I have been observing many debates about “industry going green”, but one I have not heard is … “what is the more green basic cooling utility – air or water?”
Anecdotally, I think the current (overly simplistic) answer is … if you have steam, it is probably better for you to have cooling water. If you do not have steam, you do not need to have cooling water. But does this give the “most green” answer?
This discussion could fill volumes – and I am going to limit my discussion to 2 points for and against both water and air. There are other reasons that may be specific to your location which I have not mentioned, and the items I have selected may not be relevant to your location, but I must start someplace. When you have this discussion, some of the advantages and disadvantages may be obvious, but they may not be obvious from a green reason. As society increases the value of being green (even if there is currently no “direct tangible financial benefit”), it may be to your benefit to try to stay one step ahead.
THE ADVANTAGES OF WATER
Water is easy to control. The temperature of cooling water will vary little from winter night to summer day, which allows for process stability. It is not only possible but relatively easy to optimise a process 24/7/365 because of the stability of the system. From a green perspective, optimisation means minimising waste, so water discharge and chemical consumption is reduced.
Water is also cooler than air in the summer. For temperature sensitive applications (such as refluxing a distillation column or condensing steam in a condensing turbine to make electricity) the lower temperature offered by cooling water in the summer means systems will be more efficient than systems supported by cooling air, which increases profit. In addition, because water can be cooler than air in summer, it means we may be able to either avoid a refrigeration system, or make the refrigeration system smaller. From a green perspective, this means less thermal energy input into systems, and higher efficiency (which further reduces fuel consumption).
THE ADVANTAGES OF AIR
Air is lightweight and requires no piping. The naturally distributed nature of air means it is perfect for point use and does not require collecting into a central point. For facilities where weight is an issue (such as an offshore platform), the avoidance of cold utility piping can be quite economically attractive. Low carbon footprint for manufacture, low environmental impact due to reduced size of facility, reduced water footprint due to less need to paint, etc.
In cold climates, air does not change phase – it is always air. We may have to use special chambers to keep the air temperature up, but the fact that air is always a gas makes it very predictable and easy to manage. By removing a restraint caused by physics, it may be possible to further reduce waste using economics – by removing a technical barrier, systems become more profitable.
For the disadvantages, I will simply focus on the opposite of the advantages, but there are others which are beyond the scope of this simple article.
THE DISADVANTAGES OF WATER
Water is heavy. The low temperature rise allowed in most cooling water systems means the water flowrate will be quite high. Combined with the density of water (which will probably be higher than almost any hydrocarbon fluid on the site), and cooling water can become a structural headache. In addition, the piping network will need to allow for the effects of water hammer. From a green perspective, this increases the initial construction load on water (more structural concrete) and more pipe that needs to be painted (spill management, loose paint flakes management, etc). And … the large water flowrates require large motors / engines / turbines, which usually mean a large carbon footprint to produce the power to run the cooling water pumps.
In cold weather, water is not longer water – it is ice. Freeze protection is required to keep the water liquid, and the freeze protection is usually the addition of energy (either mechanical energy to maintain circulation or thermal energy to prevent freezing). This extra energy input adds operating expenses and creates greenhouse gases (an issue for this discussion).
THE DISADVANTAGES OF AIR
The variable temperature of air creates a control dynamic that makes optimisation possible but more complex (and sometimes much more complex). Simple process control may not be adequate to squeeze the last bit of profit out of the process, so income may be slightly compromised. A lack of optimisation means the lack of opportunity to minimise waste, so discharges cannot be truly minimised.
The obvious disadvantage of cooling air is the summer daytime temperatures. Depending upon the location, the temperature can become quite excessive, which adds load to any temperature sensitive system. This higher temperature may require the use of a refrigeration system to justify operation all 12 months of the year. This increases the load on engines and motors, which in turn creates more carbon emissions.
You will note that I often focussed on the secondary effects that have green issues. Water for concrete and the increased need for paint (which can add load to total site emissions) may be a trivial or significant green issue, but it certainly is not the primary issue. The impact of process optimisation on the opportunity to optimise may be another trivial or significant green issue. The ability to optimise may result in annual emissions being the same for both air and water, but instantaneous peak emissions will vary significantly for air (and not for water).
So … is it possible that the “most green” solution is a mix of both cooling water and cooling air?