Whether you’re designing the next-generation electric bus or engineering a robust stationary power system, managing heat efficiently is critical to an application’s operation. And when it comes to cooling with refrigerants, understanding how condensing works – and the method you choose to do it – can make a major difference to system performance, complexity, and longevity.
In this article, we break down two key approaches to condensing – direct and indirect. We’ll explore how each works, where they fit best, and how they apply to real-world thermal systems.
What is Condensing in Thermal Systems?
At its core, condensing is the process of converting a gas into a liquid as it cools. In a thermal system, we use this phase change to reject heat from a refrigerant. It’s a key part of the refrigeration cycle, which is the foundation of most active cooling systems.
Here’s a high-level overview of how that cycle works (see figure 1):
- Evaporator (Heat Absorption): The refrigerant enters the evaporator as a low-pressure, low-temperature liquid. It absorbs heat from the surrounding environment, such as from a driver cabin) and evaporates into a low-pressure, low-temperature vapour.
- Compressor: The vapour is compressed, which raises its pressure and temperature. It leaves the compressor as a high-pressure, high-temperature vapour.
- Condenser (Heat Rejection): The high-pressure, high-temperature vapour enters the condenser, where it releases heat and condenses into a high-pressure, high-temperature liquid
- Expansion Valve (Pressure Drop): The high-pressure, high-temperature liquid passes through the expansion valve, which reduces its pressure and temperature. Then, the cycle repeats.
This cycle underpins active cooling systems across a wide range of applications – from driver air conditioning in an excavator to battery chillers in hydrogen fuel cell trucks. But while the cycle itself is a constant, design and system engineers are faced with a crucial decision: how, and where, to reject that heat.
Direct vs Indirect Condensing: What's the Difference?
Once you’ve established the need for condensing in your system, the big decision is how you go about it. That’s where the choice between direct and indirect condensing comes into play.
Both are viable methods for removing heat from the system, but they approach it in different ways, and the right choice often depends on the specific needs of your application.
Direct Condensing Explained
Direct condensing involves the refrigerant or working fluid coming into direct contact with the cooling medium – typically air. This setup enables efficient heat transfer as the refrigerant releases heat directly into the surrounding environment.
Common in air-conditioning units, direct condensing is relatively simple in design and is ideal for applications where a straightforward thermal connection is possible.
In many systems, airflow – typically driven by fans – is also used to support the condenser’s ability to reject heat efficiently, particularly in low-speed or high-ambient environments.
Having reliable airflow and a robust cooling system is essential for the systems performance. We'll be sure to do a deeper dive into those functions in our upcoming blogs, amongst many other topics relating to thermal management. Sign up to our Thermal Academy newsletter and be the first to find out when it's available.
Direct Condensing Applied:
A good direct condensing system example is our Compact BTMS with active cooling. The system uses direct condensing to efficiently manage heat from an application’s batteries. Using the same process outlined in figure 1, our BTMS absorbs heat from the battery and releases it directly into the ambient air, allowing us to regulate the temperature to optimise performance and longevity.
Indirect Condensing Explained
Indirect condensing takes the cycle shown in figure 1 and adds in an additional step: instead of releasing heat directly into the environment, the refrigerant exchanges it to a secondary loop.
As shown in figure 3, rather than rejecting the heat directly into the atmosphere, heat is transferred to an intermediary fluid, such as coolant. This fluid then passes through a secondary cooling system, such as a radiator, where it is rejected.
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Indirect Condensing Applied:
Grayson’s new Vehicle CTMS uses indirect condensing to manage heat across several vehicle systems, including battery coolant, power electronics coolant and cabin HVAC system.Â
The CTMS calculates the most efficient way to transfer heat in order to manage all thermal requirements of the vehicle. It will transfer heat into the power electronics coolant (indirect condensing) and reject it to atmosphere through the coolant radiator. Alternatively, it will extract heat from the power electronics coolant through the process of heat recovery or air source heat scavenging. This process enables the compact system to reduce the amount of refrigerant used, making it ideal for modern electric and hybrid vehicles.
Indirect vs Direct: Pros and Cons
Now you understand how both condensing methods operate and where they fit, you may be weighing up which approach is best suited to your design. While this depends on a multitude of different factors to determine the right solution for your application, it can help to recognise some of the broader advantages and trade-offs of direct and indirect condensing.
The below summary points can act as a useful top-level reference guide when making system-level decisions, whether you’re prioritising simplicity, control, packaging, or compliance.
| Direct Condensing | Indirect Condensing | |
|---|---|---|
|
Pros |
High Efficiency – Direct heat transfer maximises cooling performance. Simplified System Design – Fewer components mean lower complexity and reduced maintenance. Cost-Effective – Ideal for budget-conscious applications. |
Precise Temperature Control – Essential for applications requiring strict thermal regulation. Greater Flexibility – Enables heat dissipation over longer distances. Scalable – Accommodates high heat loads and complex systems. |
|
Cons |
Limited Precision – Less control over temperature regulation. Not Suitable for High Heat Loads – Struggles with large-scale cooling demands. |
Increased System Complexity – More components mean higher costs and maintenance demands. Reduced Efficiency – Additional heat transfer steps result in minor energy losses. Less Suitable for Extreme Ambient Temperatures – Indirect systems struggle at 50°C+ environments |
Choosing the Right Approach
There’s no one-size-fits-all answer when it comes to selecting between direct and indirect condensing. The right solution depends on the specifics of your application – everything from the size and location of your components to the expected heat load, refrigerant strategy, environmental conditions, and integration needs. It’s not just about what works technically – it’s about what works best for your application, your vehicle architecture, and your production goals.
Across our diverse product range, we have solutions to support both approaches. Whether you’re looking for a high-efficiency direct condensing solution like our Vehicle or Rail BTMS or need the precision and integration benefits of an indirect system like our Vehicle CTMS, we can help you find the right fit.
And the earlier thermal considerations are brought into your vehicle development process, the better. Getting ahead of thermal design early in platform architecture can help avoid compromises later – saving time, cost, and performance setbacks.
The earlier you consider thermal integration in your platform architecture, the more options you’ll have. Our team of experienced engineers has decades of thermal management experience and can support you through that process – helping you navigate cooling strategy, system selection, and design optimisation tailored to your needs.
Let's Talk Thermal
So, condensing may only be one part of the refrigeration cycle, but it plays a big role in shaping your thermal system’s performance, efficiency, and reliability. Understanding how direct and indirect methods compare is essential – but making the right choice also comes down to your specific requirements.
Whether you’re working on an electric bus, a hydrogen fuel cell truck, off-highway equipment, or stationary power systems, our thermal engineers can support you with expert insight and proven thermal solutions. If you’re navigating thermal decisions or evaluating condensing strategies, reach out – we’d be happy to work with you to define the best solution for your system.
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