Energy management

Due to increasing electricity costs I’m am under pressure to reduce operating costs for the site decribed below, any thoughts or ideas would be greatly appreciated.

I look after a cold storage site, three stores each has its own plant room. each plant is pretty much the same set up:

R22 pump circulated, flooded evaps

2 stage compound compressors with interstage cooling provided by TEV liquid injection and vapour return from subcooler/econimisers which are TEV feed,

hot gas defrosts (pressure regulators on evaps),

air cooled condensers,

no oil separators, oil is returned via a discharge line heat exchanger TEV feed,

PLC controlled.

Grenco installs 130, 195 & 288kw.

At present we run the plants with two 4 hour shut down periods per day, controlled by time clocks, the 4 hour periods are extended/reduced depending on ambient temps, time of year, store operations, etc etc (temps are very even across the stores during shut downs) Normally around -23/-24 when switched off and around -18/-19 when switched on.


We have thermostats/controllers on each evap and when a satisfied temp is reached on an evap the liquid line solenoid is closed and three of the four fans are switched off. I have tried in the past to operate using the stats but found I end up with one evap working and one compressor on 50% or compressors & evaps cutting in and out to frequently and this proved not to be as efficient as the operating method above, ie a higher kwh per day. I think this method of control is more for close control of temps?
At present these stats are set to -29 which is never reached.
The way I see it is that if you have a 90kw motor running a compressor at 50% then this has to be less efficient than running the compressor at 100%


I have Mitsubishi PLC’s controlling the plants with HMI’s which allow me to change defrost lengths, times, loading & unloading times for compressors, duty selection, and various other things, so we have good control over the plants with a lot of control options.


I am thinking of changing the control philosophy of the plant and using one thermostat per store/plant to switch plant on/off rather than evaps switching on & off regularly and compressors unloading & loading all the time. I think this way I could have the stats set to switch off the plant at -23 and back on at -19 and it may reduce running hours.


We have always maintained a discharge pressure of around 10 bar during normal running, this is increased to 11 during defrosts via a constant pressure regulator. I am considering running at a lower pressure during normal running but how low can I take it before it will start affecting the performance of the TEV’s involved in interstage cooling and oil return and will this actually achieve anything ?

Also does any one have any experience or comments on inverters for condenser fans this is another option we have been considering.

We already have a very good kwh per cubic meter ratio on this site compared with others within our company but I’m still under pressure to save and it’s getting ridiculous, some of our management (coldstore managers NOT engineers) have attended a conference last week and a presentation was given by supposedly an energy management consultant who said “in most cases the evaporators are over specced on the fans normally by 50%”
What a W?@&%R,

I now have received an e-mail from management telling us (not just my site, but all others in this region) to fill in a questionnaire about how many evaps & fans we have in each store and they (remember these guys no nothing about refrigeration engineering)
will tell us how many fans/evaps we have to run.

This is what they suggested:

If you have a store with two evaps with four fans on each can you turn off one evap or run both evaps but only use two fans per evap

Then, This one made me laugh,

Fit time clocks to the fans so at weekends or times of minimal door openings we can switch off all the fans.

So I’m now expected to run my plants at weekends with no evap fans! This should be interesting.


Sorry about the long post but I wanted to give as much info as possible,

Any criticism, ideas, advice would be greatly appreciated.

It sounds like you have as much help as you need from management!

I have never heard anyone make the comment that the fans are over specified on evaporators. First time for everything I suppose.

There are several things I can offer that relate to this however. Anytime a fan is running in a cold store, the fan motor is adding heat to the air. The fans of course have to run to maintain the desired temperatures.

When the desired temperature is achieved, the fans and liquid line solenoids can be shut off. This is one method.

Part of this problem is the temperature sensors are usually placed close to the return air inlet to the evaporators.

Air provides the cold envelope for the product. When the fans are off, the sensors are monitoring the air temperature where they are located.

If the sensors are located throughout the cold space to monitor room temperature instead of return air temperature you can have more flexible control.

I find a much better solution is to use inverters on the evaporator fan motors (one inverter for each evaporator). When the air temperature begins reach set point, start to reduce the fan speed.

The goal should be to have the fans running at some minimum speed with the liquid line solenoid ON, while still maintaining space temperatures.

Another item is to review the defrost requirements. Do they need to be defrosted every time the defrost timer is set for?

Each defrost cycle will add heat to the cold space if it is set too long or not required.

Cylinder unloading on reciprocating compressors is a decent means of capacity control from an energy perspective.

Part of the trick with energy conservation is to get the suction pressure as high as you can, while still maintaining the space temperatures. Some combination of fan speed and suction pressure will produce the capacity you need at any given time.

Inverters for the condenser motors is also a good idea. The lower limit on the TXV's is the differential pressure available during operation and the valve capacity at the lower differential.

You may have to change the TXV's to PWM electronic expansion valves to meet the control requirements at both the higher and lower pressure differentials.

The above should give you a little more information on how to approach the system, but the problem is how you deal with the energy consultant's comments and management's new directive.

Just thought of something else.

Part of the problem with 2 stage compound compressors can be the lack of sufficient capacity reduction steps. They can only unload according to the number of steps provided.

An inverter can also be used on recip. compressors to provide smooth capacity changes to meet the varying demands of the cold store. In effect, you get the smooth capacity change like a screw compressor with a slide valve.

This offers better capacity control than cylinder unloading, and certainly much better than hot gas bypass.

One thing to be careful with though... The compressor oil pump capacity will decrease with a reduction in speed. The compressor manufacturer can tell you the minimum "compressor" RPM's (not motor RPM) that you need to produce sufficient oil pressure.

You can verify the oil pressure during the lower speed operation. As the compressor slows down, the oil pressure will begin to drop off.

When this happens you have reached the lower speed limit for operation. A suitable lower speed for the inverter would be something above this speed.

US Iceman Thanks for your comments

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I have never heard anyone make the comment that the fans are over specified on evaporators. First time for everything I suppose.

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They would rather listen to this guy who nows nothing about refrigeration instead of talking to and asking advice from the 30 + engineers based at our different sites around the country, and when we all started jumping up and down and telling them this was a load of s?@T we are told that we are over reacting. They are going ahead with trials at one of our sites this week and are going to try and run a plant with only half the evap fans running, this sould have some interesting results, i'll post as soon as i here anything. i'm sure they will soon change thier minds.

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I find a much better solution is to use inverters on the evaporator fan motors (one inverter for each evaporator). When the air temperature begins reach set point, start to reduce the fan speed.

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Is this option likely to reduce energy costs by much?

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Another item is to review the defrost requirements. Do they need to be defrosted every time the defrost timer is set for?

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I'm based on the same site most of the time so this is monitored on a day to day basis and defrost intervals and lengths are adjusted accordingly.

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An inverter can also be used on recip. compressors to provide smooth capacity changes to meet the varying demands of the cold store. In effect, you get the smooth capacity change like a screw compressor with a slide valve.

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would like to cosider this, but i would imagine this would be very expensive?

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They would rather listen to this guy who knows nothing about refrigeration instead of talking to and asking advice from the 30 + engineers based at our different sites around the country, and when we all started jumping up and down and telling them this was a load of s?@T we are told that we are over reacting.

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An old engineer told me one time that the expert is the one who has to travel the farthest to get to the job. I usually find the best information comes from the in-house people myself.

Unfortunately I have seen the same thing on my side of the pond too. Management does not listen to the operators. The speakers in the front of the room can preach all kinds of nonsense and it is accepted as gospel truth. After all, they speakers were asked to talk!

The inverters on the evaporators give you a "two for one" benefit:

First, the reduce the total fan power for the coil, so you save on this account.

Second, the reduction in power input also decreases the amount of heat dumped into the cold space. This puts less load on the compressors. (If the inverters are not in the cold space. They create heat also)

The distance from the inverter to the fan motors can have an impact on installation. You may need output filters on the load side of the inverter, or the motors may have to be changed to inverter duty. The inverter supplier should be able to assist on the installation parameters.

When the temperature is at set-point, continued fan operation only serves to dump heat into the space. Reduced fan speed helps to keep the air in circulation, but does so at dramatically reduced power input.

At 50% fan speed you will still get about 65-75% of the evaporator capacity, so this works in your favor also.

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I'm based on the same site most of the time so this is monitored on a day to day basis and defrost intervals and lengths are adjusted accordingly.

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Another way to do this is to record liquid solenoid valve On time and have the defrost cycle adjust accordingly through the central control system. This can vary by time of year and operation, but a method could be worked out to suit your location I'm sure.

Larger inverters can be expensive, but I have had very good luck with doing this. The system simply runs much better and smoother.

Make sure you do your homework before mentioning this to management types. A wrong move and it is easy to get beat up.

You have to find a way to make them think it was their idea!:D

Have you considered a mathematical model of the system to calculate which bits are running at or below design specs? This is a bit involved to set up but once done you can plug new data into it and use it as a monitoring tool.

can I suggest you look into liquid pressure amplification, and / or running on oversized EEV's (with pulse witdth modulation control). This will allow you to run at lower head pressures and hense more efficient compression, look into floating head pressure. Be aware of expansion valve capacity and capability of existing TEV's and flash gas issues with long liquid lines. by the sounds of your system I would investigate LPA costs.

Regards
Darren

Yep, that's what they call it when I'm out of town... Seems the RE board has a sense of humour, I only typed the name of the town in when I signed up!

On topic though, how does the power consumption break down in a system - what percentages are attributable to which bits? You could spend an age improving the efficiency of something that only contributes 3% to your electricity bill, for example.

Johnny rod brings up a very good point.

In trying to develop an energy reduction plan you should first evaluate the major factors that contribute to the energy charges.

In a refrigeration system it is all about heat, moisture, and motors.

Heat leaking into the coolers or freezers is (normally) a fairly small percentage as this is dependent on the insulation of the room. If the insulated walls are in poor condition, this will be higher.

Moisture gain into a cold space is a large load. Door openings, wet product, etc. Obivously the door has to be opened, but, can the air entering the freezer or cooler be better dehumidified?

Heat generated in the freezer or cooler is something that can be controlled. Fans, lights, people, fork lifts, etc.

Evaporator fan motors are normally a significant factor in cold storage applications. Sometimes the total fan energy can be close to the total compressor motor power!

This does not mean that the fans were over specified. It simply indicates that a lot of energy is being used to circulate air. This is where VFD's can greatly help.

Note: I have previously uploaded a chart for VFD savings in the following link: http://www.refrigeration-engineer.co...?t=3031&page=2

The chart can be used for evaporative condenser or air cooling evaporators. Please note the chart is based on 60 hertz power.

Shutting fans on and off does decrease the energy use (very small amount). However, if the fans are allowed to run with a VFD, the lower speed greatly reduces the energy used and the heat input into the cold space.

Compressors are easy. The biggest gain comes from reducing the discharge pressure.

If your system is pumped recirculation the only requirement you have to maintain artificially higher discharge pressures is the liquid feed to several areas:

1) liquid feed to the TXV's
2) liquid feed to the low-pressure receiver (maintaining the liquid level in the vessel)

Another factor that may have an impact is the hot gas defrost pressure. Some minimum pressure will be required for the regulators and hot gas piping pressure loss.

A fast way to look at energy use is to break it down on a piece of paper. List each item that uses electricity.
Now, for each item, list the energy use.

What can you do to each item that reduces the energy use?

The best way to get management to buy into energy management is show them savings. Go after the easy items first. I call this the low hanging fruit. The easy items usually have the fastest results.

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A fast way to look at energy use is to break it down on a piece of paper. List each item that uses electricity.
Now, for each item, list the energy use.

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Plant room 1:

90kw comp drive motor.
12 x 1.75kw condenser fans, 6 running at this time of year
12 x 2.5kw evap fans
4kw liquid pump motor

Where do i go from here


I definatly like the sound of inverters and the potential savings that can be atchieved and i will be looking into this in great detail i presume this would need PLC programs to be modified as well as the inverters being set up?

thanks to you all for your advice.

Some rough estimates:

62% of the energy is used by the compressor
14.5% of the energy is used for all of the condenser fans
20.7% of the energy is used by the evaporator fan motors
2.8% of the energy is used by the refrigerant pump

This tends to show the amount of compressor motor to fan motor use. It also shows you where the biggest usage is. Now that the differences have been found we can attack this.

The refrigerant pump is so small I would not worry about it. VFD's on the refrigerant pump are not worth the installation or trouble.

Turning fan motors on & off only saves a little bit. The amount of time they are turned off only, when they would otherwise be running. Small savings. VFD's on the fan motors is the only way to go.

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I presume this would need PLC programs to be modified as well as the inverters being set up?

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Yes sir. The PLC will require some inputs from a pressure transducer for discharge pressure to control the VFD on the condenser fans. I would use one VFD for most of the fans to get the best overall control.

You might be able to use a VFD for 8-10 fan motors to allow smooth control. If the discharge pressure goes up after 8-10 fans are running at full speed, the last group of fans could be started at full speed on a pressure switch. (this might happen in hot weather)

For the evaporator fan motors I would look at one VFD per evaporator. The PLC logic would have to be changed to accommodate this.

If you mount the evaporator VFD's out of the cold space, the savings are best.

This leaves the biggest energy user; the compressor.

I would investigate the TXV's and look into using EXV's. They need to be sized for the lowest pressure difference possible. You will need to get the intercooler capacities at various condensing temperatures as the intercooler load will change. An EXV should handle this quite well.

The liquid feed to the pump vessel (liquid level control) will need to be reviewed for the same thing. Changing capacities versus available pressure differential.

I would start with the compressor and liquid feed system first. Then look at using VFD's on the evaporators. If the evaporators are fed with pumped liquid R-22 they will be OK during low load conditions.

This does not apply to evaporators fed with TXV's.

As an initial guess, the lowest amount of energy will be used when the compressor is operating at the lowest possible discharge pressure with the motors at an intermediate speed.

One thing to be aware of though...

You can have the condenser fans operating at full speed in cooler weather to drive the discharge pressure down, but...

The total power use may be higher. There is a best point of operation where the discharge pressure may be slightly higher, but the fan input energy is minimized.

What we are looking for is the lowest total power input. You can make the system use more total energy if you use all of the condenser fans trying to get lower discharge pressures.

This is not clearly evident until you have seen it before.

Hope this helps,

US Iceman

Whats VFD, iceman?


Chemi:)

Sorry Chemi,

It stands for Variable Frequency Drive. I've also seen these called VSD's (Varibale Speed Drives) or inverters.

Danfoss do a decent control system ie: ADAP-KOOL This includes. Variable speed drives, floating head & suction Pressure, night set back (including switching off of evap. fans & closing EEV valves)

Have you considered this option or a system that is similar to this?

VV