Step-by-step Guide on How to Size a Chiller

For the many years we have been in this industry, we have seen many clients buy chillers without calculating the cooling load requirements or factoring scalability.

As a result, they end up overloading the chillers, which results in compressor failure and ultimately the chiller breaks down.

It is for this reason that our engineers will take you through a detailed process on how to size a chiller. Remember, there are many chiller sizes in the market, but how can you choose a perfect option.

Steps In Sizing A Chiller

i. Identify what the chiller is cooling.

You have to ask yourself first what exactly you are cooling. Applications may range from business structures such as hospitals, data centers, and malls, to process industries and cold rooms.

You must be able to differentiate between comfort cooling and process cooling, since you may end up having misplaced sizing. For comfort cooling, weather, individuals, and time of the day are factored in, while in process cooling, the cooling is constant and predictable.

This means that you have to acquire prior data, such as the application, operating hours, and whether the load is constant or variable.

ii. Understand the meaning of Cooling Load.

We can describe it simply as the total amount of heat that needs to be removed from the system. We normally express it in kW, TR (Tons of Refrigeration), kcal/hr, or BTU/hr.

You have to consider all possible heat sources in your application, and disregarding even one may mean your chiller won’t work as expected. Some of the possible heat sources include equipment and electronic heat, chemical reactions, drives and motors, and friction losses, among others.

iii. Calculate the cooling load.

· Using heat load calculation software

We have several software tools that you can apply with the best accuracy since they factor in various aspects of the process, such as weather data and occupancy schedules. Examples of such software include:

• Carrier HAP (Hourly Analysis Program)
• TRACE, which can assist you in analyzing HVAC systems
• IES (Integrated Virtual Solutions), which analyzes the performance of an entire building

· Manual/Predictive Calculation

In instances where you are not able to apply software, you can resort to manual calculation and estimation methods. An example is using the quick estimate that factors in an area of operation.

Let’s take an example of an office that is situated in a warm temperature environment. The rule of thumb is to take 1 TR for approximately every 35m²to 45m², meaning for 900m² of office space, you will require 22.5 TR.

Remember that you can only use this method in your planning stages for approximation. You cannot apply the same in your final design.

iv. Apply the Heat Transfer formula for process cooling

The formula you will apply to achieve this is:

Q = m x C x ΔT

Where:

• Q represents the heat load in kW
• m represents the flow rate in kg/s
• C represents the specific heat capacity
• ΔT represents the difference in °C

Once you get the value of Q from the above, you will know the minimum chiller capacity that you can operate with.

v. Convert the load into Chiller capacity

The next step is to convert the heat we acquire from above into a form that any manufacturer will understand.  The main conversion to undertake here is from energy in kW to tons of refrigeration.

1 TR = 3.517 kW

As an example, if we have 105 kW of heat load, it can translate to about 30 TR. You should consider this the calculated load and not confuse it with the selected chiller size.

vi. Include a safety margin

This is probably where many designs suffer massive failures, since we guess a lot by adding very large margins in anticipation of future expansions. Your chiller system may end up performing dismally for the rest of its service life.

The best recommendations for chiller operation safety margins are:

• A 10-15% margin if you are dealing with process cooling
• A 5-10% margin if you are dealing with comfort cooling

If you got a calculated load of, let’s say, 50 TR, then getting a chiller of about 52.5 to 55 TR is sufficient for your operations. Anything exceeding that will just be a waste of energy with poor performance.

vii. Confirm The Chilled water temperature and ΔT

You can then measure and confirm the temperature of the chilled water and compare it to the supply water. This is important because it directly impacts your chiller operations, with the difference giving us ΔT.

Supply water normally ranges between 6-7°Cand return water between 12-13°C. This gives us a ΔT of between 5 and 6°C; anything below this will require you to have bigger pumps to increase the flow rate.

viii. Factor in Environmental conditions

Chillers operate in real-life conditions; therefore, the conditions of the site where you need to install yours should be verified. Factors such as constant supply of water, normal surrounding temperature, level of humidity, and altitude are some of these factors.

If you install a chiller that has a rating of 30°C in an environment having a temperature of about 40°C, it will behave differently. This means that your chiller will never reach its desired performance level and will rapidly degrade.

ix. Settle for the most appropriate chiller.

It is imperative that you decide on the type of chiller you want before settling on its size. Decide whether you want an air or water-cooled design, and the type of compressor it should come with.

You should factor in what suits your application between fixed and variable-speed technology. Variable speed may come in handy when you are operating on part-load conditions, meaning you can size your chiller nearer the real cooling demand.

x. Part-Load operations considerations

It is worth noting that the majority of chillers are never operated at their full capacity throughout their entire service life. As a matter of fact, they usually operate at around 40-70% of their capacity based on your application demands.

Ensure that you have data on your application’s peak load and how it should operate under normal load conditions. This helps you avoid excessive chiller sizing while at the same time remaining energy efficient.

xi. Factor in redundancy and future operations

Let’s take a practical example of an application that has a 140 TR load. It will make economic sense to install two chillers with 80 TR instead of having one large chiller to shoulder the load.

This means that you can easily carry out maintenance on one chiller without heavily impacting your operations. Your operations remain flexible while enhancing the part-loading efficiency.

Factors affecting the size of the chiller

Chiller sizing is determined by various aspects that we are going to look at in brief. Some of them include:

• Cooling Load: This is the total amount of heat that the chiller must remove in an operating system. If you estimate inaccurately, you may end up with an undersized chiller that won’t meet your demand, or an inefficient oversized chiller.
• Chilled Water Temperature: When the temperature drops too low, the chiller strains to cool, thereby decreasing its efficiency. You will have to go for a larger chiller so as to sustain the same cooling load.
• Temperature Difference of Chilled Water (ΔT): The difference in temperature between the chilled water entering and leaving the chiller determines the level of water flow. Having an efficient ΔT decreases the chiller’s flow needs and maximizes the efficiency of the chiller size.
• Temperature and Climate: The surrounding temperature of a chiller impacts its heat dissipation. If you are operating at high temperatures and altitude, the performance may be lower, and you would need a bigger chiller.
• Compressor Technology: Various compressor models are designed to handle different loads and working conditions. Part-load is more efficient with variable-speed compressors since you can easily size them closer to your actual demand.
• Load Change: Cooling demand normally varies over time during the year, while the peak load only happens occasionally. Understanding this ensures that you don’t oversize your chiller and enhances performance.

Common Errors When Sizing Chillers

FAQ’s

How does the type of heat transfer fluid affect chiller sizing?

Different fluids, such as water or glycol, have varying heat capacities. A fluid with a lower heat capacity will need a higher flow rate to transfer the same amount of heat, which impacts the size of the chiller.

What is the significance of flow rate when sizing a chiller?

Excessive flow rates or too low flow rates will lead to low efficiency and a lack of stable and optimum operation.

What is the impact of insulation and pipe length on chiller size?

Long piping routes and inefficient insulation contribute to additional heat in the chilled water before it reaches the load, running the risk of the chiller being undersized.

Does system pressure drop affect chiller sizing?

A significant pressure drop requires extra pump effort to sustain the required flow rate. This reduces its overall performance, meaning it will perform below its size design limit.

Can seasonal changes affect chiller sizing?

Yes, weather conditions have a direct impact on cooling loads, hence go for a chiller that works well during peak summer loads and is still efficient at lower loads during cooler seasons.

Chillerone is your Trusted Chiller Manufacturer in China

If you are looking for air cooled chillers or water cooled water chillers, Chillerone is your reliable manufacturer in China. We customize every chiller size and design to match you specific cooling needs, while also projecting the need for future scalability.

With every chiller design, our engineers work closely with your team to ensure you get a perfect chiller size – Let our team size your chiller now.