Elsevier

Energy and Buildings

Volume 34, Issue 5, June 2002, Pages 431-444
Energy and Buildings

Indoor air quality and thermal comfort studies of an under-floor air-conditioning system in the tropics

https://doi.org/10.1016/S0378-7788(01)00128-1Get rights and content

Abstract

This paper reports thermal comfort and indoor air quality (IAQ) studies of an under-floor air-conditioning (UFAC) system in hot and humid climate. Thermal comfort parameters were measured at predetermined grid points within an imaginary plane to predict the airflow pattern of the supply air jet as well as to determine the occurrence of thermal stratification in the office space. Fanger’s [Thermal Comfort Analysis and Applications in Environmental Engineering, McGraw-Hill, New York, 1970] thermal comfort index was also computed to detect the occupants’ thermal sensation. Besides, the concentration levels of dust and carbon dioxide were recorded with the intention to examine the quality of the indoor air. Statistical methods were applied to derive the relationship between air velocity and the other parameters as mentioned earlier. The main findings from the study revealed reasonable level of acceptability of IAQ associated with the UFAC system. However, occupants are likely to experience localised thermal discomfort near the supply diffusers due to the existence of large temperature gradients. In addition, a stagnant zone is discovered at sedentary level, which is caused by the parabolic airflow nature of the primary air jet.

Introduction

The concept of supplying cold air from the raised floor has been used for many years in computer rooms. However, these areas are seldom occupied with people and no real attempt is made to apply the same comfort criteria for air movement as is used for an office environment. For example, in the early 1960s in Federal Republic of Germany, the under-floor air-conditioning (UFAC) system was first introduced in rooms with a high heat production such as electric light-bulb factories, drying installation in the textile industry and data processing centres. It was later then, in the mid 1970s, that such systems began to be employed in general offices—primarily in European countries.

With the innovation of open-plan office concept, there is a restriction in changing the future layout of the office if the conventional ceiling-based air-conditioning system is adopted. The diffuser layout, in this case, is usually established before the locations of the workstations and thermal loads have been determined. In addition, partitions are placed without considering the locations of these air diffusers, often restricting the air circulation within the workspace and hence causing discomfort to the occupant [1]. By adopting the floor-based air distribution system, occupants can enjoy the great flexibility of adding, removing or relocating the supply outlets to suit their individual preferences as well as any rearrangement in the office layout. In addition, some systems have incorporated thermostats in the floor diffusers that allow occupants to adjust the required temperature. This not only provides for personal control but also gives rise to occupant-defined comfort [2], [3].

Next, with the advent of the electronic or automated office, raised floors are installed to accommodate and conceal the cables and services that are laid underneath. Since cavity is created between the concrete slabs and the floor tiles in the raised floors, this floor void can also serve to function as a supply air plenum. Thus, conditioned air can be supplied to the office space from the supply grilles through the floor cavity. This reduces or eliminates the depth of the ceiling plenum as ductwork and terminals are no longer housed there. As a result, new buildings are able to benefit from a significant height reduction or additional floors, which in turn, leads to enormous savings in the development cost [4].

Nevertheless, as people move towards information technology-based work environment, the use of computers for data storage and other job-related functions is indispensable. Together with other electronic equipment, they are probably the major contributors to the heat loads in the office space. Since the temperature of the supply air from the floor outlets is normally lower than the room air, the high-velocity air that comes from below the computers cools the equipment at a faster rate than the cold air from a conventional ceiling-based system. This effectively removes the heat in the machines, which will otherwise be built up as time proceeds. However, potential drawbacks in this case are probably the sensation of draft by the occupants and the occurrence of condensation within the machine [5].

The notion of supplying cold air from the “under-floor plenum” is relatively new in Singapore and little research has been carried out to examine the suitability of using such system in the office buildings of the city. Although the use of such system in place of the ceiling-based system has been explored in limited cases, common complaints regarding the system, e.g. cold draft, are still prevalent. This paper is aimed at exploring the performance of the system in relation to the indoor air quality (IAQ) and thermal comfort in an office space. In the context of thermal comfort, this paper is restricted in its scope to the:

  • 1.

    collection of measured empirical data on room space temperature, velocity and relative humidity;

  • 2.

    computation of the predicted mean vote (PMV) and predicted percentage dissatisfied (PPD) based on the measured values;

  • 3.

    correlation analysis and simple linear regression between the air velocity and other variables, namely:

    • 3.1.

      relative humidity;

    • 3.2.

      temperature;

    • 3.3.

      PMV and

    • 3.4.

      PPD.

Regarding IAQ, although the aim here is not to carry out an integrated IAQ audit methodology [6], air quality parameters that will provide an insight into the performance of the floor-based system are chosen for analysis. The results will also be subsequently used to support the hypothesis made in the modelling of the airflow pattern. Therefore, the IAQ study involves the:

  • 1.

    collection of data at the concentration levels of particulate and carbon dioxide;

  • 2.

    correlation analysis and simple linear regression on the following pairs of variables, namely:

    • 2.1.

      particle count and air velocity and

    • 2.2.

      carbon dioxide and air velocity.

Section snippets

UFAC system—a review

An UFAC system is comparable to a displacement ventilation (DV) system in that both systems sometimes supply cold air from diffusers that are mounted on the floor. However, for the latter system, supply air can also be supplied through diffusers that are mounted in walls near the floor where the return air is subsequently exhausted through the ceiling grilles, as shown in Fig. 1. While DV systems generally supply lower velocity air with the goal of minimising mixing and maximising displacement

UFAC system description

UFAC systems come in different designs although the location of the return air grille is limited to be only either on the floor or the ceiling plenum. Nevertheless, the test system here is designed with both the supply and return on the raised floor. It is now intended to provide a general description of the system and an understanding of the way the air is distributed in the occupied zone.

The UFAC system directly makes use of the floor void as a plenum for the distribution of air. Each floor

Description of test site

The building in which the experiment is carried out is a double-storey 6-year-old single unit factory with a gross floor area of approximately 5000 m2. However, the area that utilises the UFAC system is only about 300 m2 and this is found in the corporate office, shown in Fig. 2.

The total number of staff working in the office does not exceed 10 persons. In other words, every occupant occupies an approximate area of 30 m2. The UFAC system is not equipped with a proper outside air provision and is

Research methodology

A methodology involving the measurement of thermal comfort parameters, chemical and particulate contamination is adopted in this research study. The office space that utilises the UFAC system is divided into three zones namely Zones A, B and C with each zone served by a different conditioning unit, as shown in Fig. 3(a)–(c). As it is not within the scope to study the airflow pattern, air quality and thermal stratification at every location where the floor grille exists, two sampling locations

Temperature

The maximum, minimum and the mean temperatures for the five sampling locations are shown in Table 1 and a sample of the temperature results is presented in Fig. 5.

It is seen that majority of the values fall within the acceptable temperature range of 20–27 °C as stated in ASHRAE Standard 55-1992 [10]. Singapore Standard Code of Practice 13 [27] requires the comfort temperature to be within 23–25 °C while ENV Guidelines [18] recommends temperature range between 22.5 and 25.5 °C for acceptable IAQ.

Conclusion

A study is carried out on the UFAC system with an aim of examining its impact on the indoor office environment in relation to IAQ and thermal comfort. Experiments were conducted in an office to measure the temperature, relative humidity, air velocity, carbon dioxide and dust particulate at the selected grid points within an imaginary plane for a total of five sampling locations. Based on the empirical data, the PMV and PPD are computed. Besides this, simple linear regression and correlation

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