airlock in calorifier

[WhiteSuit]

Apologies but I have searched the forum for existing answers to no avail.

I have a horizontal calorifier under the bed fed from a variable speed water pump, no accumulator and no expansion vessel on the hot side. I know that's wrong (NRV in cold inlet). The system has worked fine for the five years we have had the boat. Having moved to a marina with no electric hook up this winter I decided to drain the calorifier to prevent freezing, not an easy task and I rigged up a system to pump air into the top connection to expel the water through the opened PRV to the cut. Now regretting my decision with the benefit of hindsight over forecast.

Have just changed the PRV, as the old one was beginning to weep last summer, and refilled the system. Took a long while to fill the calorifier expelling air through the hot taps. Now the cold system works normally but the hot obviously now has an airlock. When you open the hot tap you get a normal flow of water but it takes several seconds before the pump cuts in and the flow remains fairly constant. Shut the tap and the pump runs on for several seconds. My assumption is the calorifier with an air 'bubble' in the top has now taken on the role of an accumulator/expansion vessel. While not a problem on water flow I assume this will cut down the amount of hot water available and the length of time that water will remain hot. 

 

How do I get rid of the airlock?

The only thing I could come up with is to haul the boat out on a slope and rebleed whilst the connection end of the calorifier is higher. As I don't need the blacking done for another year this would be an expensive option.

[Boater Sam]

4 minutes ago, WhiteSuit said:

. My assumption is the calorifier with an air 'bubble' in the top has now taken on the role of an accumulator/expansion vessel. While not a problem on water flow I assume this will cut down the amount of hot water available and the length of time that water will remain hot. 

 

How do I get rid of the airlock?

 

 I reckon you are correct. It will make little or no difference to temperature or water quantity and over time the water will absorb the air.

It won't be a very large bubble. Leave it, do something more interesting!

[MtB]

8 minutes ago, WhiteSuit said:

My assumption is the calorifier with an air 'bubble' in the top has now taken on the role of an accumulator/expansion vessel. While not a problem on water flow I assume this will cut down the amount of hot water available and the length of time that water will remain hot. 

 

Yes to both of those thoughts.

 

The volume of hot water missing will be tiny though. Probably a litre or so. Won't affect the time it stays hot in the calorifier for.

 

The air bubble will slowly dissolve away into the water so no need to take any action. This is why expansion vessels need diaphragms!

 

 

[WhiteSuit]

Thank you both for quick replies, now I can find something really important to worry about!

[ditchcrawler]

6 minutes ago, WhiteSuit said:

Thank you both for quick replies, now I can find something really important to worry about!

It will also reduce the strain on the calorifer when the water heats up and expands before the pressure relief valve lifts

[Boater Sam]

 

12 minutes ago, ditchcrawler said:

It will also reduce the strain on the calorifer when the water heats up and expands before the pressure relief valve lifts

Pray explain how air pressure and water pressure differ? Does the calorifier care? It might slightly reduce shock loadings due to pump surges but I doubt it.

[Iain_S]

I think Brian means that the air will act as an expansion vessel, so the lifting of the PRV will be delayed.

[Boater Sam]

2 minutes ago, Iain_S said:

I think Brian means that the air will act as an expansion vessel, so the lifting of the PRV will be delayed.

I will need that explaining to me as well please.

 

Puzzled, not grumpy!?

[Sapphal]

When I had the same problem after re-positioning the cauliflower I loosened the immersion heater blanking plug to remove the air. I assume you have no such plug. So why not drill a small hole and fit some sort of blanking plug. Surely easier than tipping the boat on end!

 

But as said, for the small amount of hot water capacity lost, is it worth it?

[ditchcrawler]

43 minutes ago, Boater Sam said:

 

Pray explain how air pressure and water pressure differ? Does the calorifier care? It might slightly reduce shock loadings due to pump surges but I doubt it.

If you fill a vessel with water and no air at ambient pressure and temperature, then you heat it by 60 deg C (possibly more) the water will expand by about 2 lts. That will ether stretch the calorifier of stretch it until it reaches the rating of the relief valve and then lift the valve. If there is a couple of lts. of air then the water will compress the air and may or may not eject any through the RV

[Boater Sam]

2 minutes ago, ditchcrawler said:

If you fill a vessel with water and no air at ambient pressure and temperature, then you heat it by 60 deg C (possibly more) the water will expand by about 2 lts. That will ether stretch the calorifier of stretch it until it reaches the rating of the relief valve and then lift the valve. If there is a couple of lts. of air then the water will compress the air and may or may not eject any through the RV

Water expands 2lts? Why?

 

The calorifier is pressurised to the set pressure of the water pump. The air inside it then at the same pressure.

This pressure is below the PRV set pressure.

Heating the water and air will cause expansion dependant on the rise in temperature, not 2 lts.

If this pressure is still below the prv set pressure no water will be lost.

Increasing the pressure by further heating until the prv opens will increase both the air and water pressure.

The air will have no effect on the pressure in the vessel.

[Ex Brummie]

You can pressurise air, but not water. Therefore the air takes up the expansion of the water and minimises the effect of expansion on the calorifier and stops the prv from opening thus preserving the integrity of the seal. Over a days cruising, the temperature rise is quite significant.

[ditchcrawler]

2 hours ago, Ex Brummie said:

You can pressurise air, but not water. Therefore the air takes up the expansion of the water and minimises the effect of expansion on the calorifier and stops the prv from opening thus preserving the integrity of the seal. Over a days cruising, the temperature rise is quite significant.

Especially from cold to 80deg C 

Take a treacle tin, fill it with cold water, hammer the lid on and heat it up on the hotplate. Stand another one beside it half full of water and see what happens as they both heat up

[WotEver]

50 litres of water heated from cold to boiling will expand by about 2 litres. 

[ditchcrawler]

9 minutes ago, WotEver said:

50 litres of water heated from cold to boiling will expand by about 2 litres. 

Thats what I said but I was using 55 lts to 80 deg C

[WotEver]

5 minutes ago, ditchcrawler said:

Thats what I said but I was using 55 lts to 80 deg C

Yup, I was agreeing with you

[Boater Sam]

Water expands 4% between room temperature and BOILING. 55 litre calorifier? That's a bigun.

Any divers here who know how to work volumes of compressed gas? 

Pressure is pressure end of.

[WhiteSuit]

8 hours ago, Sapphal said:

When I had the same problem after re-positioning the cauliflower I loosened the immersion heater blanking plug to remove the air. I assume you have no such plug. So why not drill a small hole and fit some sort of blanking plug. Surely easier than tipping the boat on end!

 

But as said, for the small amount of hot water capacity lost, is it worth it?

I have an immersion fitted so no blanking plug. The hot water take off is however higher than the immersion so surely air will be bled from there.  Or am I missing something stupid?

[ditchcrawler]

3 hours ago, WotEver said:

Yup, I was agreeing with you

Thanks, not many people do

 

[Ex Brummie]

15 hours ago, Boater Sam said:

Pressure is pressure end of.

Pressure is pressure, and air will compress. Water will not, apart from pressure you have a physical mass that will strain its container. Ditchcrawler's example of the treacle tin is valid. There are several examples on U Tube of hot water cylinders on sealed systems exploding.