Low head pressure Versus Heat reclaim.

[taz24]

I know that in the past a lot of the larger UK based supermarkets used to reclaim heat from the condersers in the form of hot water and heated air spaces.
Because of various different reasons, such as the recomondation for smaller more localised plants and lower energy consumption, the stores have now moved to lower head pressures with no heat reclaim.

Could anyone advise me on how to work out the comparison for the supposed savings in power consumption and lower running costs of low head pressures over heat reclaim.

Thanks taz.

[US Iceman]

The way I would approach this is to make two comparisons.

1) Energy used by the compressors and condenser fans at the low head pressures. The tricky part is finding out how low you can go. Sort of like a limbo contest.

2) The energy used by the compressors and condenser fans at the normal head pressures setup for the heat reclaim.

If the energy difference between the high and low head pressure operation is greater than the energy recovered by the heat reclaim, this should tell you what you need to know.

[taz24]

1) Energy used by the compressors and condenser fans at the low head pressures. The tricky part is finding out how low you can go. Sort of like a limbo contest.

Set fire to the bar. That will sort them out

If the energy difference between the high and low head pressure operation is greater than the energy recovered by the heat reclaim, this should tell you what you need to know.

Cheers Mr Iceman.
How would you do it for a hypothetical situation or
if designing from new.

Cheers taz.

[Peter_1]

The way I would approach this is to make two comparisons.

1) Energy used by the compressors and condenser fans at the low head pressures. The tricky part is finding out how low you can go. Sort of like a limbo contest.

2) The energy used by the compressors and condenser fans at the normal head pressures setup for the heat reclaim.

If the energy difference between the high and low head pressure operation is greater than the energy recovered by the heat reclaim, this should tell you what you need to know.

US Iceman, that's exactly how you must approach this but just like Taz said, it's not so easy to do calculate this.

I tried once to poor this in an Excel file but didin't succeded. You have to do this with the matehematical function 'iteration'

But,...one of our Chinese members made a program for this i think...FlyCarpet if I remeber it well.
And may I ask to those who know well Coolpack, isn't the last option, the last Tab 'Dynamic : cooling of a room' not this an example of this? I never used this one.

[NoNickName]

The real contest is not how to compare the savings, but assessing the real usage of reclaimed heat, which is quite low in most supermarkets, or at least much lower than the available heat to be used.

[taz24]

The real contest is not how to compare the savings, but assessing the real usage of reclaimed heat, which is quite low in most supermarkets, or at least much lower than the available heat to be used.

Thats where I'm comming from.
This is not a real application just a hypothetical one and I want to prove or disprove that lowering the head pressure is more cost effective than installing and using heat reclaim .
But I would like some figures so I can demonstrate my case.

Cheers taz.

[NoNickName]

Thats where I'm comming from.
This is not a real application just a hypothetical one and I want to prove or disprove that lowering the head pressure is more cost effective than installing and using heat reclaim .
But I would like some figures so I can demonstrate my case.

Cheers taz.

Roger that. Now take a 365 days/year, 24 hours/day, 30 ambient temperature sample/hour datasheet.
Evaluate a thermal exchange coefficient, as an average of all the walls and roof of the supermarket. You will come up with something like 0.1 W m^2 / K.
Compare this against desired room temperature, ambient temperature and supermarket surface in the datasheet and calculate the actual instant heat need. Detract the heat load of lightings, display condensers and "cow stable" effect of breathing people.

That's it.

[taz24]

Roger that. Now take a 365 days/year, 24 hours/day, 30 ambient temperature sample/hour datasheet.

That's it.

Great thanks .
Thats the sort of thing.
Many thanks taz.

[US Iceman]

I was hopeing you were going to tell me to
add 10 divided by the amount of compressors and take away ambient temp or some such equation.

Well I suppose you could use this and add 10 to your result just to be safe.

Depending on the heat reclaim function (heating water or heating air) you may be able to estimate the flow and take the temperature differential to develop the amount of heat recovered. This would be your useful heat.

Then if you take the compressor performance at that operating condition kW input power, this is point one.

The second point is the compressor kW input power at the lower condensing temperature.

Point one - point two = kW difference

If this kW is larger than the heat recovery energy (again in kW), then lower head pressure would be more cost effective.

That's at least a quick method to see if this helps you before you spend a lot of time on the subject.

[Peter_1]

Thats where I'm comming from.
This is not a real application just a hypothetical one and I want to prove or disprove that lowering the head pressure is more cost effective than installing and using heat reclaim .
But I would like some figures so I can demonstrate my case.

Cheers taz.

Well, lowering the HP is indeed the biggest saving you allways can make. The disadvantage of this is indeed like you said that you lower the temperature of the warm water you can make.

But there's still plenty of heat available to preheat the water and send it then to an excisting post-heater.

Evaporating at -10°C (14°F) and condensing at as low as 25 °C (77°F)gives still discharge temperatures of 46°C (115°F) for R404a.

I think I told this already once on RE: we have a heat reclaim which is so big that the fans on then condensor became stuck because they never turned. We installed a timer in the PLC so that it runs several times a day for some seconds, just to make sure that the bearings don't stuck.

[US Iceman]

Hi Peter.

US Iceman, that's exactly how you must Approach this but just like Taz said, it's not so easy to do calculate this.

The way I looked at this is more complicated than what I suggested to taz. I generated the weather data on a per hour basis for the specific location. With this weather data I have the DB temperature for 8760 hours. Then I had to find the percentage of cooling load required in the stores (for the display cases) for each of the 8760 hours.

Knowing the DB temperature and the percentage of load required you can then estimate the compressor capacity required and the average condensing temperature.

From that, you get the kW power on an hourly basis for the whole year.

It's a little more complicated than the above, but this is the general idea.

This has been an interesting experience and hope to refine the process more on the next several jobs.

[SteinarN]

I got a bit puzzled over why you do it so complicated.

First one asumtion: The power to heat whats need to be heated costs the same as the electricity to run the compressors. This is obviously true when we use direct electricity to heat whats need to be heated.

When I, in the following, talks about 100% heat reclaim, I dont have in mind 100% of the time, but instead 100% of the available heat (desuperheating and condensing heat) when actually running in heat reclaim, even if it's only one hour pr year.

If the heat reclaim is 100% it is alvays cheaper to reclaim that heat than to dump it outside and use other energy to heating. This is regardless of any increase in condensing temperature when running in heat reclaim. When we reclaim heat 100% we get the evaporator power in addition to the power consumtion of the compressor. Even if the compressor consumed zero power without heat reclaim and a huge amount of power with heat reclaim, we would still get that huge amount of power reclaimed in addition to the free evaporator power.
IMO we will always get more power reclaimed than is the increase in compressor power consumtion caused by this very heat reclaim when we reclaim 100%.

If we, on the other hand, dont reclaim 100% then we can get a loss. Lets say we install a PHE desuperheater to reclaime some heat for heating water. If we increase the condensing temperature by lowering the condenser fan speed we obviously get higher discharge temperature and can reclaim more heat and thus heat more water to a higher temperature. In this case we may get a loss from the compressor consuming more extra power due to higher condensing temperatures than we get back in increased heat reclaim in the desuperheater.

The conclusion is that if we make a heat reclaim system dependent on considerably increased condensing temperatures, then we shall make that a 100% heat reclaim system. The less the heat reclaim factor is, the less is the allowable increase in the condensing temperature before the increase in compressor power consumtion is larger than the increase in heat reclaim power.

It is however true that we have to make "potentially low condensing temperatures at how many hours a year" calculations to determine the precise increase in compressor power consumtion with heat reclaim. Assuming not extreme increasing condensing temperature, these calculations will only to a lesser degree influence the total system efficiency meaning the increase in compressor power consumtion will be only a small part of the heat reclaimed in a 100% heat reclaim system.

Edit: I haven't taken into consideration there should be any problem regulating the condensing pressure acording to whether the heat reclaim is actually active or not. In a 100% heat reclaim system it is not any refrigerant circulation in the normal condenser in heat reclaim operation and the normal condensing temperature control is thus disabled and the heat reclaim condenser pressure control takes over.

[US Iceman]

How would you do it for a hypothetical situation or
if designing from new.

The same way I mentioned before. You should be aware that the comment NoNickName made is very valid. Heat recovered and heat used are completely different topics.

One of the current problems with low head pressure operation is the ability to get accurate data. Most of the compressor performance programs I have seen have a minimum fixed value of 21.1°C (70°F) condensing temperature for all refrigerants. That is not very low.

You need to find this data from the compressor manufacturer and their lowest possible limits. Some of this is available, but be warned, they may not release it. There is a large potential for things to go very wrong if particular tasks are not accounted for in low condensing operation. The TXV's are only one area. I had to spend a lot of time finding this information and then getting permission to use it.

Also, you need to estimate the actual load (capacity)requirements when the lower head pressure operation is possible. As the dry bulb temperature decreases, of course you can lower the head pressure. When this occurs there is also a possibility the relative humidity in the store may also be decreasing.

This lowers the required cooling capacity on the display cases, since the latent load of the potential frost forming is also reduced.

You will see this during various times of the year. In the summer all of the compressors are running. In the winter, only a few are, and intermittently at that.

The best source of information is to find the original design information on the display cases (their actual air temperature requirements & evaporating temperatures), and the compressor performance data as designed.

One of the stores I am working on right now is running their suction pressures 0.5 bar and 1 bar (two compressor racks) lower than the original design. That makes a big difference in compressor capacity and energy use too.

As you might be guessing right now, this is a slightly complicated set of tasks.

[taz24]

As you might be guessing right now, this is a slightly complicated set of tasks.

Bugger .
I was hopeing you were going to tell me to
add 10 divided by the amount of compressors and take away ambient temp or some such equation.

Oh hum back to the books then.

Cheers anyway taz.

[Peter_1]

The real contest is not how to compare the savings, but assessing the real usage of reclaimed heat, which is quite low in most supermarkets, or at least much lower than the available heat to be used.

We have a supermarket here with a high heat reclaim factor. Every supermarket can use a lot of heat but you must also be a little inventive to use the high cooling factor of water.

Theys use it in our application to make hot cleaning water, to reheat the cold corridors between the isles and wall coolers (seems that these cold corridors makes that the customers don't stay long enough in this zone and don't buy enough), they use it for heating the offices,

What I meant with inventive: we had once an application where the basement where the compressors were installed became too hot.
The first floor was a big fish selling shop and above were +/- 30 appartments for students.

So we installed a PHE on the pack and a boiler for the production of warm cleaning water.

But in summer, even with this heat reclaim, the basement became too hot.
So we had made a connection on the warm water with a thermosttaic bypass valve which made water of 35°.

We had connected this to the circuit to the flushing water of the toilets in the building. So each time some used the toilet, some fresh water was added to the boiler.

Then a second PHE was installed with a circulating pump, connected to the panel heaters in the 30 appartments.

In summer, when students are on holiday, the panel heaters were turned full open and we send the waste energy via the 2nd plate to this panel heaters.

This was installed +/- 15 years ago and it's still working.