What is wrong in this setup?

[nike123]

I have finished comisioning instalation of cooling/heating and hot water preparation (by air to water heat pump with desuperheater , solar panels and electrical heaters) in one relatively small (200kW of heating/cooling capacity 30% desuperheater, 30m^2 solar panels in sunny south Croatia) family hotel .
This is shematic diagram of setup.
2377
Is this configuration ok (serial water tanks and parallel heat exchangers in them)? I think that water tanks should also be in paralel.

[frank]

Hi Nike

I'm struggling to see why the output from one tank has to input to the bottom of another tank??

Surely, the outlets from each tank should be on a common header/outlet.

After all, you are not really going to gain anything as the energy input into each tank is the same, therefore the energy output will also be the same.

[chemi-cool]

Agree with Frank,

Tanks should be connected in parallel otherwise, all 3 will heat a little more than one.

[nike123]

Hi Nike

I'm struggling to see why the output from one tank has to input to the bottom of another tank??

Here (http://www.chinawinds.co.uk/diy_tips/installation_series_and_parallel.html) is described when it is used one or another.



After all, you are not really going to gain anything as the energy input into each tank is the same, therefore the energy output will also be the same.I am not sure in that, because temperatures in tanks are different and one tank could cool water in heat exchanger and other could heat that same water.
That is proved by poor performance of solar collectors and recuperator in these sunny days even the hotel is not operational yet. Ok, chiller works only with 1/4 of power intermittently, because there is no guests but there is also no consumption of hot water and still I cannot get more than 60°C in first two tanks and 50 in last.

There is no possibility to independently control heat exchangers and, by above article, it should be possible if we want to have serial configuration.
Solar collectors are controlled by differential thermostat whose probe is in last water tank and recuperator is controlled by thermostat in first tank (switches pump of recuperator circuit on/off).
Also, recirculation is connected only in last tank and therefore his temperature is lowest, even tho it should be highest by serial configuration.
I think that projectant here has made lot of mistakes and my solution to this would be to change to parallel tank configuration and to balance flow in tanks. Also, recirculation should be in all tanks and balanced.

[SteinarN]

I would clearly have conected the tanks in paralell on the hot water side also. And conected the recirculation to all 3 tanks.

As it is now, the water in the bottom of no.2 and no.3 is warm and thus causing dismal performance of the solar panels.

The performance of the solar panels can not be any other but dismal with this setup regardles of what the water consumption is.

[nike123]

Thanks guys, you confirmed my doubts.
I will propose these changes.

[Tesla]

Just to throw a spanner in the works. The design looks ok if you consider when the hot water is running out the last tank with highest heat is used. Its more to do with how the system is controlled. ie on cloudy lo heat source days can the last tank only be used and the other two be valved off. Then as heat source increases the other two tanks could be staged in to provide heat storage/preheat for the last tank?

[SteinarN]

At closer consideration I se the posibility for a better control logic of the system.

Can you describe the desuperheater heat exchanger? What type? Refrigerant/water flow countercurrent or cocurrent? What is the discharge gas temp? Condensing and evaporating temp? Refrigerant type? I supose the water pump is only running when the compressor is running. Is it a capasity regulated system?

[nike123]

At closer consideration I se the posibility for a better control logic of the system.

Can you describe the desuperheater heat exchanger? What type? Refrigerant/water flow countercurrent or cocurrent?
It is refrigerant/water counter current.


What is the discharge gas temp?
80-90°C


Condensing and evaporating temp?
Condensing temperature (saturated liquid) is regulated at lowest 42°C and, on hot day (38°C) could be as high as 55°C due to low-noise fans version.


Refrigerant type?
R407C



I supose the water pump is only running when the compressor is running.
No, in current configuration it runs when boiler thermostat calls but I allredy planed to insert one control contact in series to stop pump when there is no single one compressor in operation.


Is it a capasity regulated system?

Yes, 4 compressors in two separate refrigeration circuits (4 step capacity regulation).

[nike123]

[quote=Tesla;116178]Just to throw a spanner in the works. The design looks ok if you consider when the hot water is running out the last tank with highest heat is used. Its more to do with how the system is controlled. ie on cloudy lo heat source days can the last tank only be used and the other two be valved off.

Yes, it can, but that require operator. Customer wants that system is fully self controlled.


Then as heat source increases the other two tanks could be staged in to provide heat storage/preheat for the last tank?

As SteinarN said, when last tank is colder then other two (because of recirculation heat loss), then , when solar pump start, hot water in other two tanks quickly heat solar return and solar control turns off solar pump. Therefore I experiencing low solar heat performance.

That is why I think that is necessary that tanks are in parallel.

[SteinarN]

To make the best out of this system I have the following sugestions:



Connect the tanks in paralell. Recirculation also conected to all 3 tanks. Pay close attention to the design of the inlet and outlet header to allow equal water put through between the tanks.
Regulation of the desuperheater water flow. Stop the pump when no compressor is running. Regulate the water flow in relation to the number of compressor running, IE 3 compressor running requires 3 times the water flow of only one compressor running. The easiest way to do this is probably by installing a 3-phase pump and a VFD. No compressor running is a stop signal. Each compressor activates a relay connected to resistors in a voltage divider. By selecting the correct value of the resistors you get the desired voltage from the voltage divider at different numbers of compressors running. Feed this voltage signal to the VFD. The Hz and hence water flow should then be in linear relation to the number of compressors running. Dont care about temperatures. Regulate water flow only by this logic.
Ideally the water flow in the solar panel system should be regulated in a similar way. On-off pulsing could also be ok. Use one temp sensor in the tank at the upper level of the solar panel heating, and another sensor at the solar panel outlet up on the roof. If the solar panel outlet is less than say 3*C warmer than water in thank, then pump OFF. If the solar panel outlet is more than say 6*C warmer than water in tank, then pump ON. Correct temperatures found by try and error.

[Karl Hofmann]

Parallel !!!!!!

Nike.. Are your solar panels "Off the shelf" by a large manufacturer, or are the locally made/ own design?

Also if the domestic hot water circuit was pumped in a loop (Bronze pump), then each room would benefit from virtually instant hot water rather than it having to be dragged through at times when the occupancy is low

[nike123]

[quote=Karl Hofmann;116312]Parallel !!!!!!

Nike.. Are your solar panels "Off the shelf" by a large manufacturer, or are the locally made/ own design?

Of the shelf (Centrometal (http://www.centrometal.hr/eng/HR/Proizvodi/Solarni%20kolektori/Solarni%20kolektori.htm))!


Also if the domestic hot water circuit was pumped in a loop (Bronze pump), then each room would benefit from virtually instant hot water rather than it having to be dragged through at times when the occupancy is low

Yes, recirculation pump (Bronze pump) is always ON because of object purpose, and hot water is readily available soon as you open the hot water valve on any tap.

[nike123]

To make the best out of this system I have the following sugestions:



Connect the tanks in paralell. Recirculation also conected to all 3 tanks. Pay close attention to the design of the inlet and outlet header to allow equal water put through between the tanks.
Regulation of the desuperheater water flow. Stop the pump when no compressor is running. Regulate the water flow in relation to the number of compressor running, IE 3 compressor running requires 3 times the water flow of only one compressor running. The easiest way to do this is probably by installing a 3-phase pump and a VFD. No compressor running is a stop signal. Each compressor activates a relay connected to resistors in a voltage divider. By selecting the correct value of the resistors you get the desired voltage from the voltage divider at different numbers of compressors running. Feed this voltage signal to the VFD. The Hz and hence water flow should then be in linear relation to the number of compressors running. Dont care about temperatures. Regulate water flow only by this logic.
Ideally the water flow in the solar panel system should be regulated in a similar way. On-off pulsing could also be ok. Use one temp sensor in the tank at the upper level of the solar panel heating, and another sensor at the solar panel outlet up on the roof. If the solar panel outlet is less than say 3*C warmer than water in thank, then pump OFF. If the solar panel outlet is more than say 6*C warmer than water in tank, then pump ON. Correct temperatures found by try and error.


Thank you SteinarN! You helped me much here.
I will approach here in phases:
1. I will make parallel balanced tank configuration and that desuperheater pump is not working when compressor is not working. If that doesn't satisfy, then phase
2. Regulation of desuperheater flow according to number of compressors engaged.

I think that solar heat doesn't need to be flow regulated. I think that ordinary (already installed solar differential thermostat) is OK here. We have plenty of sun here all year long (11,3 hours/day in summer, 6,3 in fall, 4,2 in winter and 7,4 in spring) and sometimes, when there is not much consumption we need to cover collectors.

[yinmorrison]

I actually think that this is the correct setup because of ' recovery time' on the hot water side.The last cylinder rated with the highest heat input will always be at the highest temp in order to provide an instant maximum demand to the rooms whereas if you hook it up in parallel as soon as the hot water is used up the system will take longer to recover temperature as it is trying to heat more water from scratch.Only then should you consider the optimal control logic. Would you be happy waiting 1 hour for a shower when it is all used up?

[Karl Hofmann]

Nike, I had guessed that you had built in a loop for hot water but just wanted to be sure.. The original plumber on the project that I'm looking at now neglected to loop the system and it was a pain for me to get another pipe in to correct the job.... The reason I asked about the solar panels is because the payback time for solar in the UK is virtually never since the price for parts and installation is so high, The panels do seem to be rather expensive for what they are and would be interested in either buying direct from a manufacturer or building my own.

[nike123]

I actually think that this is the correct setup because of ' recovery time' on the hot water side.The last cylinder rated with the highest heat input will always be at the highest temp in order to provide an instant maximum demand to the rooms whereas if you hook it up in parallel as soon as the hot water is used up the system will take longer to recover temperature as it is trying to heat more water from scratch.Only then should you consider the optimal control logic. Would you be happy waiting 1 hour for a shower when it is all used up?

That rated heat input is only for backup electrical heaters.
Also, last cylinder has recirculation pump which is non-stop ON and cools the tank.

[nike123]

The original plumber on the project that I'm looking at now neglected to loop the system and it was a pain for me to get another pipe in to correct the job....

Sometimes it could be done with existing pipes, check this article (http://www.toolbase.org/Technology-Inventory/Plumbing/hot-water-recirculation)!


The reason I asked about the solar panels is because the payback time for solar in the UK is virtually never since the price for parts and installation is so high, The panels do seem to be rather expensive for what they are and would be interested in either buying direct from a manufacturer or building my own.

Here, cheapest and still OK panels of 2m^2 are about 400€ ? Vacuum tube panels are about 800€, but we don't need them in south of country. It is cost effective for hotels where hot water demand is high, but for domestic use, it is not.

[yinmorrison]

That rated heat input is only for backup electrical heaters.
Also, last cylinder has recirculation pump which is non-stop ON and cools the tank.


Ok that may be the case however it is still apparent that the last storage cylinder is supposed to have the maximum heat input on that basis.
On the schematic I see many valve symbols that I recognise , gate,double reg etc however on the flow and return manifolds there are double regs for balancing but there does not appear to be any on the primary circuits to the cylinders.Maybe you can add a valve legend as this appears to me to be a balancing prob.The hot water return pump should run all the time that is what it is for to maintaing constant temp and prevent legionella.