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Author Topic: large (1000litre ish) mains pressure hot water tank ?  (Read 3676 times)
knighty
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« on: August 16, 2017, 11:37:32 PM »

I could really do with a big mains pressure hot water tank at work.... 1000litre would be about right, but the bigger the better really

(to be heated as PV dump load to limit export, and via heat recovery from freezer compressors)


but... the only mains pressure tanks I can find big enough start at 12,000

anyone have any ideas ?


I was told by a tank manufacturer a thermal store with a hot water coil in it will be no good for me - we draw off too much hot water at peak times which cools the store water around the coil and reduces coil output temp pretty quickly
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Countrypaul
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« Reply #1 on: August 17, 2017, 12:06:47 AM »

If you used a thermal store with a FPHE rather than a coil surely that would largely eliminate the problem you describe.

How about using several smaller stores in paralell, rather than one large one, might get messy in the setup, but might suit your needs?
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brackwell
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« Reply #2 on: August 17, 2017, 08:21:38 AM »

Are you and the tank manufacturer talking about the same thing ?  You seem to be thinking of a pressurised tank where all water is physically used and the manufacturer where a coil inside the tank picks up the heat from the tank and therefore is limited to say 50% heat pick up of the tanks heat capacity.

Ken
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martin W
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« Reply #3 on: August 17, 2017, 08:20:08 PM »

Slightly off topic, I looked at a similar problem a year or two ago (more to with pressure drop due to large draw).

We actually considered an accumulator (read very big expansion vessel). IIRC 3000 litre was around 10k.

One of the problems of a pressurised storage you still need an appropriated sized expansion vessel, and pressure relief valves. You also need to consider how often the whole volume of water is changed to replace all the water with fresh water to limit bacteria. we were looking a full dump system.

From what you are saying you look like you are really looking to store heat energy for later use. I would think an unpressurised thermal store with either a very big coil for your mains water to travel through, or a flat plate heat exchanger. You could in theory pass cold mains through the thermal store (via coil, etc.) then through your boilers to top up with heat to the correct temperature.

How do you currently create hot water. How much do you use over 24 hours, how much drawn in one go? (I have a customer who averages 30,000 litres a day of hot water and could draw 1000 litres in 10mins or less, then use nought for 45-60 minutes..)

Really look at how you currently use water, draw off rate (litres per minute, usage over 24 hours, pressure drop). Work out what you need to achieve (is it just use waste heat to pre heat your water???)


akvaterm thermal stores are one brand that comes to mind that do large tanks and stores.


HTH

Martin
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knighty
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« Reply #4 on: August 19, 2017, 12:16:47 PM »

hot water right now comes from a big old boiler

we use a lot of hot (hot as in 90'C) water every day, probably over 1000 litres, mostly drawn off in an hour or so at the end of the day

I thought about a fphe but also thought if I'm pumping the store water around I'll ruin the stratification in the tank ?


it really irks me that we have big freezer compressors running non stop dumping loads of heat into the air.... and then we're running a massive gas boiler to heat hot water!    doing my best to push energy efficiency as hard as I can, but it's work, so it has to be a good financial move as much as an efficiency/green one - no good spending 12,000 on a tank which will take 25 years to recover that cost because I've got no idea what will happen in 10 years never mind 25
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A.L.
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« Reply #5 on: August 19, 2017, 03:47:45 PM »

hello,


we use a lot of hot (hot as in 90'C) water every day, probably over 1000 litres, mostly drawn off in an hour or so at the end of the day

I thought about a fphe but also thought if I'm pumping the store water around I'll ruin the stratification in the tank ?

it really irks me that we have big freezer compressors running non stop dumping loads of heat into the air....


1) Long term when the existing freezers need to be replaced look at systems using CO2 as refrigerant,  they are easily capable of heating water to 90C

https://www.ohio.edu/mechanical/thermo/Applied/Chapt.7_11/Chapter9.html

Sanyo used to make some domestic sized air source heat pumps using CO2 but they seem to have disappeared since they were taken over by Panasonic

2) Stratification can be maintained generally if water velocities are held below about 3.5cm/s, large baffle plate in tank with suitable array of holes?

or

store (and heat) the actual water you use in a tank with vent to air, use water without adding fresh cold simultaneously (avoiding mixing problems), add fresh cold water after days use and have perhaps 22hrs to reheat?

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brackwell
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« Reply #6 on: August 19, 2017, 05:27:19 PM »

I am wondering how much heat comes off the refrigerator in the first place. Instinctively what you say seems logical and irksome but we need to estimate what quantity of energy is available, it may not be as much as you think.   Anybody any ideas ?  Measure the leccy input to the fridge and multiply by 3 perhaps ?

In any case to capture this heat it is not necessary to raise to 90C. A lower fig will capture the energy and leave the boiler (modulating) just to  add a bit.  The fact the tank is stratified or not is irrelevant as as much heat as been captured as is possible.

Ken
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RIT
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« Reply #7 on: August 19, 2017, 05:41:59 PM »

The 90C output would seem to be the biggest limitation as it means you have very little headroom to over heat the water in the store and then mix down to the correct temp.

If you need to hit that target a configuration of 2 thermal stores may be the best option. The first would be a tank that raises the water to say 70C-75C and then a second tank (or even your current boilers) that completes the increase. This way you can focus a lot of the heat retention work on just the second store. I don't know the likely output temp you see from the freezer compressor, but I'm guessing that it would be well suited to raise the temp of the first store, but would not be able to get to the final 90C+ without the help from the PV driven heater (or any alternative energy sources).

One of the best options maybe to talk to a cylinder manufacturer to see what they would recommend. One company that may be a place to start is http://whcylinders.co.uk I know nothing about them apart from the fact that they are a UK company, which should mean thay can talk code and regs as well as specs.
« Last Edit: August 19, 2017, 05:43:46 PM by RIT » Logged

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knighty
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« Reply #8 on: August 19, 2017, 06:50:35 PM »

hello,
1) Long term when the existing freezers need to be replaced look at systems using CO2 as refrigerant,  they are easily capable of heating water to 90C
https://www.ohio.edu/mechanical/thermo/Applied/Chapt.7_11/Chapter9.html
Sanyo used to make some domestic sized air source heat pumps using CO2 but they seem to have disappeared since they were taken over by Panasonic
2) Stratification can be maintained generally if water velocities are held below about 3.5cm/s, large baffle plate in tank with suitable array of holes?
or
store (and heat) the actual water you use in a tank with vent to air, use water without adding fresh cold simultaneously (avoiding mixing problems), add fresh cold water after days use and have perhaps 22hrs to reheat?

I've been looking at alternative refrigeration gasses, running mostly 404A right now - which will be discontinued by 2020 because it's production produces too much CO2

(although someone on a refrigeration engineer forum told me that co2 for refrigeration is essentially made by dumping limestone into sulphuric acid and has nasty by products)

I'd love to go move to CO2 right now, but the cost to replace everything would be crazy... as in more than my house cost.   a lot of the stuff I have is old, but works just fine, and can work with newer gasses

also, CO2 looks tricky for freezers (vs fridges) something to do with the pressure/temperature chart of CO2 (I'm no expert) means you really need a cascade system, where the co2 cooling the freezer is itself cooled by more co2 instead of by fresh air, because it needs to be colder than that / you can't cool it that much in a single step without crazy pressures


I am wondering how much heat comes off the refrigerator in the first place. Instinctively what you say seems logical and irksome but we need to estimate what quantity of energy is available, it may not be as much as you think.   Anybody any ideas ?  Measure the leccy input to the fridge and multiply by 3 perhaps ?
In any case to capture this heat it is not necessary to raise to 90C. A lower fig will capture the energy and leave the boiler (modulating) just to  add a bit.  The fact the tank is stratified or not is irrelevant as as much heat as been captured as is possible.
Ken

currently have 62hp (46kw ish?) of freezer compressors  (*that includes one fridge compressor)

moving things around, new freezer has being built etc.. so pretty soon that'll be closer to 120hp

most of that being the 60hp blast freezer, where the compressors run pretty much non stop (compared to a home fridge etc. which goes on off as needed)

I know a lot of the heat is low level heat, not really hot enough for what I need - the output of the compressors is over 100'C but there's not much energy there (where it's hot gas instead of hot liquid)

if I could find some tanks I'd be tempted to have two, one as a pre-heat to the pre-heat tank if you know what I mean ?


The 90C output would seem to be the biggest limitation as it means you have very little headroom to over heat the water in the store and then mix down to the correct temp.
If you need to hit that target a configuration of 2 thermal stores may be the best option. The first would be a tank that raises the water to say 70C-75C and then a second tank (or even your current boilers) that completes the increase. This way you can focus a lot of the heat retention work on just the second store. I don't know the likely output temp you see from the freezer compressor, but I'm guessing that it would be well suited to raise the temp of the first store, but would not be able to get to the final 90C+ without the help from the PV driven heater (or any alternative energy sources).
One of the best options maybe to talk to a cylinder manufacturer to see what they would recommend. One company that may be a place to start is http://whcylinders.co.uk I know nothing about them apart from the fact that they are a UK company, which should mean thay can talk code and regs as well as specs.

there's no *requirement* for 90'C water... we just like it that way, makes cleaning etc. must quicker and easier, also makes a great job of killing bacteria etc..

it's a dog food factory so there's literally blood and guts on the floor/walls at the end of the day, when we clean down there's a guy working with a steam cleaner while others throw buckets of boiling water at/on/over things  (some things need a scrub etc. too)


I think a store (or two in series) feeding into the gas boiler is my best bet...  just can't find any tanks :-(




I didn't realise uk mains pressure could be so high - as high at 145psi sometimes (when you turn all the taps off) - I could limit that down a bit no problem
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knighty
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« Reply #9 on: August 19, 2017, 07:28:10 PM »

sod it.... why don;y I just gen a bunch of 250 or 300 litre tanks ?

I'll stick them in series, one after another, and have heat input flow in reverse to water flow through them


not a very tidy or clean solution... but it'll work

and it's a good 2 months before I'll be installing them, so I'll trawl ebay for cheap tanks too
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brackwell
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« Reply #10 on: August 19, 2017, 07:47:35 PM »

Also i am thinking that using water as the cooling medium rather than air that the efficiency will increase. Can one connect the cooling pipes directly to a coil (copper ?) in the tank/s ?
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A.L.
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« Reply #11 on: August 19, 2017, 09:50:33 PM »

hello again,

also, CO2 looks tricky for freezers (vs fridges) something to do with the pressure/temperature chart of CO2 (I'm no expert) means you really need a cascade system, where the co2 cooling the freezer is itself cooled by more co2 instead of by fresh air, because it needs to be colder than that / you can't cool it that much in a single step without crazy pressures


Sanyo claimed that their 2-stage compressor could handle this even for freezers, see attached pdf below.


* Sanyo_rotary_compressor.pdf (128.92 KB - downloaded 90 times.)
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RIT
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« Reply #12 on: August 19, 2017, 11:53:10 PM »

The 90C output would seem to be the biggest limitation as it means you have very little headroom to over heat the water in the store and then mix down to the correct temp.
If you need to hit that target a configuration of 2 thermal stores may be the best option. The first would be a tank that raises the water to say 70C-75C and then a second tank (or even your current boilers) that completes the increase. This way you can focus a lot of the heat retention work on just the second store. I don't know the likely output temp you see from the freezer compressor, but I'm guessing that it would be well suited to raise the temp of the first store, but would not be able to get to the final 90C+ without the help from the PV driven heater (or any alternative energy sources).
One of the best options maybe to talk to a cylinder manufacturer to see what they would recommend. One company that may be a place to start is http://whcylinders.co.uk I know nothing about them apart from the fact that they are a UK company, which should mean thay can talk code and regs as well as specs.

there's no *requirement* for 90'C water... we just like it that way, makes cleaning etc. must quicker and easier, also makes a great job of killing bacteria etc..

it's a dog food factory so there's literally blood and guts on the floor/walls at the end of the day, when we clean down there's a guy working with a steam cleaner while others throw buckets of boiling water at/on/over things  (some things need a scrub etc. too)


I think a store (or two in series) feeding into the gas boiler is my best bet...  just can't find any tanks :-(
[/quote]

If you have no legal requirement, but it 'helps' then just use one or more heat stores to capture the heat from the freezer compressor and the PV before the water is feed into your boilers. Such a configuration would removed all the issues of designing a system that has to handle mains pressure. Such a configuration also means that you can deploy a solution that when it works you save money, but if there are any issues you can just switch a bypass valve and burn more gas in your boilers as the input temp of the water will be lower.

One key question - are your current boilers at their end of life, or are you just trying to reduce their energy costs? If they still have a long term life you should find that preheating water with the free energy you have is a viable option.
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