Buffer tank calculation 1

[servinco]

Hi all,
I'm trying to calculate the chiller size and the tank size, based on the following facts known:
- Load of the process.
- Flow in terms of GPM's in he primary loop (process side)
- Required Delta T of the process.
Now,
Based on the above, do you have a kind of guidelines in order to approach the application?
Regards,

 

[Cooky]

We'll need a little more information.

If your process cooling load is fairly constant, the chiller capacity must match this load.

If your process cooling load fluxuates significantly over a short period, it may me possible to 'average' the chiller capacity, rather than select the chiller capacity at peak process load. In this case, you can use the buffer tank capacity to 'store' a volume of chilled water.

Let us know, I'm sure we can help.

Cooky

 

[servinco]

Thank you Cooky for your fast response. Yes, basically the load is constant along the day (no peaks)
I agree with your point regarding the capacity of the chiller in this instance,(since the load is fixed) but my point is how to select the tank size? in order to mix the two flows: hot water from the process and cool water to the process

Thanks again

 

[Cooky]

ok, so we know you have a fixed cooling load. There are two basic types of buffer vessel, the first has just two connections (in & out), this would be fitted in the pipework leading up to the chiller (or on the outlet from the chiller).

The second type of vessel assumes you have a primary and secondary water circuits, the vessel has four connections (two primary and two secondary), it is arranged to act as a very large header. From what you have said, I think this is the type you intend to use.

I have written a program that calculates the minimum capacity of this type of buffer vessel, I'll need the follwing information from you.....

chiller cooling capacity (kW)
chiller minimum step capacity (%)
minimum 'on' time for chiller (approx 6 to 10 mins)
temperature rise between 'on' & 'off' cycle (deg.C)
minimum zone capacity (kW)

Let me know the numbers.

 

[servinco]

Cooky,
Good morning, I hope you had a good one. Regarding your message, the data is the following.
-Buffer tank is that the second type that you mention.
- chiller capacity is 90 tons.
- Chiller minimum step capacity is 33% (Carrier 30GTN090--6)
- The minimum on time for the chiller (not infor avail.) actually, how you can calculate this time??
- The temperature when the chiller is off (starting to operate) in the secondary loop is 64.0C entering; and must be 4.0C leaving to the process. Is this enough for you?

Thank you,

 

[Cooky]

Hi Servinco,

Ouch!!, that's hot!!

The 30GT chiller can start and pull down with a maximum water temperature of 35 deg.C, you really don't want to exceed this value.

You need to arrange the control system to ensure the chiller is not expected to start when the secondary circuit is already at 64 deg.C

The minimum system water content should be 3,400 Litres for the 30GTN-090 chiller, assuming you don't have long pipe-runs, most of this volume should be in your buffer vessel.

Make sure the pipe system is arranged to prevent 'slugs' of very hot water entering the chiller, otherwise the chiller will cycle on & off too often.

 

[servinco]

Hi Cooky,
How can I calculate the mixing temperature in the tank?
This customer has the same application in another facility but with less amount of load; he is using a 30GTN100 with 20% of glycol which in fact (as you know) the chiller is delivering +/-75tons. Kind of background information...
This amount of water of 3,400 lts. is this the capacity of the tank? I know that's your software but can you give some rationale of this calculation?
Thanks again

 

[Cooky]

Hi, servinco,

My program calculates the size of a buffer vessel depending on the criteria based on my earlier questions, it calculates the required size of a buffer vessel when there is a mis-match between the size of the chiller and the connected load.

Because your application is different, I used a Carrier 'rule of thumb' for the 30GT chiller. This states that for a process application, 10.8 Litres of fluid volume is required for each kW of cooling duty, thus 315 x 10.8 = 3,402 Litres