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EXCLUSIVE What food processors must do to cut energy costs

08 January 2013

If you thought you knew everything you need to know about how much energy your plant is wasting – think again! In this compelling interview, David Strydom speaks to Robert Unsworth, GEA refrigeration director of sales & product development, about unnecessary costs many food manufacturers may be overlooking in their factories.

What is a bugbear with respect to energy waste in food factories?
I could spend hours talking about that! Until recently, part of my job has been to go around solving ‘issues’, whether it’s how to increase productivity in certain processes, problems with improperly installed refrigeration systems or - more often than not – reduce someone’s utility bills. A big culprit with respect to energy waste is door management. Staff members leave refrigerated storage doors open, which is a problem, particularly in facilities with low temperature cold stores / processes. Most chilled food manufacturers have to keep the environment at which they're cooling the facilities at, say, 2 or 8°C. But when doors are open, it increases the amount of energy being used to maintain the required temperature. Quite often when one calculates the total thermal load on a room, one large heat source is air ingress from outside. It’s simple - heat and moisture travels toward cold (simple heat transfer). Warm air coming into a cold room needs to be cooled down to room temperature but it also ‘carries’ moisture. The air coolers act as dehumidifiers and condense the moisture or freeze it depending on the temperature in the air cooler. This takes even more energy than cooling the air stream. The ice then insulates the cooler and decreases its performance so the refrigeration plant has to ’work’ harder to achieve the original amount of room cooling. One then ‘adds’ more energy in to defrost the ice!

How much does this cost food producers?
I calculated several years ago – with electrical cost at 8 pence per KWhr - that if the door to a cold storage facility is left open only five minutes an hour, it will cost the client more than £9,000 a year in ‘additional’ or ‘lost’ energy. That’s before you even get to the other issues. In a ‘chilled’ application, this would amount to about £2,500.

What is the solution to such a problem?
Shut the doors! I'm not being funny but it would be cheaper to employ someone to do it. His or her job would be solely to walk around and shut doors. Good door management is key to solving so many issues, not only energy reduction. The problem is, it is difficult to quantify. When you see fog coming out or going inside a cold door, unless you can ‘attach’ £ signs to it, people just don’t know. Any way of limiting air transfer through doors or any gap (and there are many) is good news.

How does a refrigeration plant's design impede its potential to use less energy?
There are an infinite number of ways the design of the (refrigeration) plant effects its performance. Actually, it’s embarrassing for me, speaking on behalf of the Refrigeration Industry, to mention the percentage of systems that we look at which have perfectly good or suitable components that are improperly installed for optimal/proper performance. We rarely see condensers installed properly and this is a crucial component in the refrigeration system with regards to performance and efficiency. This is a large failing in The ECA scheme, as you can buy any number of “efficient” products from the list, which when installed incorrectly (and many are) won’t save any energy, in fact quite the contrary!

What are the specifics of the refrigeration system and how it saves energy?
A refrigeration system moves energy (cooling capacity) from the cold end (process) to the outside (in most cases). It takes more energy (power) to move that cooling capacity to ‘outside’ (in a freezer, about half the thermal energy removed will be added to the refrigeration system as power to drive the process and about one quarter for chill). The cooling capacity and the power used to drive the system is discharged as heat to the ambient at the condenser. As explained, heat (or energy) travels from high to low (‘hot’ to ‘cold’) so if we want to discharge this energy in the form of heat to the outside, the refrigeration plant’s temperature must be hotter than the ambient air otherwise it won’t transfer. So the temperature in the condenser is always hotter than the ambient. If the temperature outside (ambient) is +20°C and we size our condenser to have a temperature difference of 10k (it’s only a heat exchanger after all) then the refrigeration plant will discharge this energy at +30 °C. If the condenser surface is fouled, a fan isn’t working or the condenser is improperly installed, it will be less effective and the condensing temperature will rise. Each degree the condensing temperature rises, the plant uses about 3% more power.

What should food manufacturers know about how their plants are designed in terms of temperature?
Half the year is night-time and the rest is divided into seasons, meaning there's great variations in outside temperature. Although the plant should be designed to run at 100% on the hottest day, it's actually only warm outside for about 1% of the plant's annual life. For the majority of its life, it's less than +10°C outside, so the refrigeration plant goes down to ‘part-load’. But the condenser is still running on the summer setting, ie the condenser fan stops or slows down at night, for instance, but the refrigeration system is still discharging heat at +30°C. Even if its -10°C outside, the condenser will still be controlling at +30°C. This makes no sense and can be rectified by simple control changes although - depending on the system installed – some minor modifications may be required. System modifications like this can easily lead to 25%-30% efficiency improvements. As mentioned, every 1°C lowering of the condensing temperature produces 3% increase in performance! (Take a look at the size of your compressor motors and do the Math). One of the most important components in the system with regards to efficiency is the condenser but for most of the year, it’s tuned off! This makes no sense!

What is the deal with the refrigeration compressor with respect to energy waste?
Fixed speed compressors fall in performance when they’re not running at 100% capacity. Screw compressors are particularly inefficient at part-load. If you fit speed control to a screw compressor, its performance still falls off – however, just not as much as it would with a fixed speed system. If you have a reciprocating compressor however and fit speed control, its performance at part-load actually increases. To simplify the understanding of this, compare it with driving a car - you get more mpg at low rpm’s than at high. Ultimately, a reciprocating compressor is pretty much the same as a diesel engine! (Typically a line I use to upset our R&D guys in our compressor factory!) The speed controlled reciprocating compressor is the only one that increases its performance at part-load. So, if you have a refrigeration requirement that has load variations, a reciprocating compressor can often be the best type of compressor to install. Even when looking at lifecycle costs, it’s still the best. If you look at servicing costs over 12 years, it’s even cheaper than a screw, which is a bit of a shock to most people.

Do you have an ulterior motive by recommending reciprocating compressors?
If the average running capacity on a compressor under chill is 50% where a screw would have a coefficient of performance (COP) of 4.5 at 100%, it would have a COP of 3.7 at 50% (3.6 with fixed speed) capacity even with speed control. A reciprocating compressor at the same condition has a cop of 5.0 at 100% and 5.3 at 50% - over 30% difference at part-load. So if your system runs for the majority of its life at part-load (and most do) this is the compressor to use (and they’re generally cheaper and less complicated to install and service). Obviously, both types of compressor have their ‘ideal’ applications; fortunately we at GEA make very good examples of both, so this advice is technically independent of any ulterior motives.

What happens when you place a frequency controller on an electrical motor?
The standard supply for electricity in the UK is an alternating (sign wave) current at 50Hz. A frequency converter basically converts this to a DC current, ‘chops it up’ and ‘rebuilds’ the sign wave at a frequency for the particular speed requirement at that moment in time or to suit the ability of the motor (most standard motors can run between 10-60Hz with little modification). (I’m sure manufacturers will get upset with that oversimplified explanation!) As I said earlier, the performance actually goes up when the speed goes down on a reciprocating compressor so in effect you get the same cooling for less power (and less wear). So, rather than running at a poorer performance at part-load, it runs instead at a better performance at part-load. Your annual power usage then goes down significantly.
What role do inverters play in energy efficiency?

An inverter or variable speed drive (VSD) is an electrical component that most equipment such as fans/pumps and refrigeration compressors use to run and optimise part-load performance. There is a loss in an inverter, so if you have a process that runs at anywhere near 100% or is off for most of its life, there’s no reason to install a VSD. One really has to look into the load profile.

Why are reciprocating compressors preferred to screw compressors?
Most of the reason is explained earlier but previously people with refrigeration systems bought screw compressors because systems got larger and screws have individually more capacity. As engineering goes, they’re a bit “sexier” too. It’s an emotive subject but generally speaking, on chill applications, reciprocating/piston compressors are more efficient. These days they’re like the diesel engine in your car – you used to have to maintain them every year but now they have monitoring packages fixed on them like your car. Depending on how you drive or how you run your refrigeration compressor, you don’t need to maintain them for the first three years or so.

That means overall cost of ownership declines dramatically with reciprocating compressors. It’s the only compressor in the world that increases its performance at part-load (suppliers may try to sell you “increased part-load performance” but these are to do with “optimising” heat exchangers and not related to the compressor at all). Lowering the condensing temperature increases the compressor performance by 3% per degree C. This is not a function of the compressor running better at part-load, but of the condenser). The others, even with frequency controllers on, decrease their performance at part-load – they just don’t decrease as much as they would if they were fixed speed.

What is your new position at GEA?
Previously I was the UK nerd for the contracting side of the business and, to be honest, have been for many years even with my previous company Sabroe in Denmark; now I’ve been made director of sales and development for the total company – ie all four business units. This would imply I’m now involved in our product sales, service, freezer and project development as opposed to just projects. There’s also been a change in drive within the business – previously, our service business was very much part of projects business, so when you bought a refrigeration plant from us, we’d ask our service colleagues to offer a quotation – through – to the end user. Now the service is an independent business unit, thus enabling us to provide market leading service support to anybody that requires this service. This enables 3rd party contractors who perhaps do not have the resource or skill set to deal with a particular issue, but have a strong relationship with the end-user, to ask for support from GEA Service and not see them as a competitor but an ally and an extension of the product.

What does this mean for the business?
Even the ‘old’ project technical support department has been realigned to a ‘shared service’, thus enabling us to provide state-of-the-art, confidential support to all our customers – whether they’re 3rd party contractors, end-users or wholesalers. Basically, we realised we have a skilled team within our company, who previously were only providing service / products to a limited sector of the market. Now, with our new model, they can provide the best products / services to all customers. In our opinion, we’re able to design the best and most innovative systems, because we have the best technical personnel available. From relatively small ‘simple’ refrigeration systems to highly complicated cooling systems and heat pumps, including all aspects of a modern facility: optimised efficiencies, optimum product quality, life-time costs and ease of use, our team of experienced engineers (locally and globally) is our strength.

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