Buffer Vessel Sizing
Buffer Vessel Sizing
Buffer vessels are required in breweries and dairies when peak loads occur for short periods of time. Fast fluctuations can be due to switching loads on and off. Alternatively this peak load may be from the CIP cleaning of heat exchangers or a heat exchanger contents and framework warming to pasteurisation temperature or due to failure of the heating system steam controls. This has the effect of the chilled fluid leaving the process heat exchanger at abnormally high temperatures and returning to the fluid chiller as a warm slug causing rapid heat load fluctuation to the refrigeration plant. Normally the plant does not have the capacity to cope with the peak loads and / or it is unable to change its compressor capacity quickly enough; therefore relatively warm fluid could be circulated out to the process.
Buffer vessels are sized to minimise this based on the peak load and the time it lasts. However the customer often has difficulty defining this load. In dairies and breweries we find buffer vessels sized to cover heat load fluctuation from 6 to 10 litres per kilowatt of total refrigeration duty are typical and work well.
Size the vessel toward the lower end of this scale for:
- Large systems, say > 2,000 kW.
- Systems with only a single steady heat load where start-up conditions are not critical.
Size the vessel toward the higher end of this scale for:
- Small systems, say <1,000 kW
- Large systems with tight temperature control (+ or – 0.5K).
- Large single heat exchangers, say >20% of total refrigeration capacity.
If a buffer volume is required to limit the temperature of the glycol onto the evaporator (to protect the refrigeration plant) then that volume must be added to the volume calculated here. Typically that warm side volume is 1/3rd of the cold side volume.
In the case above buffer vessels are sized to cope with the surge in load. On small single chiller systems a buffer vessel is often included to compensate for relatively high minimum compressor capacity and to allow the number of starts (at low duty) to be limited. In this case the compressor minimum capacity and minimum temperature fluctuation allowed should be used to calculate the system volume required:
System cold volume (m3)
15 x Min comp capacity (kW) x Min time between starts (mins)
Allowable ΔT (K) x Glycol specific heat capacity (kJ/kgK) x Glycol Density (kg/m3)
Note: This calculation is based on the worst-case time between starts; i.e. when the load is 50% of the minimum duty of one compressor. It also assumes the entire vessel is in the cold supply line.
*The pipe volume between the evaporator(s) and the nearest load on the header excluding the buffer vessel volume.