heat pump in extract

[multisync]

We got a call from an old customer who has a large supply/extract to their uni. They have asked about heat recovery as it's a 100% loss system and it shifts a lot of air.As they are at opersite ends of the building, modifying ductwork is out.

In the Uk it's estimated 75% of the year the outdoor temp is 15 deg C or less, so this system needs a lot of preheat.

I pondered if we could fit a heat pump with the evap in the exhaust flow and the condenser in the supply.

Would it work or am I missing something ?

[Tesla]

Hi multisync
Yes it will work. Take measurements of exhaust and supply then plot on a psychometric chart. Size unit to the parameters on chart or ask supplier. Allow for air pressure drop and flowrate and access to clean coils. A good control stratagy and monitoring for fine tuning for several months/seasons will gaurentee and prove results. Good Luck and keep us posted with the results

[frank]

I've never seen a heat pump fitted into a supply/extract ductwork system for heat recovery, but, I have fitted numerous 'run around' coils.
Basically, you fit a full duct size coil in both air streams and then pipe them together, with a small circulating pump and water/glycol mixture.
The extract air stream warms up the extract coil and the circulator moves the warmed water into the supply coil where it gives up it's heat.

I've received quite a few enquires recently from such places as bakeries, where the engineer wants to recover waste heat from the bake ovens to re-use in the proving ovens.
Struggling to come up with a design though as the waste heat temperature is mid 200C. I can't use water as it would turn to steam in the event of a pump breakdown.

[RefrigNoob]

Struggling to come up with a design though as the waste heat temperature is mid 200C. I can't use water as it would turn to steam in the event of a pump breakdown.

Motor oil ftw? Hell, there are so many oils that'll safely operate at those temperatures

[multisync]

Thinking on it further we suspect the controls would be the major hurdle.

There would be a constant known heat source of -say 10 m2/sec @ 22 deg'c but whilst the demand would vary according to the supply temperature.

Therefore the control would need to be done on the air off condenser not air on evaparator. How to intergrate that is perhaps beyond us at the moment.

Further to our visit there is a run around system half installed. On the supply side the coil is already installed but on the exhaust it's misssing (although the section is there ready)

We suspect it got scrubbed after being specc'ed when the cost cutting started

We have emailed the AHU manufacturer for their input..

Run around are only 50% efficient max whereas a heat pump is 3x -or are we getting confused by the 'freeheat'?

[TRASH101]

Hi multi

The control aspect isn't that bad to factor in.

e.g. if you treat the heat pump as a "block" of heating say +5c and you have your original pre-heat set at say 15c then run your heat pump at 10c air on and let the original pre-heat act as a trimmer.(works better if pre-heat is electric. This will work as long as the heat pump output is reasonabley constant i.e. the exhaust air is fairly stable temp/ volume wise. The heat pump must first in line so don't forget filters and the air velocity is all important.

Simplicity is best.

[multisync]

Hi multi

The control aspect isn't that bad to factor in.

e.g. if you treat the heat pump as a "block" of heating say +5c and you have your original pre-heat set at say 15c then run your heat pump at 10c air on and let the original pre-heat act as a trimmer.(works better if pre-heat is electric. This will work as long as the heat pump output is reasonabley constant i.e. the exhaust air is fairly stable temp/ volume wise. The heat pump must first in line so don't forget filters and the air velocity is all important.

Simplicity is best.

What happens if the air on is -5? will the system overcondense etc? Heat pumps are designed to work inside so they expect an air on around 20 deg'c If that drops too much then potentially it can cause more problems than it solves..

[TRASH101]

Yep my mistake

pre-heat first, then heat pump but still working off, in essence, outside temp.

sorry for confusion my heads a shed at the moment.

[nike123]

Just as material for thinking in controlling aspect in mind.
What if you don't use direct expansion heat pump, but instead, use water to water heat pump, air/water coils and some buffer tanks.
That way you don't need to control air flow. Just control flow rate of water.

[K.R.Iyer]

We got a call from an old customer who has a large supply/extract to their uni. They have asked about heat recovery as it's a 100% loss system and it shifts a lot of air.As they are at opersite ends of the building, modifying ductwork is out.

In the Uk it's estimated 75% of the year the outdoor temp is 15 deg C or less, so this system needs a lot of preheat.

I pondered if we could fit a heat pump with the evap in the exhaust flow and the condenser in the supply.

Would it work or am I missing something ?

Multisync,

I have seen Energy recovery wheels working in comfort applications - with limited fresh air intake. Of course not yet seen in 100% fresh air applications. It sounds logical that it should work.

http://www.drirotors.com/erv.htm. They claim to recover 65 - 75% of waste heat.

Do you think it could work? Of course you need to re-do the ducting. It will be worth the energy saving.

What is the CFM in this case?

[Tesla]

Multisync
The control is very simple - control from supply air off coil temp. The heat pump direct refrigerant heat transfer is the most efficiant. If the unit chosen will have overcondensing issues a dual supply duct can be utilised with dampers to control air flo over condenser. There may be heatpumps out there designed for lo condensing temps, It seems strange but I have seen drink vending machines opperate at below 5 degC.