Ice rink-energy efficient system

Hello
I would like to hear your opinions on energy efficient ice rink chillers. Talking with some engineers recently they advised me that inverter driven compressors are 20-25% more economical than our current screw compressors. We have a packaged chiller with 2 screw compressors, shell and tube evaporator and air condensers. I also read online that new systems with NH3 (small amount around 200ltrs) and glycol are very efficient. What do you guys think? We average 25000-30000 KWh per month on electricity with our current system for an ice area of 700 sq.meters.

Your potential energy savings from VSDs depends of how much your screws are loaded. I'm double that 20-25% will be saved. Screw compressor VSD can be helpful to recover losses related to part load operation. Your loses are not great because suction pressure isn't low. Probably, you will get 5-10% from screw compressor VSDs. If you will use NH3 + evaporative condensers, energy savings will be much greater.

Major supplier Low charge ammonia plate heat exchanger packages available from Johnson controls, using reciprocating or screw.
Would have latest controls to maximize performance & energy savings.

GEA Grasso also have similar offererings

I do not think the VSD compressor is economical, it's efficiency, better power distribution.
The original compressor rotates 2800 RPM, VSD 3600 RPM, you can save when the compressor runs at 95-100% power. Low power for the screw compressor is not economical and efficient. This is true for VSD but without VSD.

Josef.

Hi!

We have designed and supplied a few air cooled ammonia chillers with screw compressors during the last 30 years, quite a few with VSD replacing "conventional" chillers in ice rinks during the last 10 years.

The savings depend on a few things. For instance, you typically need enough capacity in the chiller to build the ice rink. However, once the ice is done, then you need only partial load to maintain the ice. How much you can save depends on a. how unloaded will the chiller be, b. the variation in condensing temperature due to changes in the ambient dry bulb, c. what is the minimum differential pressure required to maintain the oil flow through the screw compressor, and d. if heat loads need to be maintained (to keep the under-floor warm, or melting the ice pit), again limiting how much you can lower your condensing pressure.

Of course, a benefit of the screw compressor with VSD is that you can operate with an economiser on line all the time: as the slides are at 100%, the economiser flow always goes to the intermediate pressure in the compression chambers, and is never by-passed to the suction.

If you have a SCADA system, it is always good to supply a log of ambient vs operating conditions for a whole year. A good supplier can then model to this ambient conditions and provide an scenario using their chillers, so you can compare the energy consumption (and even other running costs).

Manuel,
Depending on brand of compressor, but differential pressure minimum usually 5-6 bar.
If lower a lube oil pump can be installed, full time, or on demand when differential ges to low.
Discharge pressure can go as low as you like as in your case minimum differential pressure, expansion device big enough, other as you have mentioned.
As far as load variations, you may have 1 compressor to handle normal load & another kicks in for high loads.

Hello, Ranger1!

You are correct. Provided the installation of a pump is possible.

When a traditional ammonia chiller is used, a compressor pack is matched with an evaporator and a condenser of some sort, and they are piped together in a base frame - or site assembled. In this type of installation, the compressor pack includes an oil separator, vertical or horizontal, and enough head is allowed to install, if necessary, an oil pump.

A modern "factory" chiller is expected to minimise the ammonia charge, footprint (and costs!) using a more integral design. In our case, we use in our chillers our screw compressors with integral oil separators (similar to what you have in semi-hermetic screws), at the same time minimising external oil pipework and oil charge. We add a DX evaporator, PHE economiser, VSD drives for the motors and air cooled condenser fans. In this type of chillers, you avoid using an oil pump as it defies your main objectives. In our case, we have used differential pressures as low as 3 bar in certain projects.

Last March in Mostra Covegno, GEA presented their new water cooled ammonia chiller following the same principle of integral oil separator, combined in a semi-hermetic ammonia compressor and a flooded evaporator, so kg/kw are probably higher. Nothing wrong with this, as soon as it meets the design criteria and customer needs.

In case of dividing the load between two compressors, it is possible. However, extra is advised on the control strategy. You may end with a compressor fully loaded and another at very low partial load. So, you may (or may not!) be better off running both compressors in parallel. Again, some thought (i.e. number crunching) is required to find the best strategy.

Thank you for your comments! Have a great day!!

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Originally Posted by mcamacho

Hello, Ranger1!

You are correct. Provided the installation of a pump is possible.

When a traditional ammonia chiller is used, a compressor pack is matched with an evaporator and a condenser of some sort, and they are piped together in a base frame - or site assembled. In this type of installation, the compressor pack includes an oil separator, vertical or horizontal, and enough head is allowed to install, if necessary, an oil pump.

A modern "factory" chiller is expected to minimise the ammonia charge, footprint (and costs!) using a more integral design. In our case, we use in our chillers our screw compressors with integral oil separators (similar to what you have in semi-hermetic screws), at the same time minimising external oil pipework and oil charge. We add a DX evaporator, PHE economiser, VSD drives for the motors and air cooled condenser fans. In this type of chillers, you avoid using an oil pump as it defies your main objectives. In our case, we have used differential pressures as low as 3 bar in certain projects.

Last March in Mostra Covegno, GEA presented their new water cooled ammonia chiller following the same principle of integral oil separator, combined in a semi-hermetic ammonia compressor and a flooded evaporator, so kg/kw are probably higher. Nothing wrong with this, as soon as it meets the design criteria and customer needs.

In case of dividing the load between two compressors, it is possible. However, extra is advised on the control strategy. You may end with a compressor fully loaded and another at very low partial load. So, you may (or may not!) be better off running both compressors in parallel. Again, some thought (i.e. number crunching) is required to find the best strategy.

Thank you for your comments! Have a great day!!

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As you mentioned your screw compressors (without oil pumps) can operate at 3 bars differential. Who is manufacturer? You have air condensers. For ammonia refrigeration pants I saw only evaporative condensers. What is your summer condensing pressures?

Hi! Sorry for the delayed reply.

We use HallScrews (www.jehall.co.uk). Other manufacturers use low pressure differentials, too. In fact, the KDC valve from Danfoss, which is quite normal to control pressure difference in screw sets, is always closed by a minimum differential pressure of 1.5 to 2 bar (and opens about 2.5 bar dP).

Air cooled condensers are becoming more and more common (in plants up to 1~1.5 MW) because in many cases they represent a lower capital cost and use lower charge than evaporative condensers - even if tubing is used instead of microchannel. Also, water and chemicals are avoided, as well as the water distribution system. Draining refrigerant from air cooled condensers could be easier than draining from evaporative condensers.

For instance, with a typical HS3200 compressor, it is possible a SST of 0C an a SDT of +55C. Then, ambient is a matter of approach in the condenser: with a 10K dT, your max ambient is +45C. If need be, an adiabatic condenser can be applied, limiting the water usage to a few hours of the year (and does not require chemicals).

That been said, with large site assembled systems, evaporative condensers are still preferred. Multiple air cooled condensers (a.k.a. "farms") can be extensive and prove very tricky to install. Levelling is critical, and even solar incidence and wind can cause balancing problems. There are ways around these issues, but the systems become more and more expensive. So, these farms are mostly used when water is extremely scarce or highly polluted.

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Originally Posted by Segei

As you mentioned your screw compressors (without oil pumps) can operate at 3 bars differential. Who is manufacturer? You have air condensers. For ammonia refrigeration pants I saw only evaporative condensers. What is your summer condensing pressures?

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I'm concerned about SDT +55C. It will be condensing pressure 320 psig. As far as I know every ammonia refrigeration plant in Canada has PRVs set to 250 psig. 320 psig is new world for ammonia refrigeration plants.
Second concern is energy use. Most likely at 320 psig condensing pressure a plant will use twice more energy compare to 150 psig. Today everybody focused on energy savings. It is good if you will be able to save 10-20%. At 320 psig you will waste additional 100% of energy.

Quote:

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Originally Posted by Segei

I'm concerned about SDT +55C. It will be condensing pressure 320 psig. As far as I know every ammonia refrigeration plant in Canada has PRVs set to 250 psig. 320 psig is new world for ammonia refrigeration plants.
Second concern is energy use. Most likely at 320 psig condensing pressure a plant will use twice more energy compare to 150 psig. Today everybody focused on energy savings. It is good if you will be able to save 10-20%. At 320 psig you will waste additional 100% of energy.

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Hello, Segei!

Your concerns are quite valid. My two cents on the arguments would be to put things in a different perspective.

Designing refrigeration systems at 300 psig or more is not new. Industrial trans-critical CO2 systems already available over a 100 years ago were designed at 1500 psig (although they were not common in North America), Today, small commercial transcritical CO2 systems are designed at 2000 psig.

The need for designing ammonia systems for higher pressures has built gradually. Frick (US) introduced 300 psig besides 250 psig as standard in their ammonia offering about 20 years ago. And, companies such as RFF and Danfoss have industrial valves designed for 580 psig.

This is out of need. Today's ammonia heat pumps have design pressures between 580 psig to 750 psig to obtain water at +70C to +80C (we just commissioned a 2.5 MW ammonia heat pump in Switzerland supplying water at +70C ~ +158F).

Now, designing an air cooled system that can run at 320 psig does not mean that it will be running at 320 psig. It will just run at the pressure the ambient temperature allows it to condense to. Many systems are chosen based on the COP at "design conditions", which really happen just a few days during the year, running inefficiently the rest of the time. This is why A/C systems have moved to evaluation of SEER and similar measures. In EU, condensing units and chillers, A/C or refrigeration, have to comply with Eco-Design Directive, which sets a minimum acceptable level of efficiency at part load and different ambient conditions.

In practice, one of the first things which has to be looked at when designing a system, is the profile of the heat sink which will be used. For instance, ground bore pipes could represent a fairly stable temperature heat sink, while ambient air temperature could vary a lot. In most cases, air or water are used as heat sinks, and that means a yearly profile of the dry-bulb temperature, the wet-bulb temperature, or both, has to be generated. If possible a profile of the load should be created, too, and matched. Then, different plant concepts can be analysed to determine their energy usage. Normally, this requires some type of simulation. The total cost of ownership can then be estimated for each alternative, and a decision can be made with all facts available.

The importance of energy consumption is relative to the cost of energy, how valuable is the process, the risks associated with failure, the cost of plant space, the cost, direct and indirect of refrigerant charge and many others. Some examples:

1. Last time I was in Saudi, filling a normal saloon car with fuel was about 10 USD. So, customers in that part of the world were not particularly interested in energy consumption. However, water was (still is) a premium, and a big concern to them. In such place, they tend to like air cooled systems.

2. In many chemical and petro-chemical processes, refrigeration systems run non-stop for periods of up to two years or longer, regardless if there is load or not. The value of the product requires the systems to cool as soon as the process demands it. If a system is off, pushing the start button does not guarantee that the equipment will run. Having a back-up is of no use. Again, it cannot be guaranteed that the back-up will start when the button is pushed. However, a running system is running, and can be monitored in detail and be ready to cope with the duty when need calls. We recently supplied a project of this sort, where the customer would not allow us to touch the chiller to set up the economiser into operation as the cooling process was generating $750.000.00 of profits daily. They could live with the extra energy consumption.

3. Some of the most challenging systems to design are the chillers for critical operations in Navy ships (weapon systems, radar, communications, and others). The height of the decks are restricted, the footprint is pre-defined, there is limited power generation, and water pumping capacity for the heat rejection is also limited. In this case, an important issue is start-up currents, as the network is limited, too. However, if you lose the cooling, you may loose the weapons, radars and communications, lose a ship with the crew, lose a battle, and lose a war... So, reliability and envelope have to be balanced against energy efficiency, as the consequences are enormous.

4. Another real case project. A chiller to cool down the packaging machines of a chocolate factory. Each packaging machine packages six metric tons of processed chocolate per minute. So, if cooling is lost for a minute, it creates a expensive mess. Again, reliability has to be balanced against energy efficiency.

5. Jet-engine test facilities. Very large systems are required, but they may run only two weeks in a year. In this case, you have time to prepare and check the equipment, and be sure that the start button will do the trick. The capital cost to make a highly efficient system could have a very long payback time. We have similar customers in the food industry, where process equipment runs maybe two weeks in a year, and capital cost has to be balanced with energy efficiency.

At the end, as my International Law professor thought me, "the best answer for any question is 'it depends' ".

We did a distribution centre for a very large worldwide distribution company to be used by one of the top three food manufacturing companies in the world, and they chose independent ammonia air cooled chillers over a central machinery room (ambient temperatures between -10C/+35C) as the advantages were just overwhelming in that particular project: no machinery room, a fraction of the refrigerant charge (0.7 lb R717/TR), less challenges with local environmental authorities and emergency services, no need of water network or additional chemicals, factory assembled chillers delivered as "boxes" for outdoor location on simple plinths, in built redundancy (just added another chiller), great system part load efficiencies... The results just confirmed all expectations.

And, please don't get me wrong. We still have a contracting division which makes site installs with large machinery rooms, evaporative condensers and all the bells and whistles. But, every project has a different set of needs.

Best regards,

-Manuel.

Hi Manuel.

I agree with many of your points. However, I worked on the ships and they have a lot of power (auxiliary engines)and water pumping capacity(a lot of sea water). I think that it is possible to design and run plant which will be energy efficient and reliable. We can design plants for high pressure but very often local regulation will not allow us to build this plant.
About Saudi. Several months ago one company decided to save energy and they asked me to optimize operation of their refrigeration pant. Surprisingly I found that this plant with evaporative condensers (located in Riyadh) has optimum condensing pressure of 150 psig. The same like in Toronto. However, air condensers will give them summer condensing pressure of 320 psig. Actually they are interested in energy savings as well.
About distribution centre. Recently in our area one cold storage was built similar to yours. Everything looked good. However, nobody mentioned to the owner that this plant will have summer condensing pressure 225 psig. This is a lot of additional energy. Typically, old condensers don't perform as new ones and in several years this pressure will be higher. PRVs are set to 250 psig. You mentioned about redundancy. Assume that cold storage has 4 freezers and 1 compressor per each freezer. In compressor room you need 5 compressor (1 as back up). In your case you need 8 chillers.
Personally I prefer central refrigeration plant with evaporative condensers.