Evaporator fan control

Hi all,

I would like to hear different opinions on effective evaporator fan control. When I first joined this company the fans were set to be on 24/7. Using the danfoss controller I set the fans to come on when cooling/heating is asked for and a few minutes after cooling/heating is stopped. There were many advantages to this method but mainly the cost saving - £5000 in a two week period.

However, after this two week period some management and customers started to panic that there was something wrong with the equipment and even after I fully explained the method behind the set up they still did not like the change. They asked for a consultant to analyse the data from the two weeks and compare it to last years data to make sure the fruit was not being compromised. Whilst waiting for this I had to revert back to the original set up.

The humidity given off by some of the fruit in the higher temp stores generally holds the correct temperature a lot of the time and so the fans are not required, they are simply circulating the ambient air around the room. Some stores are set to 4 degrees and so the fans are required to be on most of the time but the average saving across all of the stores along with the other benefits surely makes the set up very desirable to any company. When I spoke to a colleague about this they said this method is very popular with supermarkets who have huge chilled warehouses and is not a new idea at all but is a very effective way of energy saving and cutting unnecessary costs.

We have had the report back from the consultant which concluded there was no temperature difference between the two set ups, but they didn't have any summer time danfoss fan control data to compare against last years records so I will hopefully be testing it again in June to gather some data.

There are 9 stores here ranging from 4-14 degrees with a total of 179 3PH fans that can be controlled by danfoss. I would be interested in hearing if anyone else is in control of a similar site and what method they use or anyone who may know of any side effects of using this method, thanks in advance.

Martin,
Where are thermostats located so all product can be held at correct temps

Martin,
Are the fans fixed speed, or VSD?
If VSD, how is it controlled

Hi, fans are fixed speed and the temperature is controlled by the air on probe on the evaps

Quote:

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Originally Posted by Martin Davies

Hi, fans are fixed speed and the temperature is controlled by the air on probe on the evaps

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leave one fan on permanent for circulation and the rest on temp control.

Yes that's what I'm going to do as soon as management gives the go ahead

I just didn't know if there were any ramifications of not having the full air circulation even if the air temperature is correct?

Sometimes certain humidity should be kept. To achieve this, certain minimum air circulation should be kept.

That sounds like a big saving,but it also sounds like a lot of product being cooled is there not a data logger? one of customers uses a system of logging where the logging devices are so cheap to buy plus when the fruit is delivered from all over Europe to the site the same loggers are used which gives them so much information plus when they deliver the product to their client a temperature log is from start to finish so to speak.

It's the humidity levels that I'm concerned about as I'm not sure of the relation between air flow, humidity and heat given off by the fruit. I have a sophisticated testo data logging and alarm system in every store to constantly monitor temps which sends out an email if the temp breaks the boundaries set.

It's the data generated by the testo system that the consultant measured and from that decided if the intermittent fan control was acceptable or not. But the data does not include humidity levels.

Hi Martin
The reason your stores use Evaporator Pressure regulators is to help keep the humidity high by keeping the td at a low level. Its been proven that by simply controlling the suction condition, the humidity remains high as against using ON/Off thermostats with pump down which causes the humidity to drop as the evaporator temperature falls to cut out point.
Many Danfoss controls and I suspect many others also use a simple fan cycling in their controllers as a means of energy saving. Many of the fruit stores use VSDs on their evaporator fans as a means of reducing both energy during the off cycle and also reducing the fan motor heat load inside the stores. Some stores simply cycle the fans during the off cycle something like 5 minutes On 5 minutes OFF, or if using VSDs simply reduce to say 25 hertz during the Off cycle. This saves energy and also helps stop frost burn to the fruit. The CC550 controller as used in many of the supermarkets has this facility to fan cycle , heat rail cycle etc etc to reduce energy wherever possible, these settings are adjustable to the individuals taste, but Ive never had complaints from fruit storage facilities using evap fan control. Remember fans with VSD work on the Cube rule ie at half speed only 12.5% of the FLC = big savings

Hi Glenn I was hoping you would respond. As we don't have VSD's and are stuck with the more basic 210 controllers I think the only option is to leave 1 evap out of 4/6 (depending on the store) in constant fan operation and have the others controlled via the temp?

And on the subject of evap pressure regulators - what is the correct set point of the regulator controller if the air sensor is reading air off instead of air on?

It should be set at 4 deg C for your application and adjusted for other fruits etc

Even if the 361 is reading air off?

Using an EPR valve you are controlling the evaporating condition to give a constant off coil temperature. The expansion valve looks after itself with the change in signals it receives, but the EPR modulates the evap pressure/ temperature to give a constant off temperature this has the effect of keeping the delta T small and the humidity high around the product a win win situation

I don't understand why I was told to set the controllers to -5 degrees lower than room temp!?

Hi Martin
I was reading your comment on the supermarket bulk holding stores and I would always be wary on using this as a reference, they usually look to short term product storage with high turnover. I always walk past fruit isles and cringe at the lack of understanding on fresh product holding and storage and then you see similar regimes applied to bulk storage.

How did you get to your fan energy saving cost? Seems a very high saving?

As Glenn says the design should be for a very close TD, but I differ in the opinion that air movement is an effective energy saving mechanism. The best energy saving is achieved by the full use of your heat exchanger. This comes down to original design and effective surface area and product load in storage.
The fan design and throw would need to be sufficient to remove all of the heat of respiration across all product. The longer a product is stored the higher the impact poor air flows will have on product and you always need to consider that the product value is higher than the operating cost or there would be no storage facility.

Your air changes levels will vary from pull down, to holding levels but I would anticipate that your stores would have been designed on holding only as they are transitional storage. Fan cycling and reduction from design should be approached with extreme caution as you will need multiple readings of air and product core temperatures from across the storage facility to give a true overview of conditions. Air movement not only improves heat transfer rates but will also minimise, moulds, yeasts, ethylene levels etc. Also type of fruit and condition needs to be considered.

I prefer secondary systems to DX systems for real close control as there is always an element of overshoot and undershoot in the coil itself with the refrigerant and this can be compounded by changing air levels.

VSD savings always seem to forget the energy expelled as heat by the inverter. The best energy saving is done at design level but capital cost usually always outweighs the long term savings that could be achieved with good design. Just look at layout and position of evaporators and you will immediately see compromise with space usage, work flow and storage capacity.

Hi Hookster and thank you for your input. I was in contact with the finance manager before and after the two week period and asked him to let me know what difference in electricity costs the changes made. He sent me a very detailed report and work out that the bill was £5000 less over the period than normal and than the year previously.

The problem I have, which you have pointed out, is the store temperature is only monitored via the evaps - air on, off and coil temps. The temperature in the centre of the store is not monitored. So if the fans are off then the temperature being monitored is only going to be directly under the evap as there will be no air flow across the store.

However, as mentioned before I will strategically isolate particular evaps in the store to provide some air flow from one end of the store to another and see how I get on, possibly keeping 25% of the air flow on 24/7 operation.

To show a working out of the energy saving; fans are all between 1.5 -1.9 kW, we pay 10p per kW/h, 180 fans are danfoss controllable, 180 x 1.7kw x £0.10 x 24(hours) = £734 x 14(days) = £10,280. Obviously this is the maximum cost for fans being run continuously for 2 weeks so to save £5000 shows that on average 50% of the fans in all of the stores were not required in that period. Now, that period was a couple of months ago now so the ambient temp was quite low anyway and the majority of the stores are kept at 14 degrees for bananas so the need for cooling was quite rare, obviously at different stages thoughout the year the savings will vary as the need for cooling changes.

You will need to apply your power factor correction to your calculations and assume motor efficiency.The measurable data for comparison would be ideally be two similar stores operated simultaneously on alternate regimes. I have done some energy projects with reduced airflows and found that although savings were being made on motor inputs there was excessive start currents on motors and compressor packs which not only nullified energy savings they actually increased physical wear.

I have found that an excellent energy saving can be had with ensuring the coils are clean and equipment is operating correctly i.e bearings and fan blade selection / conditions.

It would be worth investing on an energy meter and working your way around the system gathering data it not only shows high energy consumers but it makes an excellent ppm prediction regime.

Coil cleaning has dramatic measurable impact.
The low hanging fruit of energy saving is what makes the real impacts!

I've kept a couple of the evaps running constantly in all the stores, with the rest controlled via the store temp and will keep it like this for June, I'll comment at the end of June with the savings generated

I would monitor conditions closely towards end of June when ambient conditions will rise to our seasonal peaks. With bananas you want close control in your 13 - 14 range (I know door control murders it!)
Fluctuations above will increase your ethylene production and if your circulation ventilation is not sufficient your will get localised premature ripening in these areas. The more stable your conditions the better it is for this fruit.