Air dewpoint

If the air dewpoint is higher than the water temperature (or more accurately, the surface temperature of the drops), water vapor condenses from the air on the surface of the watet drops. Now the water warms up and the air cools down and at the same time dries up in other words, the cooling tower recovers heat from the outlet air. We will now consider the operation of a cooling tower more closely with the notations of Fig. 4.19. 

Most dryers are designed to produce a drjdng air dewpoint temperature in the -40°F range under normal drying conditions. This will usually be more than adequate, since most moisture-sensitive pol5nners dry very well with a dewpoint temperature in the 0 to —20°F range. 

Figure 1 illustrates the overall block diagram. To raise S03/air with a volumetric content between 4 and 7% SO3, the process air should be dried to prevent the formation of sulphuric acid mist. Filtered ambient air is compressed (about 0.6 bar gauge), chilled (about +5 C) to remove the major part of water by condensation and subsequently dried with a desiccant (silica gel. Alumina) to arrive at a process air dewpoint of about -60 C. 

Notwithstanding the low process air dewpoint, some sulphuric acid/oleum mist condenses in the coolers following the converter tower at temperatures of about 45-50 C. This highly reactive mist can affect the quality of the subsequent sulphonation reaction and therefore a Wgh-efficiency demister is installed before the actual sulphonation step. 

The importance of the air dewpoint on sulphuric acid/oleum formation in the gas raising plant can be seen from the following table ... 

An alternative to sulphur burning and subsequent SO2 conversion to SO3 is the use of stabilised liquid SO3. The use of stabilised liquid SO3 is restricted by the proximity of a supplier. This chemical should be transported and handled with great care. A simplified flowsheet is presented in figure 14. The stabilised liquid SO3 is transferred by a membrane metering pump from the day tank through a filter and into the vaporiser. The gaseous SO3 thus obtain is filtered to remove any remaining mist and then mixed with dried process air (dewpoint -60 C) to the required SO3 level. 

The amount of sulphuric acid (oleum 20%) collected in the bottoms of the SOs/air cooler system and in the SO3 mist eliminator, is directly related to the dewpoint of the dried process air (see 5.2.2.). With an air dewpoint of -60 C, only 2 kg oleum per 24 hours will be formed for a 1 ton AD/h sulphonation plant. With increasing dewpoint the amount of oleum collected will rise sharply. For sulphonation plants working with an SO3 absorber (see section 5.2.6) it is recommended that the collected amounts of sulphuric acid/oleum be discharged into the vessel at the SO3 absorbing system. The acid coming from the various parts of the gas raising plant should be collected via gravity flow in an intermediate tank with level control from where the acid is pumped off with a membrane pump to the SO3 absorber. The use of compressed air is not recommended for safety reasons. 

The control system requires the values of T and AT obsei-ved during the first minutes of operation to be stored as the basis for the above calculation of end point. When the exhaust temperature then reaches the value calculated, diying is terminated. Coefficient K can be estimated from models but requires adjustment on-hne to reach product specifications repeatedly. Products having different moisture specifications or particle size will require different settings of K, but the system does compensate for variations in feed moisture, batch size, air moisture, and inlet temperature. Some exhaust air may be recirculated to control the dewpoint of the inlet air, thereby consei-v-ing energy toward the end of the batch and when the ambient air is especially diy. 

Dewpoint means the temperature of saturated water vapor that has the same vapor pressure as the humid air in question. When the total pressure is constant, the constant vapor pressure means the same as the humidity x. In other words, dewpoint is the temperature of saturated air that has the same humidity as the air being considered. 

By cooling a certain surface so cold that water starts condensing on it and measuring that temperature, the dewpoint can be measured. Combining this with the measurement of the dry bulb temperature, the state of air can be defined. 

The total pressure thus has no importance. If this result is. sought from a Mollier diagram by finding the intersection of the humidity line (jr = humidity of air = constant) and the saturation curve, which gives the dewpoint temperature, a diagram constructed for a pressure of 9.S0 mbar should be used. A decent ap-proximation can be found from a diagram constructed for pressure p 1 bar. 

When the airflow meets a surface whose temperature is lower than the dewpoint, water vapor from the air condenses on the surface of the cooling coil. If all air comes into contact with the cold surface, the state of the air after the process will be at point 3. Some air always escapes the cold surface, and therefore the state of air after contact with the coil is a mixture of saturated air (3) and escaped air (1). The mixing point (2) lies on the line connecting points 1 and 3, as shown in Example 8. The nearer point 2 is to point 3, the more effective is the cooling coil. 

Cooling towers are commonly used for water cooling, but they can also be used for heat recovery from outlet air. If the water temperature is higher than the dewpoint of the air, water will cool in the tower. Cooling is caused by vaporization on the surface of the water drops. The vaporization energy comes from the inner energy of the water and in a certain phase, when the water temperature is lower than the dry bulb temperature of the air, also from the airflow. When the water temperature drops to near the air wet bulb temperature at the observation point,... 

Saturated air is difficult to measure accurately a deviation of 3-5 K must be accepted. In some air conditioning systems, very humid air may condense on surfaces below the dewpoint temperature of the air. Poorly insulated ducts containing humid air can cause serious problems. See Fig. 9.57. 

The dewpoint hygrometer detects the dewpoint temperature of air by cooling a surface in contact with the air to the dewpoint temperature. There are several ways to achieve cooling and to observe the formation of condensate on the surface. The early dewpoint hygirometers were cooled simply by applying the vaporization of ether or some other suitable liquid. Condensate formation on the surface was determined visually. Other cooling methods are to use a refrigerant flow in direct or indirect contact with the back of the surface, or to use electricity with a (thermoelectric) Peltier element. 

The dewpoint hygrometer is claimed to be the most accurate instrument tor measuring air humidity. When properly calibrated, the inaccuracy can be 0.5% RH.- On the whole, the dewpoint hygrometer is a reliable lundamen-tal instrument suitable for many ventilation applications, but is more expen sive than other humidity instruments. 

If the one-point calibration in ambient air is not sufficient, the next best approach is to use the calibration box method.- The air state is created in a closed box made of nonhygroscopic material, like metal or plastic. A controlled state of humidity is maintained by exposing the air in the box to a liquid surface of a saturated salt solution. In practice, a dish containing the saturated water solution of a salt is placed on supports at the bottom of the box. The air in the box is circulated by means of a small fan. The box should be airtight and positioned in a constant temperature environment. The calibrated instruments are placed in the box. A dewpoint hygrometer can be used as a reference. A wide range of humidity can be created by using solutions of different salts. Table 12.5 shows a few examples of equilibrium humidities achieved with different salt solutions. 

Measure, at regular intervals, the air temperature, steel temperature, relative humidity and dewpoint in the area where blast cleaning or painting is to take place. 

The dewpoint for the flue products from gas is a maximum of 60°C and will, in most cases, be lower because of excess air. There is no acid dewpoint because the sulfur content is negligible. It would be thought that condensation would be most unlikely, and this is so for the bulk flue gases. However, the temperature gradient across the flue wall can be such that the skin temperature at the inside wall can be considerably less than the bulk temperature, and condensation will take place. 

Boilers and air heaters can be designed to operate in the condensing mode, that is, the outlet temperature of the products entering the flue can be at or close to the dewpoint. The usual design criteria for flues which attempt to avoid condensation are no longer applicable and it is necessary to incorporate additional features. These will include ... 

The design of the economizer must be robust enough to survive occasional excursions beneath the acid dewpoint and the effects of the methods used to clean the economizer periodically. This may take the form of rapping equipment to shake off deposits, sootblowing by steam or air and water washing with lances. 

Condensation occurs whenever moist air encounters a surface, which is colder than the dewpoint of the air. The dewpoint is the minimum temperature at which the air can contain the amount of moisture within it, and it will vary with moisture content. Condensation can also occur in the air when warm moist air meets cold air, when it is known as fogging. 

If now cooled to 14°C the air would be at its dewpoint (i.e. saturated). If cooled further, moisture is condensed out, the sample remaining saturated as it cools. If now reheated back to 25°C its rh would be lower than 50 per cent. If cooling had not been continued to condense moisture its rh would return to 50 per cent at 25°C. 

The temperature at any intermediate point is proportional to the R total to that point. This may be used to decide whether that point is above or below the dewpoint of penetrating air. 

If the water is chilled, cooling of the supply air takes place together with control of dewpoint and hence humidity of the room treated. Saturation efficiency is given by ... 

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