It has always been a staple of energy training related to catering that the doors of fridges and freezers should have tight seals and effective closers, and that walk-in freezers should have insulated strip curtains to supplement the proper door when it needs to be kept open temporarily. Most of us would assume that this advice relates to preventing the ingress of ambient air, but that’s not the whole story. When room air gets into a freezer, something like a quarter of the energy needed to cool it down goes into condensing and then freezing the water vapour it was carrying. The amounts involved are not huge: something like 0.02 kWh per cubic metre of air overall. What is significant is that the internal vapour pressure will plunge. So even after the door is closed, ambient moisture will pour in through any gaps in door seals, adding continuous cooling load as the condense-freeze process continues. Meanwhile the resulting ice build-up will be clobbering the energy performance.
It’s atmospheric moisture that you need to keep out.
Most people who understand the physics of heating systems will understand why the claims for energy-saving boiler-water additives ring hollow. But similar products (possibly the same ones with different labels) are now being touted for central chiller systems and it is evident that some users have fallen for them. Time to look at these new claims.
First, as a quick potted revision of the context, I’ll describe an air-cooled chiller with ordinary basic control:
Heat is abstracted from the building by means of a closed loop of chilled water leaving the chiller at a set temperature and returning at a higher, and variable, temperature.
The water boils refrigerant in a heat exchanger called the ‘evaporator’ which operates at a temperature just below the chilled-water set point (the difference, called the approach temperature, is typically 2°C or less).
After compression the refrigerant (now at elevated temperature) passes through an air-cooled heat exchanger called the ‘condenser’ which runs maybe 10-15°C hotter than ambient. Here latent heat is lost from the refrigerant, which condenses back to liquid.
Thermodynamically the key thing is the temperature ‘lift’ in the refrigerant between the evaporator and condenser. As users we are interested in the coefficient of performance (CoP) of the machine, which represents the ratio of cooling power out to electrical power in. The theoretical CoP is given by the formula:
Where Tc and Th are the absolute refrigerant temperatures in the evaporator (cold) and condenser (hot). Let’s put some numbers on this as an example:
Chilled water set point: say 6°C. This is self-evidently fixed.
Evaporator approach temperature: let’s say 2°C. This is the thing which we might be able to influence by improving heat transfer.
From (1) and (2) above we have an evaporator temperature (Tc) of 6-2=4°C or 277K
Ambient air temperature: let’s go for 35°C
Condenser approach temperature: let’s say 12°C
From (4) and (5) we would have a condenser temperature (Th) of 35+12=47°C or 320K
So our theoretical CoP is
Tc/(Th-Tc) = 277/(320-277) = 6.44
(actual CoPs are always lower but that won’t matter if all we want is a comparison)
Now let’s repeat the calculation with an evaporator approach temperature of 1°C instead of 2°C. This means Tc will go up from 277 to 278K and our new CoP will be:
Tc/(Th-Tc) = 278/(320-278) = 6.62
This is slightly less than a 3% improvement, but even that could well be an exaggeration because it assumes (a) that there was scope to reduce the approach temperature in the first place and (b) that poor heat transfer from the chilled water was the cause. Actually if the chilled water system is clean and properly treated it’s unlikely to be the problem: it’s a closed loop so contaminants won’t be getting in. In fact if the evaporator approach temperature is too high the reason is far more likely to be loss of refrigerant, or unbalanced distribution in the chiller, or oil in the circuit, none of which is affected by water treatment. It follows that if you suspect that your evaporator approach temperature is higher than it should be, look to ordinary maintenance steps first before anything else.
Vendors of additives will point to possible heat-transfer improvements within air-handling units or room air conditioning units. This is a red herring. These don’t affect the cooling load presented by the building, which is entirely and solely dictated by its heat gains and control settings.
Finally if anyone shows you a case study demonstrating an improvement, be skeptical. It’s far more likely they made the gains by cleaning the condenser coils, enabling evaporative cooling, servicing the chillers, or raising the chilled-water set point. These proven solutions are all things which perhaps are opportunities for you to exploit today.
The Government’s Energy Technology List (ETL) is a highly recommended resource. The ETL was devised to complement the Enhanced Capital Allowances scheme (a meagre tax concession for installing energy-efficient equipment) but it is useful in its own right because products cannot get onto the list without rigorous scrutiny from expert assessors. This makes it useful for filtering out bogus products.
Of course not every manufacturer, supplier or product that could be on the list is necessarily there. But the ETL itself, which lists products, is complemented by an Energy Technology Criteria List which lays down the criteria for inclusion within each product category. This will help you establish what to look for if you are doing your own evaluation of unlisted products (or if you just want to know what attributes are important, or what performance thresholds separate good products from the rest).
“Efficiency” in our business means the ratio of the useful output energy to total input energy. Unfortunately, when evaluating combustion performance, there are two versions of the input energy because any hydrocarbon fuel has both “gross” and “net” calorific values (GCV and NCV).
To understand the difference, you have to appreciate that the products of combustion include water vapour, and that it takes energy (latent heat) to vaporise water whether it happens in a kettle or as part of the combustion process. In a condensing boiler you get that latent heat back. A fuel’s GCV counts all its chemical energy but its NCV disregards that fraction (10% in the case of natural gas) that will be absorbed as latent heat. So when you calculate efficiency on the basis of NCV you get a higher value than if you had used GCV, to the extent that you see condensing boilers advertised as having over 100% efficiency. That is actually true on an NCV basis, but only because there’s energy in the fuel that NCV ignores.
Why does this matter? Because when you look at a combustion test report from a maintenance contractor it may well be on an NCV basis, which somewhat flatters the performance. I prefer to use the GCV basis. Some combustion analysers also make an allowance for boiler standing losses in an effort to give a supposedly more realistic overall efficiency figure, but that just clouds the issue in my mind.
If you want to be sure you are getting results (a) in GCV terms and (b) without deductions for standing losses, you can take the raw measurements from a boiler test and feed them into this on-line calculator, which incidentally lets you try changing the input assumptions for a side-by-side estimate of the savings that would result.
Recent changes to legislation means that operators of HVAC chillers and refrigeration equipment should review their equipment and the availability of replacement condenser fans.
In the past few months, we have had several enquiries to supply replacement AC axial fans for air cooled chillers from end users unable to obtain direct replacement fans, or where the cost has become prohibitive. Their predicament is not unusual and the situation is likely to get worse, with many fans no longer being manufactured, or only manufactured in short production runs.
Since 2017, all new condenser fan motors used on new chillers and condensers are required to meet the International Electrotechnical Commission (IEC) motor efficiency regulations and the Energy-related Products Directive 2015 (ErP). This has pushed manufacturers to look at the overall efficiency of fans and account for the entire fan, including the control electronics, motor, bell mouth and impeller and to define minimum efficiency requirements for the fans.
New Equipment manufacturers cannot use products that do not meet the regulations on new equipment but can continue to sell motors or fans that do not meet the regulations as spares to existing equipment.
Fan manufacturers and OEM’s have benefited from the spares market at the expense of the end user by selling replacement parts at ridiculously high margins. In addition to these high costs imposed on the end user, the end user should avoid trying to repair motors, as rewinds further reduce efficiency.
For many clients, we have removed all the existing AC fans and replaced them with the latest design IE4 Super Premium EC fans which have built in speed controls making them perfect for HVAC applications.
With speed control built into each motor, it allows all the fans on each circuit to operate together, modulating the speed to maintain accurate discharge pressure dependent on the cooling demand and ambient air temperature.
When you consider the CIBSE guidelines HVAC equipment life cycle is 15-25 years and that AC fans typically last 5-8 years it makes the retrofitting of EC fans a viable option to re-life an asset whilst improving efficiency and making use of the latest technology without the disruption and total cost of replacement.
Postscript: condenser cleaning
Dirty condensers can also have a drastic effect on compressor efficiency. To clean an air-cooled condenser correctly the fans should be removed and the coils cleaned in the opposite direction to airflow. This is rarely done due to cost and disruption, but incorrect cleaning in the direction of airflow can bury debris deeper into the coil, further reducing airflow and efficiency.
The correct deep cleaning of condenser coils can economically be undertaken at the same time as fan replacement as access can be gained during fan removal.
Excalibur Energy is willing to provide fully costed proposals with an energy analysis showing how performance can be improved for refrigeration energy-saving projects in connection with ESOS assessments which includes air-cooled chillers and condensers, dry air coolers and evaporative condensers. Contact:
Unit 115 Rivermead Business Park, Swindon SN5 7EX.
This article may upset some of my friends in at energy publications and associations, but we have a problem which people need to be aware of. It is that we can no longer trust awards for energy-saving products as indicators of merit.
I get asked for advice about dubious products by my newsletter readers and often they’ll say “I smell a rat but it has an award from [insert name of prestigious body]“. How can something bogus get an award that it does not deserve? To answer that you have to understand how award schemes work. In particular you need to appreciate that their promoters are driven by profit. The commercial imperative is simple: get as many bums on expensive seats as possible at a gala-dinner awards ceremony. To do that, they need to have a lot of short-listed candidates, because those are the people who will pay on the off-chance that they get to pose as a winner with the celebrity host. Having a big shortlist means putting an awful lot of entries in front of the judging panel (44 for one panel I sat on). But these judges are unpaid, and as volunteers they simply cannot spare enough time to scrutinise entries thoroughly, even though some do take it seriously and try to be diligent. They aren’t helped by the fact that candidates often submit little more than rehashed sales blurbs full of unsubstantiated claims — a short-cut which promoters condone in the interests of maximising the number of bums on seats.
Some judges, moreover, will have been selected more for their celebrity than their knowledge (celebrity judges equals more bums on seats), and will lack the ability to spot snake-oil propositions or even to understand counter-arguments from more knowledgeable fellow-judges. The majority of any panel will be easily swayed by the plausible nonsense in the entries, will not question the credibility of testimonials, and will naively assume that no competition entrant could possibly have criminal intent.
It is asymmetric warfare. The snake-oil peddler just needs to keep plugging away with award entries because the spurious credibility that they get from their first award is too valuable to forego. Once they have landed one award, they are effectively immunised against rejection by judges for other awards and probably even have their chances boosted.
I don’t want to tar all awards with the same brush: in an honest world they would all work to everyone’s benefit, and no promoter is knowingly complicit in the occasional fraud that slips through the net. But sadly a few bad apples have devalued energy awards and my advice would be this. If you have doubts about a product, seeing the phrase ‘award-winning’ should put you on alert.
This kit of parts appears in my latest “Kitchen Table-top Talk” on energy topics. It demonstrates the principle of evaporative cooling, a technique which can reduce the temperature of air in a space without the need for active refrigeration. To view the video please visit my YouTube channel.
In situations where it is necessary to keep a building’s outer doors open, you will sometimes find “air curtains”, fans which blow a sheet of air down across the width of the doorway. These are an effective way of preventing dust and insects getting in through the door: they are entrained in the outer layer of airflow, and where the jet hits the floor it splits, with the outer layer discharging the contaminants back outside.
Some suppliers of air curtains claim that they conserve energy as well. The basis of this claim lies in what would naturally happen in an open doorway in still conditions, namely convective circulation in which warm air at high level flows out to be balanced by cold air flowing inwards at low level (right). This effect will be especially marked with high doorways. The claim for air curtains is that they disrupt the flow of escaping warm air, forcing it down to floor level where the jet splits, with the warm inner layer returning inside.
However, even in still conditions there is a problem here, because the fan is drawing air from high level inside and at floor level only half of it returns inside. 50% of the internal air drawn into the fan is diverted outside when the jet splits at floor level (left).
A further problem with pedestrian doorways particularly is that the air curtain usually needs heating to prevent the perception of cold that the air’s velocity would create. If the building actually doesn’t need that heat, it is all a waste of money. Even if it does need the heat, half of what is put in goes straight outside.
In windy conditions the argument for air curtains as heat barriers really breaks down. A moving sheet of air is simply not as effective as a door. If there is any differential pressure whatever, that sheet of air will be displaced, and the problem is exacerbated if there are open doors or windows on the far side of space – or extract fans. In one instance I visited a restaurant that operated an open-door policy. Their air curtain had a 20kW heater that ran continuously, but the downjet did not reach the floor: about 60cm above the floor it turned inwards along with a layer of cold air at floor level, thanks to the kitchen extract depressurising the space.
The exhaust from a natural gas appliance contains about 0.15 litres of water per kWh of gas input, and about a tenth of the thermal output is lost because that water is emitted as vapour. Condensing boilers are a good idea in theory because they can condense the vapour and recover latent heat from the products of combustion, boosting output by around a tenth.
In practice, too few condensing boilers achieve their potential because they cannot cool the flue gas below its dew point (around 59C ). Result: plumes of vapour outside. This one resembles what you’d see boiling a 2-3 kW kettle in the open air, and that’s a measure of how much energy is being wasted.
The truth is that so-called condensing boilers need to be installed in heating systems with low return water temperatures. Underfloor heating, or systems with oversized radiators for example. Only then will they get sufficiently-low temperatures in their heat exchangers to get the exhaust vapour to condense.
It’s like a curse. Waking up last Wednesday to a view of the full moon reflected off the Adriatic in the pale light of dawn, I opened the sliding door to the hotel balcony to take a snapshot. As I did so I heard the air conditioning fan stop and there in the pale light of dawn I saw a magnetic reed switch on the door frame, evidently linked to the fan-coil unit control. “Brilliant” I thought: “a picture of that will be perfect for my session at our conference on energy in hotels”.
To be fair to myself I did photograph the moonlit Adriatic first and the interlock switch later.