27 replies to this topic

[mastergii]

Hello everyone,

 

I am designing a closed looop cooling circuit of water, and want to use a cooling tower in Hysys, but as far as I know, this is not possible?

 

I want to cool down water, so what I am thinking is to use air cooler, are there any better suggestions?

 

My data is:

Air temp: 308 K

Wet bulb temp: 300 K

 

Water temp inlet: 328

 

This is 55 relative humidity, right? So what I'm asking is, which temperature will I have to use as inlet air temp in my air cooler, and how I can, in the best way possible, "simulate" a cooling tower in Hysys ?

[thorium90]

Perhaps the easiest way would be to use the cooler module. The duty of the tower wont change significantly with the changes in the process and using the cooler would therefore be a reasonable assumption.

[mastergii]

Perhaps the easiest way would be to use the cooler module. The duty of the tower wont change significantly with the changes in the process and using the cooler would therefore be a reasonable assumption.

Ah,Okey. The main purpose of my simulation is to compare different cooling systems. And since I need to use the air temperature for the air cooler (I don't think I have that possibility in the cooler), I thought maybe an air cooler would be the best choice.

 

I was wondering if you could help me with a small thing. As my first post says,  Air temp: 308 K, Wet bulb temp: 300 K, what would be the right air temperature to use in the air cooler?

 

Was thinking about mixing air at 308K with water at the same temperature. I used the diagram and found that when you have 55% relative humidity, you would have 20 g water/1000 g air. This will give me an temperature of about 288K. Is this thinking right?

[PingPong]

A wet cooling tower is just a simple column (with a few theoretical stages) with the warm (328 K) water entering at the top and the cooling air (308 K) entering at the bottom. Should be easy to simulate that in Hysys, or any other simulator.

 

The cooled water leaving the bottom of the cooling tower can never become colder than the adiabatic saturation temperature (wet bulb temperature) of thye entering cooling air. In actual practice the cooled water is usually about 2 - 3 ^oC warmer than that, so in your case expect 302 - 303 K.

[mastergii]

A wet cooling tower is just a simple column (with a few theoretical stages) with the warm (328 K) water entering at the top and the cooling air (308 K) entering at the bottom. Should be easy to simulate that in Hysys, or any other simulator.

 

The cooled water leaving the bottom of the cooling tower can never become colder than the adiabatic saturation temperature (wet bulb temperature) of thye entering cooling air. In actual practice the cooled water is usually about 2 - 3 ^oC warmer than that, so in your case expect 302 - 303 K.

Okey, Thank you for your answer!!

 

What if I chose to use a WSAC - wet surface air cooler. How could I simulate that, and how will the temperatures be now?

[PingPong]

What process fluid will be cooled inside the tubes? What are its inlet and outlet temperatures?

 

Or is the closed cooling water inside the tubes, meaning that you cool the closed cooling water with open cooling water? That would not make much sense to me.

Edited by PingPong, 01 December 2013 - 06:03 PM.

[mastergii]

What process fluid will be cooled inside the tubes? What are its inlet and outlet temperatures?

 

Or is the closed cooling water inside the tubes, meaning that you cool the closed cooling water with open cooling water? That would not make much sense to me.

 

Okey, I have a process on a platform A, with a refrigerant going through compressors and pumps. I use a HX at platform A to cool down the refrigerant. I will be using water to cool down my refrigerant in the HX. The water is then, through pipes, transported to platform B. At this platform, I want to cool down the water with an WSAC, before it is sent back to platform A. So my water comes in with a temperature of 328 K. I want to cool it down to 313 K. 

 

The air temperature is 308 K, Wet bulb temp: 300 K. Relative humidity is 55 % 

 

My mind has always been thinking that a WSAC is almost like a cooling tower, isn't that correct? How can I solve this problem?

[curious_cat]

 

 

My mind has always been thinking that a WSAC is almost like a cooling tower, isn't that correct? How can I solve this problem?

 

 

Is it? I'd think a WSAC is more like a cooling tower with a HEX in series. i.e. the water outside the tubes can only get say 3 C above the 

 adiabatic saturation temperature

 

But after that you still have your regular HEX wall deltaT (say 5 C). because of the wall metal & film resistance on either side.

 

I've never designed one but I'd intuitively expect you cannot approach the AST as close as you can in a cooling tower. 

[PingPong]

And moreover a WSAC is much more expensive than a simple cooling tower.

A WSAC is in my opinion only interesting if the process fluid (in this case the refrigerant) can be run through the tubes in the WSAC.

 

On a platform I would expect that seawater is used as a cooling medium, either direct, or indirect by cooling a closed cw system.

 

I use a HX at platform A to cool down the refrigerant.

What is the refrigerant inlet and outlet temperature of this HX ?

[curious_cat]

 

A WSAC is in my opinion only interesting if the process fluid (in this case the refrigerant) can be run through the tubes in the WSAC.

 

 

Agreed. Don't know if I'm right, but I think of s WSAC as an improved Air Cooled Heat Exchanger. You save cooling tower loads than using a conventional Cw loop yet get a better HTC & target temp. than using an ACHE alone.  In a hot dry climate this may help? But on a humid rig I doubt it. 

[mastergii]

 

And moreover a WSAC is much more expensive than a simple cooling tower.

A WSAC is in my opinion only interesting if the process fluid (in this case the refrigerant) can be run through the tubes in the WSAC.

 

On a platform I would expect that seawater is used as a cooling medium, either direct, or indirect by cooling a closed cw system.

 

 

 

I use a HX at platform A to cool down the refrigerant.

What is the refrigerant inlet and outlet temperature of this HX ?

 

 

Refrigerant inlet: 378 K

Refrigerant outlet: 308 K

Water inlet : 303 K

Water outlet: 318 K

 

Is this possible?

[PingPong]

In message #7 you wrote that the cooling water from the refrig HX had to be cooled from 328 to 313 K.

Now you write that the cooling water in the refrig HX warms up from 303 to 318 K.

That's a 10 ^oC difference in temperature level of the cooling water. That makes a lot of a difference.

 

It seems to me that it would have been better to use a somewhat increased refrig flowrate so that its HX outlet temperature would be 320 K. In that case the HX could simply be an aircooler, as an air temperature of 308 K can easily cool the refrig down to 320 K.

 

The question whether a WSAC can cool water in tubes down to only 3 ^oC above the wet bulb temperature of the air, can only be answered by the vendor. It is not something you can simulate in Hysys. Simply send them an email.

Vendor websites imply that they can, but what they will actually guarantee when you order their WSAC is another matter.

[mastergii]

A wet cooling tower is just a simple column (with a few theoretical stages) with the warm (328 K) water entering at the top and the cooling air (308 K) entering at the bottom. Should be easy to simulate that in Hysys, or any other simulator.

 

The cooled water leaving the bottom of the cooling tower can never become colder than the adiabatic saturation temperature (wet bulb temperature) of thye entering cooling air. In actual practice the cooled water is usually about 2 - 3 ^oC warmer than that, so in your case expect 302 - 303 K.

 

Okey, thank you for your help! I see that I have been a  bit misleading when it comes to the temperature. 

 

Anyways, can you help me simulate a wet cooling tower? I cant make it to work. So my "correct" data is:

 

Air temperature = 35 C

Air humidity = 55 %

These two parameters give me an wet bulb temp on 27 C and a dew point temp on 25C

 

The warm water comes in to the tower with a temperature of 45 C. I want to cool it down as low as possible, which I understand is to 30C,(2-3 degrees higher than the wet bulb temperature.). The mass flow is 400000 kg/h

 

Okey, so the air entering the tower contains a bit of water vapor, because of the humidity, right? So to take advantage of this, would I want to cool down the air before it enters the tower? Cool it down to the dew point temperature?

 

In addition, I guess I need some make up water, since the tower uses evaporation of water as cooling method. Where do I add this? I ampretty new to Hysys, so I'm having a hard time figuring this out, so I would really appreciate it if someone could help me.  

[PingPong]

I want to cool it down as low as possible, which I understand is to 30C,(2-3 degrees higher than the wet bulb temperature.).

In a normal cooling tower that is possible.

In a WSAC it may be very costly as it would require a very big coil area. Only a vendor can tell you whether it is feasible in your case.

 

A WSAC is a sort of heat exchanger with the process medium on the tubeside and air+circulating water on the shellside. Hysys and other simulators do not have the right correlations to calculate U and determine required A.

 

I don't really see the point of trying to simulate a WSAC in Hysys. One can de the overall heat and mass balance just as well by hand. But If you feel the irresistable urge to do so I suggest you use a Heat Exchanger. Send the closed cooling water flow (400000 kg/h @ 45 ^oC) through the tube side, and air + circulating water through the shellside. Water from HX shellside oputlet is recirculated to the shellside inlet. Where you add the make-up water in the circulating water loop does not matter.

Specify for the HX a minimum temperature approach of 3 ^oC (or what you want) between hot and cold streams and see what required UA, and what tubeside outlet temperture, will be calculated. By specifying that UA (from the final simulation run) in subsequent runs you can see the impact of process changes would on the performance of the WSAC.

Edited by PingPong, 03 December 2013 - 12:06 PM.

[mastergii]

 

I want to cool it down as low as possible, which I understand is to 30C,(2-3 degrees higher than the wet bulb temperature.).

In a normal cooling tower that is possible.

In a WSAC it may be very costly as it would require a very big coil area. Only a vendor can tell you whether it is feasible in your case.

 

A WSAC is a sort of heat exchanger with the process medium on the tubeside and air+circulating water on the shellside. Hysys and other simulators do not have the right correlations to calculate U and determine required A.

 

I don't really see the point of trying to simulate a WSAC in Hysys. One can de the overall heat and mass balance just as well by hand. But If you feel the irresistable urge to do so I suggest you use a Heat Exchanger. Send the closed cooling water flow (400000 kg/h @ 45 ^oC) through the tube side, and air + circulating water through the shellside. Water from HX shellside oputlet is recirculated to the shellside inlet. Where you add the make-up water in the circulating water loop does not matter.

Specify for the HX a minimum temperature approach of 3 ^oC (or what you want) between hot and cold streams and see what required UA, and what tubeside outlet temperture, will be calculated. By specifying that UA (from the final simulation run) in subsequent runs you can see the impact of process changes would on the performance of the WSAC.

 

 

Okey, I thought you wanted me to use a simple cloumn with some stages for simulating a cooling tower? I have understood that simulating a WSAC is useless, so want to use my data from post #13 to simulate a cooling tower. And i guess that is done in a different way than described in your last post?

[PingPong]

If you want to simulate a normal cooling tower (not a WSAC), then you use a column as I mentioned in message #4.

Simply feed the 400000 kg/h warm water to the top stage, and the air to the bottom stage.

Adjust number of stages and airflow to get the desired cooled water outlet temperature (say 3 ^oC above wet bulb).

 

 

I was in message #14 under the impression that you now wanted to simulate a WSAC.

[mastergii]

If you want to simulate a normal cooling tower (not a WSAC), then you use a column as I mentioned in message #4.

Simply feed the 400000 kg/h warm water to the top stage, and the air to the bottom stage.

Adjust number of stages and airflow to get the desired cooled water outlet temperature (say 3 ^oC above wet bulb).

 

 

I was in message #14 under the impression that you now wanted to simulate a WSAC.

 

Is it that simple? Dont I need to consider the relative humidity in the air? Because I want to simulate cooling by evaporation.. 

Edited by mastergii, 03 December 2013 - 02:21 PM.

[PingPong]

Yes, it is simple.

 

You need to specify the temperature and composition of the air stream to the column, so including that 20 g water vapor/1000 g air.

[mastergii]

Yes, it is simple.

 

You need to specify the temperature and composition of the air stream to the column, so including that 20 g water vapor/1000 g air.

 

Okey, cool! When I do that, I have some water loss, so I guess I need to add some more to have the same mass flow. Is it more realistic if I do that before or after the column?

[PingPong]

Does not really matter, but I think it is normally added in the basin below the tower.

 

But for your first simulation(s) you need not worry about that. Just leave out the closed cw circulation and have just a feed water to the top stage and an air feed to the bottom stage. When you have a satisfactory result you can see how much water is vaporized and thus how much make-up water you need. There will also need to be a bleed (blowdown) on the system to avoid that the TDS (total dissolved solids) level of the circulating water gets too high (due to some salts in make-up water), so actual make-up = vaporization + bleed.

[mastergii]

If you want to simulate a normal cooling tower (not a WSAC), then you use a column as I mentioned in message #4.

Simply feed the 400000 kg/h warm water to the top stage, and the air to the bottom stage.

Adjust number of stages and airflow to get the desired cooled water outlet temperature (say 3 ^oC above wet bulb).

 

 

I was in message #14 under the impression that you now wanted to simulate a WSAC.

 

Is it possible to calculate how much plot size my absorber will occupy? If yes, how? and will this area be the same as a cooling tower? 

[PingPong]

Absorber? What absorber?

 

Do you mean a WSAC ? But that is not an absorber.

 

I do not know how to calculate the required plot space of a WSAC, but in view of the fact that the air fans are located next to the wet cooling section, instead of on top, one can assume that the plot space will be larger than that of a cooling tower:

 

You best send an email with your WSAC questions to Niagara Blower Company http://www.niagarablower.com/

Edited by PingPong, 08 December 2013 - 03:22 PM.

[mastergii]

Absorber? What absorber?

 

Do you mean a WSAC ? But that is not an absorber.

 

I do not know how to calculate the required plot space of a WSAC, but in view of the fact that the air fans are located next to the wet cooling section, instead of on top, one can assume that the plot space will be larger than that of a cooling tower:

 

Oh, okey. When you said use a column to simulate a cooling tower, I thought  you meant the absorber column in hysys? I dont see what other column equipment I could use?

Edited by mastergii, 08 December 2013 - 06:13 PM.

[PingPong]

A cooling tower is simply a column without a reboiler or condenser.

So I guess an absorber object in Hysys can be used to do the necessary number crunching.

 

How you simulate something in Hysys has no impact on the required plotspace.

It is the cooling tower vendor who determines the required plotspace.

[mastergii]

A cooling tower is simply a column without a reboiler or condenser.

So I guess an absorber object in Hysys can be used to do the necessary number crunching.

 

How you simulate something in Hysys has no impact on the required plotspace.

It is the cooling tower vendor who determines the required plotspace.

How can I calculate the amount of air needed in the actual cooling tower? I guess the amount I get with the absorber in HYSYS in not the right one?