A Chilled Water Buffer Tank Sizing Guide for Optimal System Performance

If you’re looking for the best approach to sizing a chilled water buffer tank or vessel, you’ve most likely seen chillers cycle because the buffer vessel or tank is too small.

Have you ever been annoyed by temperatures that fluctuate as your load changes? As the load decreases, the room’s temperature drops abruptly.

The system fails due to a low temperature fault, necessitating a service call to the location to complete the reset. In addition, there is a possibility of flooding back and oil buildup in the compressor, among other potential issues.

Perhaps your problem was the opposite: going much over budget for an 8,000-liter buffer tank when you only needed half that volume.

Calculating the buffer tank size required for your chilled water system may appear to be a daunting undertaking, but getting chilled water buffer tank sizing right is essential for your chillers and other equipment requirements for your hvac system and customer satisfaction.

The chiller manufacturer should be your first stop, because all chillers can have nuances. Nobody knows their chillers better than they do, so our advice is to contact them first for a recommendation.

When is it appropriate to use a chilled water buffer tank?

A chiller requires a particular system volume to function in low-load mode without cycling.

Cycling can occur in low-load situations, such as when everyone who works leaves the premises at the end of the day.

The building load significantly decreases, necessitating an increase in the system’s volume to avoid cycling and subsequent damage to the chiller.

Identifying scenarios that require a buffer vessel or tank for chilled water

Water chillers require a certain system volume to operate in low-load conditions without cycling.
If the total system volume (TSV) is less than the chiller manufacturer’s requested capacity, a buffer tank is required.

The benefits of using a buffer vessel or tank in chilled water systems

Prevents temperature fluctuations and system cycling.
It ensures optimal chiller performance, improves energy efficiency and prevents potential damage.

Buffer Tank Sizing Fundamentals

Understanding the variables that affect the size of the buffer tank is crucial.

Determine the kW rating of the chiller and the necessary system capacity.
The total system volume (TSV) encompasses the water capacity volume of the chiller, pipe system, and buffer tank.
Flow rates and desired temperature fluctuations.

The formula: calculating buffer tank size accurately

First,the chilled water buffer tank sizing rule of thumb

Chiller manufacturers can differ in their system volume needs; however, four litres per kW seems to be the normal ratio for air conditioning system comfort control applications. Thus, chiller kW x 4 litres = required system capacity (litres).
However, multifunction chillers can be up to 11 liters/kW. This is due to the need to balance the heating and cooling within specific limits, which complicates the system and necessitates the additional volume.

Then the accurate calcualtion is:

Required System Capacity (L) – Total System Volume (L) = Required Buffer Tank Volume (L).

V=N x 60 x Z / 4.18dt

V = total system water content (litres)
N = the chiller’s first capacity step (kW).
Z = the minimum allowable running time (minimum 5 minutes).
dt represents the temperature difference at the minimum part load condition, which is, for example, 2 °C.

This allows the chiller to operate smoothly at low load capacities, and it should provide sufficient thermal storage to offer at least 5 minutes of operation after the unit has stopped running.

The Role of Bypass Valves in Buffer Tank sizing

The positioning of the bypass valve is important; it can’t be too close to the chiller.

To avoid damage from cold water flowing back into the chiller, position the bypass valve far enough away from the chiller.
The valve should be positioned to ensure the volume of water enclosed in that section of pipe meets the specification or minimum volume of water required by the chiller.

How do bypass valves affect the total system volume?

The TSV used in the buffer tank volume calculation should be adjusted based on the length of pipe required for the water to come out of the chiller and return through the bypass.

Avoiding Common Sizing Mistakes

The consequences of undersizing a chilled water buffer tank are significant.

There are temperature fluctuations, system cycling, and potential damage to the chiller.
Adjustments to the BMS and installing additional valves may help temporarily, but they cannot solve these problems.

Strategies for preventing undersizing

Use the correct formula to calculate the required buffer vessel or tank volume.
Consider variables such as flow rates, desired temperature fluctuation, and chiller kW rating.

Choosing the Right Buffer Tank

The process of determining the buffer tank capacity involves taking into account the stabilisation of the system and the actual water level.

Calculate the water required for system stabilisation using the formula:M1 = QT / (C △ T), where Q is the unit’s heating capacity (in KW), T is the defrost time (in seconds), C is the specific heat capacity of water (4.187), and △ T is the maximum allowable drop in water temperature (in °C).
The actual water volume of the system is the sum of the water capacity per meter of the floor heating pipe and the total length of the system pipe, including the water capacity of radiators.

Then look at this webpage for our range of chilled water buffer vessels or tanks and select your appropriate buffer vessel for chilled water

Material considerations, installation guidelines, and maintenance tips

Consider material options such as carbon steel or stainless steel.

Ensure proper installation and maintenance to prevent corrosion and damage.

Regularly inspect and clean the buffer tank to maintain optimal performance.

Chilled Water Buffer Tank Sizing