Calculating Buffer Vessel Size

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Buffer Vessel Capacity

A quick overview of buffer vessel sizing.

A buffer vessel is a vessel that holds water, increasing the overall volume content of the heating distribution system. The additional water volume absorbs heat (thermal storage) produced by the heating appliance in low load conditions, which the building or system does not yet require principally to prevent plant short cycling.

An overview of buffer vessels ∞ Scroll down for a comprehensive buffer vessel download

Why Use A Buffer Vessel

A buffer vessel or thermal store, captures heat to provide a buffer between load variations in order that biomass boiler appliance efficiency can be optimised, thus alleviating slow response issues.

A buffer effectively prevents cycling of the boiler when the system demand is less than the boiler minimum output. Reducing boiler cycling is important to minimise harmful emissions of CO and NOx.

There are two vessel variations which can be defined as:

Buffer Vessel: Used to capture residual heat on shut-down to improve system efficiency

  • Dissipates heat from boiler on shutdown
  • Protects boiler from overheating
  • Improves overall efficiency
  • Stored heat can be used by boiler on start-up

Thermal Store: Enables a small boiler to serve a system with a higher capacity

  • Boiler can be sized at less than 100% of the system heat demand
  • Allows boiler to operate continuously for long periods
  • Will serve function of a buffer vessel (takes heat on shut-down and feeds boiler on start-up as required)
Buffer Vessel Sizing

Vessel size will depend upon the application, heat output, fuel quality, minimum acceptable on-time cycle of compressors (ASHP & GSHP), the operating temperature differential of the [vessel] controls, hours of operation, etc. The following general definition will determine a vessel size:
Vessel capacity = (required system volume) - (actual system volume)

Where the required system volume is that necessary to accommodate the thermal output of the plant and the actual system volume is the water volume in all pipework and heat emitters.

There are a number of rule of thumbs which can be used to determined the vessel size depending on various factors such as the heat source and fuel quality. Here are some variations:

Method 1 - Litres per kW: 10 litres x Plant size = storage volume (L)
where 10L·kW-1 is a typical lower limit for biomass.

Method 2 – Minimum Operating Time: t= mcΔT/Q
Full explanation in our detailed guide

Method 3 – Smallest Zone: m= Qmin - Qzone x t /cΔT
See our buffer vessel guide download for a full explanation - link below.

Method 4 – Flow Rate Percentage: 10% x water flow rate = storage volume (L)
for a single compressor unit.

Where the symbols used are:

Symbol Definition
t Time in seconds (s)
m Mass of store water (kg) & 1kg = 1 Litre
c Specific heat capacity of water kJ/kgC
ΔT Operating temperature differential (°C)
Q Heat input (kW)
Qmin Minimum heat output of heat source (kW)
Qzone Heat demand of smallest zone (kW)

Now download the detailed free buffer vessel sizing guide 

Sizing For Different Heat Sources

Different heat generating plant have different requirements. Here are a few basic tips.

ASHP: BS EN 15450:2007 suggest sizing the buffer storage volume at 12 to 35 L per kW maximum heat pump capacity.

Biomass: Size between 10 and 20 litres/kWth plant capacity (CTG012); a lower value may be used where loads do not not fall to zero, for pellet fuelled boilers which are more responsive and for boilers with a low turndown capability. Where low grade wood with a high moisture content is used, the storage volume may need to be sized at up to 40 litres/kWth.

Chilled Water:A rule of thumb is around 2.5 to 8 litres per kW or 8 to 14 litres per kW when temperature accuracy is critical.

GSHP: Intermittent use heating load (kW) x 25 (L) and continuous use heating load (kW) x 80 (L)

Now download the detailed free buffer vessel sizing guide 

Accumulator Tank A buffer vessel or thermal store
Buffer vessel A vessel that captures residual heat on boiler shut-down preventing frequent boiler start-ups to improve system efficiency. It will also reduce the energy required from an auxiliary boiler. Usually has a simple on/off control strategy.
Thermal Store A vessel that is charged by the biomass boiler when the boiler output exceeds the load demand enabling a small biomass boiler to meet demand. A progressive control strategy using several temperature sensors will control the boiler output.

Also see buffer vessel sizing in the Carbon Trust tool for biomass boiler selection

Sizing a Buffer Vessel - last updated 13th March, 2016 by Corny