Thermal efficiency of natural gas balanced-flue space heaters: Measurements for commercial devices

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Abstract

The performance of the most common gas space heater used in residences in Argentina was experimentally investigated. The furnace is a simple device with a burner (ranging from 2 to 7 kW according to the model) set into a metal chamber, and this separated from the ambient by an enclosing cabinet. This type of individual gas heater is installed indoors with both inlet and outgases chimneys connected to the outside. For the same furnace, two chimney configurations are regularly provided: a vertical one that intakes and exhausts through the roof, and a horizontal one with short concentric tubes connected to outdoors through the wall. To assess the thermal efficiency of the equipment, we have measured several running parameters, like temperatures, airflows, and gas consumption. Thermal efficiencies for indoor heating in the range of 40–60% were found, for vertical and horizontal chimneys, respectively. The main design characteristics leading to the poor performance were identified: low emissivity of the combustion chamber surface (leading to poor radiation transfer), tight enclosing cabinet (affecting radiation and convection transfer), and no chimney flow control (leading to large outgas losses). The findings are in agreement with previous works on the excessive amount of natural gas consumed in residences in the area studied.

Introduction

The amount of energy required for heating is primarily established by the weather pattern, the comfort level, and the thermal quality of buildings and equipments. Conservation techniques can greatly lower energy demands, even for locations with cold weather [1], [2]. Nevertheless, due to the large variety of fuels and designs available, heating device efficiencies remain a concern for residential use. Recently, Lutz et al. [3] reviewed the efficiency of heating furnaces used in the US and, from a baseline efficiency of 80%; they studied the impact that the cost of improvements could have on households.

Depending on the availability of technologies and resources, the need for space heating is satisfied in a large variety of ways. Centralized furnaces and district heating are common in developed countries, used mainly in apartment buildings but in some areas are also available to one-family houses. Though, individual furnaces, ranging from 1 to 10 kW, are increasingly common and have several advantages, both on building design and consumers’ preferences [4]. In Argentina (as well as in other developing countries), due to low cost and rather easy installation, individual heaters running on natural gas are the most commonly used. The counterpart is that they are less efficient and more polluting than centralized systems. At present, technologies to built high-efficient individual furnaces for almost every kind of fuel are available; however, there exist large regional differences according to degree of development, codes enforcements and energy subsidies.

In a recent work, the use of natural gas in the residential sector in a cold city of Patagonia, Argentina, was investigated [5]. More than 80% of households in the survey have installed individual space heaters of the balanced-flue type. Basically, these devices are gas burners enclosed in a metal chamber. The hot surface of this combustion chamber transfers the heat to the room air by thermal radiation and natural air convection, and the inlet and flue pipes are connected to the outside. In this way, the device does not take oxygen from the room and then it can be installed in closed spaces without extra requirements for ventilation. Since the device is simple, its overall cost for installation is the lowest for space heating in Argentina, where natural gas is the main fuel for household use [6]. In average, around 60% of households in the country are connected to a nationwide gas-pipeline network, ranging from urban areas where the percentage is higher, to a large region of the country with no provision of gas by pipelines. Due to national and regional subsidies, there is a striking difference in prices paid per unit of energy either for natural gas or for other fuels. Compared to bottled gas, electricity, diesel or kerosene, network natural gas has a price in a range of 6–14 times lower, depending on regional subsidies. Firewood is the second cheapest fuel in the country, and can be bought by households at a price three to five times higher than network natural gas.1

In the area studied here (the city of Bariloche: mean annual temperature 8 °C, 3600 HDD [7], [8]), one-family households connected to the natural gas and electricity networks used around 85% of direct energy (excluding transport) in space heating. Electricity is used only for lighting and domestic appliances, being its annual consumption as much as 20 times lower than that for gas. These figures contrast with other locations with similar climate, like Sweden (slightly colder) or Holland (slightly warmer), where the percentages in heating are around 60% [9], [10]. Compared to Stockholm, the consumption of energy in space heating of one-family houses was found to be three times higher in Bariloche [5]. In a much warmer location in Argentina, for instance in the city of La Plata near Buenos Aires (annual mean temperature around 16 °C), the consumption of natural gas was still found to be eight times higher than that for electricity, being the percentage for heating around 30% [11].

The above-mentioned facts motivate the present research. For the cold areas studied here, the low quality thermal insulation in buildings lead to a high-energy demand. Nevertheless, as we shall show below, the furnace efficiency is another relevant concern, usually underestimated. The improvements in both, buildings and equipments, must be done if a better use of energy and lower greenhouse-gas emissions shall be achieved.

In this work, we present an experimental investigation of the most common commercial balanced-flue space heater running on natural gas in Argentina. The residential sector, and small and medium size businesses like hotels, and some office buildings, is mainly equipped with this type of furnace. These furnaces are also exported to 30 countries worldwide, mainly to Latinamerican and Eastern Europe (see for instance two of the manufacturers’ sites: www.emege.com.ar and www.orbis.com.ar).

To assess the performance we have measured the temperatures at the surface of the combustion chamber, and for both the air inlet and exhaust gases, as well as the airflow through the chimney. Two standard chimney configurations (horizontal and vertical) have been considered. The room-heating efficiencies obtained were: 60% for the horizontal chimney and down to 40% with a regular 5 m vertical one. The low values achieved are due to poor thermal radiation and convection transfers to the room, and a lack of flow controller in the chimney (the airflow intake greatly surpasses the required for the combustion reaction involved). We will finally discuss the impact on the residential sector, and the fact that running these types of low efficiency furnaces can only be afforded by users connected to a subsidised natural gas network.

Section snippets

Heating device description

A commercial balanced-flue space heater was used for the present study (brand Emege, model 9030), which is provided with a gas burner of 3.000 kcal/h (3.48 kW). Essentially, this type of heater use a long and narrow burner made by multiple individual Bunsen's like burners, and placed within a combustion chamber made out of steel sheet. It draws air from outdoors for the gas combustion by a primary chimney, and let the fumes exit through a secondary chimney. In Fig. 1, a scheme of the device is

Radiation and air convection heat transfer

Temperatures at the combustion chamber and cabinet surfaces were measured at difference points (see Fig. 2), for maximum and minimum burning power, obtaining a range of 70–570 °C and 40–300 °C, respectively. The highest temperatures are found on the front and the back surface of the combustion chamber.

The infrared photo in Fig. 3 was obtained for maximum flame. It shows the high-temperature gradients throughout the front of the combustion chamber. The sides and lower parts from the burner present

Conclusions

We have investigated the running parameters and thermal efficiency of one of the most common type of space heating furnace in use in Argentina. The heat is transfer to the room living area by radiation and convection. We have set up an experiment in order to determine thermal efficiencies for different flame intensities and chimney configurations. Thermal efficiencies of 40% for vertical chimneys and 60% for horizontal ones were found.

We have identified different causes for the low performance:

Acknowledgements

This work has been done with a grant by Programas Especiales of the Secretaría de Ciencia, Tecnología e Innovación Productiva, Argentina, under number PyPEsE283. Technicians C. Catenazzi and J. Morales at CAB are acknowledged for their permanent support. Engineer S. Gortari has also given support in certain stages of the experiments. The calibrated gas meter was a donation from the manufacturer Elster AMCO of Argentina, and the laboratory of thermalhydraulic at CAB has also given support.

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