Cool Efficiency: Sustainable Heat Transfer Part 1

jamesalexanderfawc
Feb 5, 2024
9 min read

Updated: Feb 6, 2024

Included:

Looking down the barrel of 2024, the more savvy of us will be preparing ourselves for the year to come.

These quiet months give us a great opportunity to self reflect and get our houses in order. One components that more than likely requires some attention is your chiller.

Perhaps not the most pressing of concerns now that its cold and there is less brewhouse throughput, but refrigeration is responsible for between 30-40% of our total electrical bills. In many cases it is the single most energy intensive aspect of the brewhouse. In a warming world, we can expect this energy requirement to increase. (IBD, Module 3, 2024).

So while we have the time, some consideration seems appropriate.

Under The Hood

Let’s remind ourselves quickly what a chiller is.

To cool something down we need to remove heat. The purpose of the chiller then is to remove heat.

It does this by manipulating a refrigerant using pressure and temperature.

The 4 components of the chiller that allow us to do this are the Evaporator, Compressor, Condenser and the Expansion Valve. This cycle is call the the OVC (ordinary vapour compression cycle) and is present in all chillers.

Evaporator

This is the Cooling Capacity of the chiller. In other words, it is the stage that dictates the amount of heat able to be transferred from your target heat source (a fermenting FV) to the refrigerant. The heat energy is “stored” in the refrigerant by making it change phase, from liquid to gas. Substantially more heat is required to change a fluids phase rather than to just heat it up which is why Latent not Sensible heat transfer is used within the refrigerator. This in part dictates the type of refrigerant used. The temperature band of the refrigerant’s phase change needs to be the same as the required heat transfer environment, ie, the heat source needs to be able to evaporate your refrigerant (more important with heat pumps).

Compressor

Once the refrigerant has fully evaporated and is a gas, it is compressed to increase its enthalpy (heat energy). When things get compressed, heat energy increases. Imagine you have particles flying around a space, each time they collide with the wall they create more energy. If you reduce the space, the particles hit the walls more often, generating more heat. The compressor is often called the workhorse because it is where most of the electrical energy used to run the chiller is allocated. This is where Coefficients Of Performance are calculated (a COP of 3 means that for every 1 kWh of electrical energy that is used, 3 kWh of heat energy can be displaced).

Condenser

Cold air or water is subjected to the compressed refrigerant gas to condense it back to a liquid. This is another phases change and allows for the heat energy to be transferred to the ambient air or water.

Expansion Valve

The cold liquid is then expanded to drop the temperature even lower. It helps to actually think of this as a release of the compression as it is often the realisation of the work done by the compressor. This extra cold state allows the refrigerant to take up more heat energy when it passes the heat source, evaporating and starting the cycle again.

Chiller In Action

Now let’s look at how this works in the brewhouse.

Because the primary refrigerant used in the OVC cycle is often toxic, a secondary refrigerant is used to transfer heat energy from the heat source (your fermenting FV) to the chiller. This secondary refrigerant (commonly glycol) passes the primary refrigerant in the evaporator where it transfers the heat energy.

If the purpose of the chiller is to remove heat, then we can assess the likely requirements of the chiller by identifying our main heat sources:

Controlling fermentation temperatures
Wort cooling / CLT cooling for pitching
Crashing during cellaring (flocculation and carbonation)

Q (heat transfer) = m (mass) cp (heat capacity) ΔT (temperature difference)

These are our Intentional Heat requirements. They are the heat that we are actively trying to remove.

The next thing to get to grips with is the Cooling Capacity of our chillers. This is the amount of heat that the OVC cycle can displace over time.

Q = (V p) Cp * (glycol temp @ Inlet - glycol temp @ Outlet)

Q = Heat transfer kW

V = Volume flow rate m^3/s

p = Glycol Density kg/m

Cp = Specific heat capacity

C = Celcius

K = Kelvin

By dividing the Intentional Heat by the Cooling Capacity, we can quickly see how much our chiller really should be on.

Duration = Intentional Heat / Cooling Capacity

And if we divide the Cooling Capacity by the amount of Power Drawn by the chiller, we can determine the Coefficient of Performance (COP).

COP = Cooling Capacity / Power Drawn

This is pretty neat if your chiller displays all the information, as you’ll be able to quickly assess the efficiency of your chiller as well as understand if it is on longer than is should, highlighting unwanted heat transfer from somewhere else perhaps.

If your chillers does not present this information you will need to manually collect some data to do this. This will likely involve getting readings for volumetric flow rate and the power drawn by the chiller. Your ability to do so may be hindered by access.

Power Drawn Monitor (to be installed on your fuse board)
Manometer or digital equivalent (to attach to the glycol line - if there’s a fitting for it)

For those of you planning expansion or updating some kit in the brewhouse, getting a chiller that gives you all this information would be an excellent way to improve your energy efficiency.

The Real World

Inevitably, our chillers are going to be on more than we have calculated. Despite our best efforts, unintentional heat will transfer and the chillers will struggle with efficiency. But there are certainly things that we can do bring our chillers usage closer to the ideal.

Unintentional Heat Transfer

Insulation

Obviously if we can reduce the amount of unintentional heat being transferred to the system (via the glycol), there will be less demand for our chillers to be operating. Effective insulation is essential to reduce unintentional heat transfer. Ensuring that all the glycol pipes are lagged and the insulation is in good condition is the single most impactful way you can increase chiller efficiency, by up to 25%. (IBD, Mod 3, 2023).

When inspecting your insulation, ensure that there is no gaps between the pipe and insulation. If there is, moisture in the air can condense and freeze on the pipe wall acting as a heat conductor. Thermal conductivity of ice is 2.18 Wm-1K-1 where as your insulation is 0.020 - 0.040 Wm-1K-1. Be mindful of particularly damp areas of lagging and ensure that the insulation material is structurally sound. If the your lagging is disintegrating, so will it’s insulating potential.

Radiation

Many of us have our chillers outside. This makes sense as outside ambient temperatures are generally lower. However, with warmer and longer summers on the horizon, it may be time to reconsider this placement. As well as outside ambient temperature exceeding internal ones, radiation also comes into play. If your chiller is in direct sunlight, radiation may contribute to unintentional heat transfer by warming the condenser and evaporator. This in turn will diminish the amount of available evaporating potential of the evaporator, reducing how much intentional heat can be transferred.

COP

As stated before, the compressor is the main consumer of electrical energy within your chiller, and so is the main function of calculating your COP. With this in mind, the more we can do to reduce the degree or frequency that our compressors are used will reduce the power drawn by it and therefore increase it’s COP.

Low Cost

Evaporator glycol inlet should be as warm as possible

This increases the amount of heat transferred to the primary refrigerant which also increases the pressure just before the compressor. As a result, this reduces the load on the compressor. This is commonly known as reducing stretch or the compression ratio as you are reducing the difference between the pressure entering and leaving the compressor.

Rule of thumb is that compressor efficiency can increase by 1-2% per degree C that the glycol inlet temperature is increased. You should check with the manufacturer what the temperatures bands for this are. (engineering mindset)

Condenser air/water should be as cold as possible

If you can decrease the temperature of the air/water being used to cool the condenser, you will allow more heat to be transferred out of the primary refrigerant, allowing more heat to transferred from your heat source to the primary refrigerant in the evaporator. The more heat you can transfer per cycle, the less cycles you need to run.

This is why keeping your chiller in a poorly ventilated box is bad. If the rejected hot air cannot easily escape, it is reused to cool the condenser, which inhibits how much you can reduce the refrigerant temperature. Finding a way to decrease your condenser temperature could increase efficiency by 20%. (engineering mindset)

Clean heat exchange and vents

By cleaning the heat transfer components on our chiller (mainly the glycol line within the evaporator) we can increase the rate of heat transfer, therefore increasing efficiency. For reasons mentioned above, we should also be ensuring that the grill of our fans are clear to allow good ventilation.

Internal cleaning should be done every 3 years. Averagely 1-3% increase of efficiency can be achieved through cleaning; up to 10% if it’s never been cleaned. (engineering mindset)

Capital Investment

Depending on your chiller, there may be options to upgrade components. You should be checking with your chiller manufacturer for updates; always look to improve rather than just replace.

Variable Speed Drive (VSD)

Depending on your chiller, you may be able to install VSDs, or replace the compressor with one with variable speed capabilities. A crude understanding is that compressors running 20% slower are 50% more efficient, so being able to dial the compressor or perhaps fan rate back seems attractive. (DS Energy Group, 2023)

However, VSDs are a bit of buzz word when it comes to chiller efficiency and may not always be appropriate. Here is a check list to see if they will be right for you:

Have you addressed efficiency solutions mentioned above?
Does you chiller work at part load often? (is it often off)
Are the bearings and moving components fixed to a set speed? (Check with manufacturer. It may also be necessary to keep the rate locked as other factors may be reliant - humidity regulation may require a set fan speed for example)
Will you have to increase flow rate of glycol to compensate for reduced cooling load? (And so negate any efficiency savings)
Is your chiller large enough to warrant the investment? (Check with manufacturer)

Electronic expansion valve

Some of your chillers will be installed with thermal expansion valves. These may be able to be upgraded to electronic expansion valves which are able to measure the pressure and temperature after the evaporator which allows the compressor pressure to be reduced at part loads. This can produce up to 14% increase of efficiency. (engineering mindset)

If you are in the market for a new chiller, a good rule of thumb is to get something that is larger than your maximum requirement. Chillers don't run efficiently at 100% load capacity, and if your new chiller has some reactive features as mentioned above, you will be able to take advantage of energy savings from dialling back.

Beyond COP

As well as considering energy efficiency, it’s important to recognise the other maintenance your chiller requires.

GWP

If your chiller is old, the primary refrigerant used within it may have a very high Global Warming Potential (GWP). This is how much more heat the refrigerant is able to trap in the atmosphere than CO2. A common primary refrigerant is R134a for example, which has a GWP of 1,300, so it’s 1,300 times more effective at contributing to global warming than CO2.

Even older refrigerants like R11 also deplete the ozone layer, as well as having a higher GWP. This both traps heat, and removes the atmospheres radiation protection, so double bad. Make sure you are aware which you are dealing with, and avoid using chillers with older refrigerants.

Although these refrigerant are being phased out with natural options like ammonia, it’s important to realise their impact when disposing or if your old chiller leaks. If your chiller holds 10kg of R134a, and this all leaks out, that’s 13000kg CO2e released into the atmosphere. That’s about the same CO2e embedded with 1 ton of pale malt.

(Idemat 2023: World Food LCA Database 3.5)

Mechanical Friction

Mechanical and fluid friction can cause a reduction in efficiency by a factor of 2. If your chiller is stop/starting regularly, it might be worth installing a soft starter to reduce the torque stress and resultant mechanical ware on your compressor. These are not always compatible and you will need to check with the manufacturer.

Conclusion

Getting to grips with your refrigerator is an essential step in your sustainability strategy. To neglect it is to risk needlessly damaging both your wallet and planet and great savings can be made to both. It is also the gateway to realising the potential of your latent heat in the brewhouse. In the years to come, understand and utilising latent heat will be an essential component of the craft brewery and we’ll cover this in the next article; Warming Up to The Heat Pump.

Too many of us avoid the subject as it is too complex or we believe that nothing can be done. In this article we have shown this is not the case.

We have looked at the basics of how your chiller works and how this relates to the brewhouse.

We have looked at ways to determine KPIs as well as efficiency assessments.

Perhaps most importantly, we have looked at options to improve our chillers efficiency and to use them in an environmentally considerate way.

I hope that this will enable you to make improvements to your brewhouse and empower your journey towards sustainability. As always, let me know if I can help.

Good luck out there,

James