This sounds like a simple question, but in fact this is just the tip of the iceberg when it comes to heat rejection in a system. Every process has its own ideal operating conditions, and while many are similar they are rarely the same, so cooling systems need to be designed to provide the right amount of cooled water at the correct temperature.
Couple this with environmental factors such as the ambient temperature of the region where the cooling system is required, allowable noise levels and the quality of the water and a 2MW cooling tower selection could vary significantly.
Heat rejection can easily be quantified using this simple equation.
q = m x Cp x ΔT
kW = kg/s x KJ/kg.K x K = KJ/s
q = This is the rate of energy released, in Joules/second or better known as Watts
m = Mass flow rate of the fluid in kg/s
Cp = Specific Heat Capacity of the fluid in KJ/kg.K, Informally, it is the amount of energy that must be added, in the form of heat, to one unit of mass of the substance in order to cause an increase of one unit in temperature, this is usually around 4.18KJ/kg.K for water, although seawater is lower for example.
ΔT = Temperature in – temperature out, this is the change in temperature
If we consider that a process creates 2MW of heat when operating at peak efficiency what could the cooling tower this look like for differing flow rates and temperatures?
Water Flowrate and Temperature
Different equipment can be cooled in different ways, with external heat exchangers, internal heat exchangers, cooling jackets or submersion in the cooling fluid just some of the methods that can be used, though there is no right or wrong way to do this. Subsequently, cooling flows can vary across a range of equipment.
For example, 2MW could be equal to the following:
2MW = 50kg/s x 4.18 x 9.57K; at a flow rate of 50kg/s the cooling water will pick up 9.57K in heat.
2MW = 100kg/s x 4.18 x 4.78K; at a flow rate of 100kg/s the cooling water will pick up 4.78K in heat.
What could these systems look like?
For the purposes of this we are looking at the Marley NC8400 series for comparison, based on a wet bulb of 20ºC and a cold water temperature of 25 ºC
At 50kg/s the cooling tower selection would look something like this:
NC8403 – 18.5kW Fan motor, 5.5m wide by 2.6m long
At 100kg/s the selection would be:
NC8405 – 30kW Fan Motor, 6.1m wide by 3m long.
Why are these selections different, when the heat rejection of the system is the same?
As the cooling water enters the equipment or process, this will begin to heat up. In order for the equipment to operate at the correct temperature, it is not just how cold the entering water temperature that is important, it is also how hot the exit water temperature will be and this is related to the flow rate through the system.
In the above example, 50kg of water heats up by nearly 10 ºC and twice that volume of water will heat up by half that amount which is nearly 5 ºC in this case. The relationship between flow and temperature is inversely proportional.
Because the hot water temperature from the equipment is higher when the cooling water flow is lower, the temperature difference to the cooling air flow is much bigger, as air heats up it can carry more energy, so the exit air temperature will be higher meaning you need less cooling air to reject the heat.
In the case of the higher water flow rate, the temperature difference at the hot water inlet temperature is lower, and therefore you need more cooling air to remove the same amount of heat, so the cooling system becomes larger and the airflow rate increases to compensate.
There are of course upper and lower limits to this, if water flows are too low then creating a suitable surface for heat exchange to occur can be difficult, which will lead to efficiency issues and very high flowrates can result in the cooling equipment becoming abnormally large to create a large enough heat exchange surface, resulting in equipment with very low inlet velocities that are susceptible to wind problems, resulting in loss of efficiency.
Approach to Ambient Air Temperature– Temperature difference of the cooling air to the outlet water temperature
The other critical factor is how close to the external ambient temperature the cold water temperature needs to be. Providing the correct cold water temperature to a piece of equipment or process can be the difference between operating things with higher efficiency and having to reduce output or even needing to shut down due to overheating.
As the approach temperature reduces more cooling air is required to remove the heat at the water temperatures required, this leads to a cooling tower needing to become bigger, or motor powers to increase to achieve the desired cold water temperature.
What happens if my 2MW cooling tower is undersized?
Heat is created by the process, the cooling tower plays a very small part in how much heat is created, although it is often possible to increase efficiency of a process that is suitably cooled or can be further cooled by its cooling system.
If the cooling system is not sufficiently sized to achieve the cold water temperature that it was designed to achieve, then the 2MW of heat will still be removed from the system, but the water temperatures will increase to do this, as shown in the graph earlier. Rather than your cooling tower achieving 2MW at 23ºC cold water temperature it may only achieve this at 25ºC cold water temperature which can affect your process in a number of ways such as a reduction in output, or leading to overheating of your equipment.
Other factors can affect the size of cooling towers, including the cooling water used, seawater for instance has a lower heat capacity than mains water, low noise applications often lead to larger equipment to reduce the noise created by the mechanical equipment, or allow for additional silencers to be installed, and various access an maintenance options can increase the pressure loss through a cooling system leading to larger fan motors to maintain the necessary airflow.
The upshot of this is that cooling should be driven by the process and what benefits your equipment the most, to get the best throughput for the majority if the time. One cooling system is not the same as another and bigger may not necessarily be better so it pays to take time to correctly size your cooling systems to help you achieve the best results.
DHD Cooling Ltd have a wealth of knowledge in defining the correct selection of cooling systems and can support you in the selection of your cooling equipment to offer robust high performing solutions that meet the needs of your equipment and processes, for more information please visit our website www.dhdcooling.co.uk.