Injection Water Cooling
The warm water from the condensers needs to be cooled to the lowest practical temperature before being re-used. The cooling process is carried out in cooling towers or spray ponds after which the water is pumped back to the condensers. In cooling towers or spray ponds the exchange of heat between the warm water and ambient air is by,
- conduction between the fine droplets of water and the surrounding air
- evaporative cooling, which is by far the most effective factor.
The efficiency of the system is mainly dependent on the relative humidity of the air. The efficiency should be between 50% and 70%, 60% being a satisfactory average, and the efficiency can be calculated from
η = (Th - Tc) / (Th-Twb)
- ηis the efficiency of the system
- This the temperature of warm water entering the system, °C
- Tcis the temperature of cooled water leaving the system, °C
- Twbis the temperature of wet bulb, °C
An approach temperature of between 8°C and 10°C is possible. Approach temperature is defined as
Tapp = Tc - Twb
Two criteria are involved namely
- depth and
- surface area
This has virtually no influence on the cooling of the water. An average depth of 1 metre should be adequate provided there is sufficient water to fill the flumes, seal wells and flood the injection and export pump suctions at the starting up of the plant. There is no advantage in exceeding a depth of 1 metre, since the increase in the mass of water in the circuit has only a negligible influence on the cooling. The surface area only is important.
Area of the pond
The area of the pond is important on account of the necessity of arranging the nozzles so that the curtains of water formed by them do not overlap or interfere with each other, and so that air may circulate between the sprays. The area of the pond is deduced from the quantity of water which it can treat per hour per unit area of the pond. Tromp suggests 120 lb/ft2/h (585 kg/m2/h), Webre and Robinson 150 lb/ft2/h (732 kg/m2/h). Other figures given are (lb/ft2/h)
Among French manufacturing firms, Fives Lille bases its calculations on 164 lb/ft2/h (800 kg/m2/h). We consider that the best value to adopt is: 750 kg/m2/h
Do an online sizing calculation for a spraypond
The main design features are that nozzles should:
- deliver water in a cone, and
- be of simple design, easily dismantled with no portion of smaller cross-section than the orifice.
Quantity of water delivered
This is calculated from
q = C·S √(2gH)
- qis the capacity of nozzle in m3/s
- Cis the coefficient of contraction, normally 0,5
- Sis the cross-section area of orifice, m2
- gis the acceleration due to gravity 9.81 m/s2
- His the pressure head of water = 0.5m
Nozzles are mounted on pipes called laterals at right angles to the main pump delivery line. Laterals decrease in size towards the end.
These are best grouped in a star pattern with the following dimensions given as guidelines:
- branch diameter to star = 50 mm
- nozzle orifice diameter = 41.3 mm
- length of branch to nozzle = 1220 mm
- distance between cluster centres = 4 m
Spacing between pipes
- a = spacing between laterals, m
- b = spacing between nozzles, m
then ab = q/750, in which q = output/nozzle, in kg/h
Loss of water
- by entrainment 3%
- by entrainment and evaporation 5 - 6%
- A spray pond should be long and narrow to improve the efficiency at the centre, but cost might be prohibitive.
- All laterals should be provided with quick acting flushing valves at their ends, easily accessible.
- Pump suctions should be adequately protected from blockage by tank, bagasse and other flotsam. Easy access for cleaning screens must be provided.
- The arrangement of nozzles may not always conform to a star pattern as cost will be an important consideration but efficiency might suffer.
- Due to loss of water from the pond, a fresh water make up system operating on pond level is required.