[TRNSYS-users] Capacity of buffer tank

[Zhe Li]

Dear David,

 

Thank you very much for your reply. 

 

I did a number of calculations followed by your recommendation in designing
the correct size of buffer tank. However, The answer I am getting is far
from the size of buffer tank that I am allowed to simulate my model. 

 

The rated flow rate I am using is 1548kg/h and time step is 0.1h. For one
time step, the volume is 154.8kg. I would think the buffer tank having a
size of 0.2m3 is sufficient to hold one step volume liquid. However, I
always get error when performing the simulation. The minimum size of buffer
tank that I have to use is 0.37m3 in order to carry out the simulation
without error occurring.

 

I have also worked out the total amount liquid is in the loop. I am using a
pipe inside diameter is 0.022m, the total length of pipe is 60m, this gives
me a total volume water is 22.8kg, I am not sure how this is related to the
size of the buffer tank. 

 

Hope you don’t mind to explain to me a little more.

 

Thank you very much.

 

Zhe Li

 

From: David BRADLEY [mailto:d.bradley at tess-inc.com] 
Sent: 03 December 2012 17:56
To: Zhe Li
Cc: trnsys-users at cae.wisc.edu
Subject: Re: Capacity of buffer tank

 

Dear Zhe,
  In order for the simulation to be numerically stable, the buffer tank must
be large enough to hold at least one time step of liquid. So the answer to
your question depends upon the rated flow rate of the pump in the loop and
on the time step that you have chosen. In the actual system, there is liquid
volume contained in the piping network. I would recommend that you determine
how much liquid volume there is (knowing the inside diameter and length of
each pipe section), then create a buffer tank of equivalent size and reduce
your time step accordingly.
Kind regards,
 David



On 11/28/2012 11:07, Zhe Li wrote:

Dear David,

 

Thanks for your time.

 

As we discussed before, due to the fact that the current air-water  heat
pump (Type 941) is a single stage heat pump, a buffer tank is used to
prevent overheating when a imposed thermal load is employed. Even though the
desired system is not working with a buffer tank, this does solve the
original problem I had. However, I have discovered a new problem. What would
be the right capacity of this buffer tank to built into in order to simulate
the actual system (system without using buffer tank)? I have tried to
simulate using varies capacity tank ( from 0.3m3 to 1m3) as the buffer tank,
the electricity consumption can be different as much as 5% which it is
significant as far as I am concerned. 

 

Also, I understand there is an auxiliary heater built into the air-water
heat pump component. Would you recommend to use this auxiliary heater or use
an external heater built into the circuit where after the buffer tank which
makes it more realistic?

 

Hope I have explained the problem clearly.

 

Thank you very much.

 

Zhe Li


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***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA
 
P:+1.608.274.2577
F:+1.608.278.1475
d.bradley at tess-inc.com
 
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