The concentrations of dissolved gases were analysed in this study as described in Section 3.4.1. The three gases that are known to predominate in such systems were considered, these being oxygen, hydrogen and nitrogen.
4.2.1 Oxygen gas
Fig. 4.1 illustrates the decay in the dissolved oxygen concentration as a function of time after the system filling with tap water. Upon filling, the average system dissolved oxygen concentration was measured as 7280 PPB equal to 7.2 mg/L water. This concentration suggests that at atmospheric pressure, the tap water used was quasi saturated with oxygen as the oxygen solubility coefficient for water at the measured temperature is 7.9 mg/L water (Gerrard, 1976). Relatively high dissolved oxygen concentrations are expected in water as a result of the fact that circa 21 % of atmospheric air is made up of oxygen gas. It is worth noting that tap water could result in a reduction in the dissolved oxygen concentration when compared to still water exposed to atmospheric air. This is due to the possibility of deaeration in storage tanks and oxidation reactions in the pipework. Therefore, the actual concentration of dissolved oxygen in tap water could depend on the actual supply.
A substantial drop in the oxygen concentration was observed during the first 48 hours after filling, when an average dissolved oxygen concentration of 102 PPB was recorded. After circa 4 days, with a daily system operating time of 8 hours and a flow temperature of 75oC, the dissolved oxygen concentration reduced to 11 PPB. A consistent average concentration of 11 PPB was measured during the subsequent days, thus implying that all oxidation reactions stopped. The system was in operation for circa 8 hours daily. Hence, the water cycled throughout the system in a consistent manner and therefore, the system water volume was in constant contact with the untreated steel surface in the radiator and buffer vessel. Central heating systems are known to incorporate untreated steel surfaces that result in an oxidation or rusting reaction (Heat, 1998).
As indicated in Eq. (2.35) apart from the presence of an exposed steel surface, oxidation requires the presence of both oxygen and water. The latter react to
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form ferrous hydroxide that in turn reacts to form ferrous ferrite, water and hydrogen. The formation of black ferrous ferrite as a substitute to the red/brown iron oxide occurs due to the presence of a limited concentration of dissolved oxygen in the water. The presence of heat is also known to increase the rate of oxidation (Davis, 1987). Therefore as reported by Heat (1998) and Lamers (2005), upon filling with fresh tap water, a certain degree of oxidation is expected. This reaction is expected to last until the bulk oxygen content is used up, thus eliminating one of the reactants necessary for the oxidation reaction to take place and consequently terminating the reaction.
Figure 4.1: Dissolved oxygen concentrations over time after system filling (No error bars shown on graph due to the limited error of 1% in the dissolved gas concentration sensor
reading).
Davis (1987), reports similar trends in the depletion of dissolved oxygen as a function of time. Davis also reports that some additional oxidation could occur due to the leakage of air into the system during long term usage as a result of thermal cycling. Such leakages could originate at pipe joints and connections and at the flanges on the negative side of the circulator. Leakages could also occur due to the presence of an undersized or faulty system expansion vessel. A visual inspection of the system water upon filling is also proof to the fact that after circa 100 hours from the system filling, any form of oxidation stops. In fact, the presence of black rust particles in the water resulted in a limited degree of
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cloudiness in the system water generated during the first 3 days from filling. No increase in the level of water cloudiness was observed after this time period, thus further proofing that the oxidation process was terminated upon the depletion of dissolved oxygen.
4.2.2 Hydrogen gas
Fig. 4.2 illustrates the dissolved hydrogen gas concentration in water as a function of the time lag after the system filling with tap water. As expected, the concentration of dissolved hydrogen in tap water is low and does not result in saturation conditions. Hence, considering the solubility coefficient for hydrogen gas dissolved in water at the dissolved gas testing temperature of 40oC and at atmospheric pressure, the maximum concentration of hydrogen gas is 1,490 PPB or 1.49 mg/L water (Baranenko and Kirov, 1989). Low concentrations of hydrogen gas in water, are inherent to the fact that hydrogen is found in very limited concentrations in atmospheric air.
Figure 4.2: Dissolved hydrogen concentrations over time after system filling.
Upon filling, a dissolved hydrogen concentration of 11 PPB was recorded. This increased to an average of 20 PPB after circa 3.5 days from filling. The near doubling in the hydrogen concentration is attributed the oxidation reaction in Eq. (2.33) where hydrogen is one of the by products from the final reaction that leads to the formation of ferrous ferrite. Davis (1987), Heat (1998) and Lamers (2005)
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also report similar trends in the increase in dissolved hydrogen. The ferrous ferrite forms a layer on the internal steel surface of the radiator and buffer vessel. This layer is very thin and consequently could break down into fine particles which float in the system water (Davis, 2000). The latter could be attributed to the limited cloudiness observed in the water. This process exposes the steel surface to water and dissolved oxygen thus ensuring that further oxidation reactions take place until the bulk oxygen gas concentration is consumed thus leading to a termination of this reaction.
4.2.3 Nitrogen gas
Fig. 4.3 illustrates the dissolved nitrogen gas concentration as a function of time. The experimental error due to the TGM system amounting to ±1.1% is illustrated in the form of error bars on the data points. An average dissolved gas concentration of 18,800 PPB or 18.8 mg/L water was measured. As expected, and considering the solubility coefficient for nitrogen at the testing temperature, the tap water was saturated with nitrogen gas at atmospheric pressure. Throughout the measuring period the dissolved gas concentration was relatively constant. However, some cycling in the nitrogen dissolved gas concentration was evident.
This phenomenon can be attributed to the system cycling where gas is deaerated due to high temperatures, hence during the heating phase. When the system is idle, the gas pockets generated in the radiator and buffer vessel as a result of deaeration, dissolve back to the water due to a higher solubility of nitrogen in water at lower temperatures. Hence this results in a higher dissolved gas concentration.
Other dissolved gases in water such as argon and carbon dioxide are known to amount to less than 1% of the dissolved gas content in water exposed to atmospheric air. This suggests that these gases are present in very low concentrations in their dissolved form in water. Therefore, in agreement with the findings of Davis (1987) and Lamers (2005), the present study suggests that nitrogen is the dominant dissolved gas in a central heating system.
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Figure 4.3: Dissolved nitrogen gas concentrations over time after system filling.