Optimizing refrigerant charge has always been a key part of designing cooling systems and balancing their efficiency, reliability, performance, and cost. But recently, that balance has shifted, and refrigeration engineers are once again looking at refrigerant as a key piece of the puzzle.

There are two main reasons for this.

First, refrigerant prices are rapidly increasing in certain parts of the world. This is largely driven by dwindling supplies as traditional options are phased down under European F-Gas regulations, and other measures to limit the production of greenhouse gases under the Kyoto, Montreal, and Kigali protocols.

As a result, some refrigerants — such as R-404A — have risen in price by more than 500 percent in Europe since 2017.

Where once refrigerant may have been a relatively minor cost compared to a system’s components, now the charge has a far larger impact on its overall production and installation price – making every saving valuable.

Second, the move to reduce GWP has resulted in the growing use of flammable alternatives. In such cases, having less refrigerant charge materially increases the number of applications where a system can legally and safely be used.

So, in the current climate, reducing refrigerant charge is a key  part of gaining competitive advantage — for manufacturers and installers alike — satisfying end users, and maintaining profitability.

The advantages of reducing refrigerant abound

Refrigerant costs seem unlikely to fall in the near future. Supplies of traditional refrigerants are being reduced, and new alternatives being developed will likely carry the price premium that comes with having limited suppliers and competition.

But it is not only the increase in prices that is an issue. Fluctuating costs and supply mean that designing for lower refrigerant charge also reduces a manufacturer’s exposure to risk should things change at short notice.

Reducing charge can also significantly improve installation flexibility. A reduced charge means A2L, A2, and A3 refrigerants can be used in a greater range of settings too — as it becomes easier to satisfy standards like EN 378:2016 and ISO 5149:2014.

We’ve identified seven approaches engineers can take to reduce refrigerant charge without the need to compromise on safety, efficiency, or cost.

1.Reduce internal volume by reducing piping

Internal volume, of course, is an important factor for refrigerant charge, since there is a direct correlation between the two.

Because internal volume is dictated by the size and number of components, minimizing the length of piping or removing it altogether is vitally important. And the smaller diameter you can practically use, the better. 

This is especially true in the liquid line. Each refrigerant has its own ratio of liquid to vapor density, but in call cases, the liquid refrigerant density is significantly higher than vapor. So even though most of the volume in a system might be gas, the vast majority of its mass is in the liquid phase, which means each reduction in liquid volume has a disproportionately high impact on the overall charge amount.

A potential solution is to move some components closer to the condenser, or design reversible heat pump systems with bi-flow expansion valves, instead of bypassing it by adding parallel piping with check valves.

As long as refrigerant remains as a liquid before it reaches the expansion valve, and as long as the valve has sufficient capacity, reducing the diameter of the liquid line and the associated increase in pressure loss won’t affect system performance.

2.Improve heat transfer efficiency

A high proportion of your charge will be in the heat exchangers, so their designs will have a significant impact on your system.

An efficient heat transfer process in modern plate and microchannel heat exchangers can have a positive impact on system design and can improve system efficiency.

A microchannel heat exchanger (MCHE) uses flat tubes with small channels that not only increase heat transfer efficiency, but also reduce the internal volume and refrigerant charge by up to 70 percent compared to fin and tube heat exchangers. In applications where MCHEs aren’t a viable solution, fin and tube coils with smaller diameter tubes can be used.

Heat exchangers in refrigeration systems have a two-phase mixture of liquid and vapor refrigerant. In the evaporator and condensation processes, the amount of vapor changes from the inlet to the outlet of the heat exchanger. A smart heat exchanger design minimizes the volume taken up by liquid refrigerant and charge in the heat exchanger. 

An asymmetric plate heat exchanger design will reduce internal volume on the refrigerant side and the amount of refrigerant in the system — without an adverse impact on water-side pressure. As a side benefit, this will result in improved heat transfer performance.

3.Consider system architecture

Traditional flooded evaporators require a large pool of refrigerant to work. In falling film evaporators, however, refrigerant is sprayed on the tube bundle, and only a small portion of the tubes are submerged in refrigerant, resulting in significant charge reductions.

In DX systems, the refrigerant flows inside the tubes using flow boiling and condensation processes. DX systems will typically have less refrigerant charge than flooded systems but will also be less efficient.

New DX heat exchanger technologies such as micro-plate heat exchangers work with a very small temperature difference, and offer a similar performance to flooded and falling film systems.

In some applications though, it isn’t feasible to have a packaged solution. The evaporator and condenser sections can only be connected by long refrigerant lines, and they require a significant charge.

Alternatively, a water-cooled condenser and a brine loop can be used to carry the heat from the condenser to a remote cooler, which eliminates long refrigerant pipes and will significantly reduce system charge.

4.Take advantage of new compressor technology

A system design engineer has few tools to meet ever-increasing efficiency requirements. One method can be to use a larger  heat exchanger with a smaller temperature difference. But while this is a reliable way to increase system efficiency, it uses more refrigerant.

High-efficiency compressors can improve efficiency not just in full-load situations or applications, but also in part-loads.

This is particularly true for variable-speed compressors, and those which use an intermediate discharge valve to prevent over- compression in part-load conditions. Using an oil-free centrifugal compressor with variable-speed functionality can significantly increase compressor efficiency.

By taking advantage of new compressor technologies, it’s possible to meet efficiency requirements without increasing charge.

6.Reduce receiver, accumulator, and filter-drier size

Large accumulators and receivers offer opportunities for substantial refrigerant savings if they’re reduced in size or eliminated from the system altogether.

When you consider the damage that can be caused to a compressor in a hot and cold system when there’s no accumulator, or it’s not fit for purpose, it is entirely understandable that many engineers play safe, by specifying a larger component than is strictly necessary.

However, it is a far safer approach to run tests on the accumulator to verify the exact sizing required — preventing compressor damage and excess refrigerant charge at the same time.

Alternatively, you could opt for a compressor design that can accommodate intermittent liquid flow.

Meanwhile, modern filter-drier designs can provide effective filtering with far lower internal capacity, further reducing refrigerant charge on the liquid line.

7.Consider better economizers

Economizers serve an important role in many applications by increasing energy efficiency and system capacity.

Some systems are designed with flash tank economizers that are highly efficient and can handle increased capacity. However, due to the size of the tank and the amount of charge they use, they’re not always appropriate.

Alternatively, an economizer system with a brazed plate heat exchanger can be used. These can achieve almost the same level of performance as flash tanks, but crucially, requires far less charge.

Match the solution to your requirements

Clearly, there is no one right answer covering all applications. But as refrigerant prices continue to play an important role in system design decisions, and as safety continues to be an increasing concern for those using flammable refrigerants, it’s likely that engineers will use a combination of methods to improve cost, efficiency, and competitiveness.

For more information, visit www.danfoss.com.

Publication date: 4/8/2019

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