District Cooling - water requirements 2

[BRIS]

We are tasked with providing infrastructure for a major new development and are required to make provision for the future installation of district cooling.

We estimate cooling requirement at about 50,000 tons.

The plant will be located on the coast in the Gulf and the only source of cooling water is sea water. Sea water temp 30C

my question is what are the problems of using sea water for cooling and what volume do we need to make allowance for (abstraction, flushing, return)?

 

[quark]

50000 tons is about 175850 kW and if you consider a 5⁰C temperature difference across the cooling coil, for potable water, the flow rate will be,

Q = mCpdT or 175850 kW = m x 4.182kJ/kg ⁰C x 5⁰C

Therefore, m = 8410kg/s or 30276 m³/hr

The specific heat of saline water is lower than potable water and density of saline water is higher than potable water. The first one requires you to have more mass flowrate for a fixed TD and tonnage than potable water where as the second factor requires more pumping energy. Once you check the specific heat and density, you should compare the running cost of the cooling unit vis-a-vis investment and running cost on water treatment system.

Plus, you should consider scaling and corrosion issues.

 

[BRIS]

quark

Thanks for your reply. I am a civil/hydraulic and out of my depth on this topic. My responsibility is to provide the infrastructure which will allow the client to contract district cooling to a supplier at a later date. I need to size the land area and utilities to supply the Plant. we have loacted the plant next to the sea. Potable water supply to the development is by deslaination. Hence the desire to use sea water.

You say we need 30,000 m3 hour - is that the flow of sea water we need to supply to the plant for cooling or the flow rate of coolant through the plant. I understand that a method of plant coooling is via evaporation cooling towers. It is the supply needed to the plant for cooling through evaporation that I am looking for. I realise I may be off track.

 

[quark]

There are two things to consider. First one is evaporative cooling where you reduce the air temperature by evaporating water. This requires lower ambient RH which is uncommon in coastal areas. If this is the system your people are thinking, I require some more details viz., 0.4% and 1% values of maximum wet bulb temperature and mean coincidental dry bulb temperature and room conditions that are to be maintained. This kind of data is available from ASHRAE or local weather stations. I can check from my resources if you mention the place where you are working.

Second one is the chilled water (recirculation) system and my earlier calculation was based on this. 30000 m³/hr is the fluid quantity that you have to recirculate and this requires one time purification.

However, replenishment is required for cooling tower makeup. If the cooling load is 50000 tons, with the best chiller, your compressor power at full load will be roughly 0.8 kW/TR. This is about 40000 kW.

So, total heat load on cooling tower will be 175850+40000 = 215850 kW. With the evaporation of 1kg of water, you can remove 2417.92 kJ. So, total quantity of water that is to be evaporated is (215850 kJ/sec)/(2417.92 kJ/kg) = 89.27 kg/s or 321.37 m³/hr. Plus, you should consider 0.1% drift and windage losses on circulating cooling water(34536 cu.mtr/hr). This totally becomes 356 cu.mtr/hr.

So, your one time purification quantity will be 30276 cu.mtr/hr (for chilled water) and 34536 cu.mtr/hr for cooling water.

Your continuous purification requirement will be 356 cu.mtr/hr.

My experience(for maximum capacity), so far in 12 years, has been with 1/10th of your requirement and with good quality water. Though I am not wrong on my calculation, I am also not experienced with high saline systems. Silicates of magnesium and carbonates of calcium are detrimental to the system in terms of physical deterioration and poor heat transfer as well. We ideally prefer TDS below 100 ppm and TH below 5 ppm and maintain 5 cycles of concentration in the cooling towers.

Please let me know if I can be of any assistance further.

 

[BRIS]

Quark - Thanks for your informative reply - We are located in the Arabian Gulf. Shade temperature 45C RH 60%.

From my point of view I think I have two problems 1) to provide a supply of about 350 m3/hr and 2) to provide for a piped recirculation system to 35,000 m3/hr. Both are problems.

You have suggested a 5c temperature difference across the cooling coil. The equation is linear so if we allow a 10c difference then presumably the required flow reduces by 50% but no doubt there are many other factors to consider?

I stess my job is to make provision within the infrastructure for future installation of pipes. However, I am interested to find out more - I have searched google but can't find any useful sites - any ideas?

 

[quark]

Your logic and question, both are good. Note that 350 m³/hr is the requirement for the cooling towers for replinishment of evaporated water. This depends upon overall heat rejection from the controlled space and will not change with respect to flowrates.

Regarding circulating cooling water, there are cooling towers that operate on 10 deg.C difference and are used for generators. But in your case, you have a clear restriction by nature itself. The water can't be cooled down in a cooling tower below the wetbulb temperature of air, conceptually. In practice, it can't be cooled down below wet bulb temperature + 2 deg.C (minimum possible) and this is called approach of the cooling tower.

At the given condition, the WBT is 39.3 C and this means, your minimum leaving water temperature is 41.7 C. If you want a difference of 10C to reduce the cooling water flowrate to condenser of the chilling system, the leaving temperature will be 51.7 C. This will run your compressor at very high discharge pressure and what ever savings you have in the investment will go for a toss in the operation, with huge expenditure on energy.

For chilled water, air handling system coils are generally manufactured for 5C. This is optimum difference with respect to sizing of the system. Detailed study in terms of heat load (sensible vs latent) should be done before checking the possibility for a 10 C system. In any case, this will be a customized system.

As you have to only facilitate the system for a future design, my suggestion is to go with the conservative design.

Regards,

 

[BRIS]

Quark - Your response is very much appreciated - thanks.

regards

 

[KiwiMace]

BRIS, I think you are looking for an indirect seawater cooling system. This saves water consumption from your desal plant - a cooling tower system on this sized plant will evap up to 190 L/s!

Often titanium plate heat exchangers are used with a closed treated desal water system cooling the chillers. The seawater is pumped through the heat exchangers and then dumped back out to sea - subject to resource issues. Or you can submerge H/E's out to sea and pump cooling water through them.

There area number of installations of this type - Changi Naval Base, Singapore and the Purdy Wharf Office, Halifax, Canada spring to mind. Google them.

With your limited knowledge and such a large project, I'd recommend hiring a consultant with experience - My old consultancy, Beca Singapore, has recently opened an office in Dubai. You might wish to try them.

 

[BRIS]

CinciMace

Thanks for your reply your 190 l/sec is in the order of other estimates and the contact.

Water cooling is clearly an option. We are also looking at a need to reduce groundwater levels by pumping groundwater from a network of wells to the sea and it may be a solution to pump this through the heat exchanger. groundwater is highly saline with a temp of about 35c. I assume that for a recirculated water cooling system with sea water pumped through the heat exchanger we will need to exchange 5c. Does flow of about 30,000 m3/hr so I would be providing for flows of 8m3/sec for each plant. Is this the right order of magnitude ?

 

[KiwiMace]

I like the 30degC seawater option over the 35degC groundwater. The condenser supply temp needs to be as low as possible to get the best efficiency from the chiller plant.

Your numbers look ok to me. Your chilled water flow will be about 7,600 L/s and your condenser water closer to 9,000 L/s (based on a typical 5.5K delta-t). Remember that the power consumed by hermatic chillers is rejected as heat in the condenser water. I've added power using a moderate chiller efficiency of 0.6kW/RT.

My conservative number for cooling tower loss is 2% of condenser water flow. There are much better towers than this in reality, and numbers less than 1.5% are not uncommon. A high efficiency tower would be a valid design goal under your constraints.

Pipe sizing for CW is in the order of 1.3m dia! from memory of a similar scale project.