Geothermal Design 2

[AndyHX]

Hi All!
House is going ahead , and i will be building ground source heat pump.
Question i have is this , to desuperheat or not? My idea so far is to have a seperate condenser/desuperheater before reversing valve . In heating 90% heat going to floor through main condenser and 10% to domestic hotwater. Water flow through desuperheater will be controlled with head pressure regulating valve , when it comes to summer cooling , turn off main condenser water pump and desuperheater takes over and takes all heat from a/c and puts in to domestic water. When domestic to temp , switch back on main cond pump and heat to waste and maybe desuperheater scavenging remains? Any thoughts or over sights on my part?

[AndyHX]

Or perhaps one condenser and a 3 way valve to divert water to domestic or heating.

[flyinkiwi]

Or perhaps one condenser and a 3 way valve to divert water to domestic or heating.

Only if you're happy with your space heating being interrupted while your domestic is heating, and head pressure being *much* higher. I think your COP would be better with the desuperheater plan.

What about a HR valve ahead of reversing valve, diverting discharge into desuperheater when DHW requires heat?
That way you don't have an idle condenser being heated to high heaven with no water flow - sounds like a good way to scale a HX.

[AndyHX]

Hi Flyin!
With the main condenser pump off , the desuper heater would take over the load , water regulating valve open wide and increase water flow so condenser would not get hot at all.
What im trying to do is use the heat when unit is cooling in summer time.

[flyinkiwi]

Why not dump the excess your desuperheater can't handle back into your ground loop, and store the heat for winter? Or at least, later?

Have you considered using a temp controlled water valve to give desired DHW flow temp?

If you're talking whole house sort of capacity, eg. 12-15kw, then you may have a very high water flow for the DHW side when heating only domestic. OK if you use a cylinder with a coil (preferably a 20m one!), not so if you are recirculating potable water - high flow would disturb your stratification in the cylinder.
At any rate, you may struggle to move your THR into your DHW system without running at silly head pressures.

[mad fridgie]

No Issues.
You need a slightly special cylinder Superheat in Chch. Slighly larger than you need tall and skinny, with a baffle plate about 1/3 the way up. (i would also fit a element close to the top)
Desuperheater water loop (you should have by law a double barrier heat exchanger, but who really cares),
You draw from above the baffle plate and re-introduce just above (100mm), unlless you are going to control theoutlet temp from the desuperheater then it can be introduced into the top (i would not advise this as you have to ensure always a high discharge temp)
Now the excess energy in summer.
You can either fit a coil below the baffle and run you closed (heating) loop water through it, or go for an external heat exchanger and pump the potable water , whilst the closed loop is also pumped. (i would advise the second, for max efficiency)
The baffle stops excessive mixing. But allows for natural stratification.
As the bottom starts to heat and rises so will your discharge temp, increase further the stratification in the top of tank.
Two stats are required, at at the bottom to control the main flow and and one above the baffle plate, to control the desuperheat pump. (I should say also one for the element, but that normally comes fitted)

[MikeHolm]

We've had this argument for years in North America where propylene glycol is one one side of the HX and water is on the other side and it is bogus. When the limited volume of glycol or refrigerant is introduced to the water after a breech, it is diluted so much as to be almost irrelevant. There are uses for double wall HX but when both sides have non toxic components, there should be no problem using single wall.

How would you put a baffle in a standard, ready made tank. Do some of your tanks come with removable heads?

[AndyHX]

Thanks for comments!
There is no chance to store heat in ground as im using a stream as the source, 50metres from house so criminal not to.
Mad what do you think about a larger desuperheater , controlled with the water regulating valve so consistant temp out but varying flow , pumped to cylinder and using wet back connections? high in and low out?
Also using multi speed pumps on condenser and desuperheater , switch condenser low speed for summer , high speed for desuper ? Trying to keep it simple .

[mad fridgie]

We've had this argument for years in North America where propylene glycol is one one side of the HX and water is on the other side and it is bogus. When the limited volume of glycol or refrigerant is introduced to the water after a breech, it is diluted so much as to be almost irrelevant. There are uses for double wall HX but when both sides have non toxic components, there should be no problem using single wall.

How would you put a baffle in a standard, ready made tank. Do some of your tanks come with removable heads?

Thing are slightly different in NZ, a lot of cylinders are made to order any way (not must held in stock), so a baffle is fitted during production. (Superheat is a maufactures brand in christchurch)

[mad fridgie]

Thanks for comments!
There is no chance to store heat in ground as im using a stream as the source, 50metres from house so criminal not to.
Mad what do you think about a larger desuperheater , controlled with the water regulating valve so consistant temp out but varying flow , pumped to cylinder and using wet back connections? high in and low out?
Also using multi speed pumps on condenser and desuperheater , switch condenser low speed for summer , high speed for desuper ? Trying to keep it simple .
You are totally reliant on having high discharge temps to achieve high water outlet temps. If you have low compression ratios, (for example start up), then your discharge temp will be low, so you are unlikley to have any flow.
In the way i described you will always absorb energy, even if the discharge temp is low(ish)
The size of the desuperheater should be based upon minimum refrigerant pressure drop, when there is NO water flow

[AndyHX]

Hi there Mad , ive been thinking over your design idea and agree it will give best performance. Do you have any external hx in mind ? Ive been talking to Vaportec in Napier about ground source hx , the plan is 50mm spiralled stainless steel , 3 x 5m lenghts which will give me 2.35 m2 surface area so i should get 10kw at 5k according to

http://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html

Forced liquid (flowing) water - Forced liquid (flowing) water : 900 - 2500 W/m2K (heat exchanger water/water)

Would you agree with this calculation?

[mad fridgie]

Hi there Mad , ive been thinking over your design idea and agree it will give best performance. Do you have any external hx in mind ? Ive been talking to Vaportec in Napier about ground source hx , the plan is 50mm spiralled stainless steel , 3 x 5m lenghts which will give me 2.35 m2 surface area so i should get 10kw at 5k according to

http://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html

Forced liquid (flowing) water - Forced liquid (flowing) water : 900 - 2500 W/m2K (heat exchanger water/water)

Would you agree with this calculation?

I do use a quite a few vaportec heat exchangers. I find them to be good for my tested standard application. For non standard (non tested) I found that results are not quite as close as I would like, from a design point of view.
So my advice would be to leave the design to them, flow rates, pressure drops and heat transfer, reducing your risk. For these type of applications I use SWEP, a little expensive, but the design software is just about spot on. and that is important for what I do.

best of luck

Mad