Airflow cooling

A nominal resiilt of this techniqne is that the reqnired airflow rate and eqnipment size is abont two-thirds of that when evaporative cooling is not nsed. See Sec. 20 for eqmpmeut available. 

Operating co.sts. Power requirements for air-cooled heat exchangers can be lower than at the summer design condition provided that an adequate means of air-flow control is used. The annual power requirement for an exchanger is a function of the means of airflow control, the exchanger seiwice, the air-temperature rise, and the approach temperature. 

Evaporative Cooling of the Turbine. Traditional evaporative coolers that use media for evaporation of the water have been widely used in the gas turbine industry over the years, especially in hot climates with low humidity areas. The low capital cost, installation, and operating costs make it attractive for many turbine-operating scenarios. Evaporation coolers consist of water being sprayed over the media blocks, which are made of fibrous corrugated material. The airflow through these media blocks, evaporates the water, as water evaporates, it consumes about 1059 BTU (1117kJ) (latent heat of vaporization) at 60 °F (15 °C). This results in the reduction... 

If, instead, the air is damped adiabatically with the wet cloth, so that the state of the air varies, the cloth will settle to a slightly different temperature. Each state of air (0, x) is represented by a certain wet bulb temperature 6, which can be calculated from Eq. (4.116) or its approximation (4.123), when the partial pressures of water vapor are low compared with the total pressure. When the state of air reaches the saturation curve, we have an interesting special case. Now the temperatures of the airflow and the cloth are identical. This equilibrium temperature is called the adiabatic cooling border or the thermodynamic wet bulb temperature (6 ). 

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,... 

The quantity aAp is defined separately for each type of cooling tower. It depends on many variables jet pressure, jet division, airflow velocity, and others. The total energy balance for a cooling tower is (see Fig. 4.19)... 

Similar studies were conducted by Troyanovsky, who concluded that to maintain the airflow pattern in rooms with heated or cooled air supply as in isothermal conditions, it is necessary that the rise of horizontally supplied jet does not exceed Ay = 0.1 BH at the distance from the outlet X = 0.15K BH) -. From this assumption the following equation for the maximum air temperature difference was derived ... 

Airflow near the hood can be influenced by drafts created directly by the supply air jets (spot-cooling jets) or by turbulence of the ambient air caused by the jets, upward/downward convective flows, moving people, and drafts from doors and windows. 

It can sometimes be easier, inside a specific volume, to use supply than to use exhaust air to control contaminants because of the much longer range of influence air from a supply opening has when the openings have the same size and the same airflow rate. For some contaminants and in some processes it may also be necessary to supply additional breathing air to the workers and this air could then also be used to control and transport contaminants. For spot cooling of workers or work pieces it is possible to use point jets originating from specific supply inlets. 

Supply Air When designing workbenches, it is essential that the supply air face area be large enough to cover the contained area. Therefore it is important to have some indication of the operator s range of movements for all intended operations. Moreover, for efficient protection the supply airflow must be adequate to get a stable flow field that will not be affected by ambient disturbances. In industrial applications the suitable mean supply air velocities are typically between 0.2 and 0.45 m s h Low velocities should be used when the distance between the supply air unit and the operator is small or for cool supply air. High velocities are applicable at greater distances and in hot environments, with thermal comfort being considered. 

FIGURE 10.90 Suppiy airflow patterns with (o) isothermal supply air. b) cool supply an. and (r). warm supply air. 

Airflow in space, air Cjuality at all points in space, local age of ait, entraininent of jets, heating. and cooling by airstreams (heat transfer), buoyancy of tvarm and cold jets, thermal comfort at arbnra v points... 

Combined thermal and multizone airflow models are needed for problems such as thermal comfort analysis in naturally ventilated buildings, determination of heat-removal capacity by natural ventilation, design and evaluation of passive cooling by nighttime ventilation. This is outlined in more detail in Section 11.5. 

LESOCOOL is an easy-to-use computer program for the determination of passive cooling potential by nighttime ventilation. 21 [t is based on a simple combined airflow and thermal model, originally described in Van der Maas and Roulet. -... 

To save energy, many HVAC systems employ a mechanism for regulating the flow of outdoor air called an economizer cycle. An economizer cycle takes advantage of milder outdoor conditions to increase the outside air intake and in the process reduces the cooling load on the system. Controlling the rate of flow of outdoor air appears simple, in theory, but often works poorly m practice. The small pressure drop required to control the flow rate of outdoor air is rarely controlled and monitored. Quite often, the damper system used to regulate the airflow is nonfunctional, disconnected from the damper actuators, or casually adjusted by building operators (Institute of Medicine, 2000). 

A typical performance range of capacides of rotary lobe vacuum pumps is shown in Figures 6-48A, 6-48B and 6-48C. Another set of curves for rotary lobe pumps, shown in Figure 6-49, provides the brake horsepower, airflow at inlet (CFM) (referred to as their standard volume at 70°F and 29.92 inch Hg abs discharge pressure—essentially atmosphere), and the temperature rise through a non-cooled (no internal or external cooling) vacuum. All data... 

The running of a cool airflow under a warm wind is another cause of temperature inversion. As a rule, the presence of an inversion implies a highly stable atmosphere one in which vertical air movements is rapidly damped out. In such a situation, fog and airborne pollutants collect, being unable to move freely or be dissipated by convection. 

Provide fresh air. For maximum effect, a ducted fresh-air inlet system will provide cool air in summer and, if fitted with a recirculation unit, allow a flow of tempered or warm air in winter. The air outlet should be positioned to ensure that the airflow does not cause a draft on the back of the neck. 

Where the pan is in the air stream the condition downstream of the pan has increased moisture content (kg/kg) found from the airflow and moisture input. On a psychrometric chart, this will lie on a line of sensible-to-total heat ratio of 0.3. Thus, the psychrometric plot shows a steep rise in moisture content with a small rise in dry bulb temperature. The latter is a disadvantage when cooling is... 

Where it is available the source can be a separate boiler plant, but common practice is to employ purpose-made electrode boilers within or adjacent to the plant. The latter reduces sensible gains to the plant but, being essentially saturated steam, condensate return pipes are required. In addition to the rise in moisture content of the air (kg/kg) being dependent on airflow and steam-injection rates, there is a very small increase in dry bulb temperature by the cooling of the vapor to the air temperature. The rise in total heat is total heat of steam (kJ/kg) x quantity supplied per kg air. 

Lowe, H. J. and Christie, D. G. Inst. Mech. Eng. Symposium on Heat Transfer (1962). Paper 113, 933. Heat transfer and pressure drop data on cooling tower packings, and model studies of the resistance of natural-draught towers to airflow. 

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