhaust duct

Also, in the case of two cases during the survey, it was found that the range hood was not functioning properly because the duct was damaged or the connection with the hood part was incomplete. To improve range hood performance, duct inspection should be preceded.

Buildings over 10 years have a high rate of remodeling. In this process, the range hood is mostly replaced as part of remodeling. In this process, the range hood is installed at a low price for cost reductions. As a result, the performance of the range hood was found to deteriorate.

Recently, range hoods which were classified as kitchen appliances has been emphasized and many products with differentiated designs have been released. Some of hood with new designs are superior in terms of aesthetics, high in price and users’ preference is relatively high, but performance was measured to be less than ordinary hood.

The range hood exhausts the contaminants that were sucked through the internal exhaust fan to the outside through the duct and takes a common exhaust duct system or a method of installing an exhaust port (exhaust port, cap grill, hereinafter exhaust port) on the outer wall part and the type is divided into 6 major types (Fig. 8). Depending on the installation type of these various cap grills, the performance of the range hood may be greatly affected.

Fig. 8. 
Types of exhaust port

In the past apartment, type 4 and co-duct type were mostly used as hood duct finishing. Recently, however, when designing facades of apartment, there is a tendency to prefer a flat exhaust port for the sense of unity with the outer wall. As a result, apartment using type 1 ~ 3 when finishing the hood exhaust increased more than in the past. In fact, the results of the field survey showed that the percentage of type4 the most used in the past was the highest, 38.7% but the percentage of apartment using type 1 ~ 3 was high around new households (Fig. 9).

Fig. 9. 
Frequency of mounted exhaust port

For example, the diameter of the exhaust duct commonly used for household range hood is 125mm(0.0123m²), whereas the exhaust port area is reduced by 1/3 of the exhaust duct area, depending on the exhaust port type. In this case, no matter how high the performance of the range hood, the performance deterioration of the hood at the exhaust port portion may inevitably occur. The percentage of the exhaust port installed in the target apartment is as shown in fig.9. Of these, narrow exhaust port (Type 1~3) that 25.8% has an outlet area of less than 0.0060m² is installed.

If this type of exhaust port is installed, the exhaust efficiency of the hood may be adversely affected. The use of such a narrow exhaust port may have an impact on efficient kitchen exhaust.

Fig. 10 shows the performance of the range hood with flat exhaust ports (Type 1 ~ 3). As a result, the performance of the range hood connected to Type 1, 2, 3 was measured to be less than half compared to the rated flow. Given the relatively low years of use of buildings and range hoods using flat exhaust ports of the type, the area of the exhaust port is considered to affect the range hood performance.

Fig. 10. 
Air volume of range hood with flat exhaust port