Conditioner power estimation

My understanding is different electrical equipment will have different heat dissipate, some equipment's catalogue will provide this info. CIBSE guide books provide these heat info on general area. Based on this heat dissipate info, the HVAC side can calculate the cooling load and select the AC unit.

1kw of electrical power expended in the room is about 3400 BTU. 12kbtu = 1 ton of AC. Then add 500-600 btu for each person in the room plus the nominal building load (something around a ton per 300-400 sq/ft of office space) You really need a "manual J" calc for the building load. A lot will bepend on ambient outside air temp ans sun load for that.

That is the way we calculated HVAC in computer rooms

Yes please tell me how to keep into account the fact that we want to keep the room at least 8 celsius (46 fahrenheit) below the temperature of the outside air.

Thank you

more precisely: max 32 Celsius when outside is 40 Celsius

Do a search on "manual J". I believe there are a few sites with this calculation in an excel spread sheet. This is how you size HAVC systems for the weather, sun exposure etc. It willl ask you about the R factor of walls and ceiling, window openings and direction, glass type etc. Usually using 300 sq/ft per ton will work in all but the hottest climates and worst performing buildings as long as you also factor in all the internal heat loads.

BTW a delta of 8c is only 14f (24c = 75.2f, 32c = 89.6f) you don't add the 32 unless you are speaking of an absolute temperature. The 46f threw me for a second.

On Thu, 27 Mar 2008 09:47:44 +0100 kunt wrote: | snipped-for-privacy@aol.com wrote: |> |> 1kw of electrical ...... |> ...A lot will bepend on ambient outside air temp ans sun load for that. | | Yes please tell me how to keep into account the fact that we want to | keep the room at least 8 celsius (46 fahrenheit) below the temperature | of the outside air.

When calculating _differences_ in temperature, conversion between celsius and fahrenheit is slightly different. For every 5 degrees of difference in celsius, there is 9 degrees of difference in fahrenheit. So if you want the temperature to be 8 C less than outside, it will be 14.4 F less than outside (not 46).

You have to know the rate that heat will flow into the room from outside. This depends on so many factors that you pretty much just have to test it and see. It's hard to test this with everything running, which you most likely must do. One reason is the test varies depending on what the outside temperature is.

Do the existing A/C systems run continuously or do they cycle on and off in a normal manner? Is the temperature stable now, or does it go up and down rather extreme? Is it just too hot in there and the A/C you have now just can't cool it any better?

OF COURSE! Stupid me! I used the google calc conversion, I didn't pay attention to the fact that the C/F conversion it is not like all other conversions because of the zero problem

Is this the only problem with the 8 degrees difference?

I thought that the biggest problem was the conditioning system working less efficiently if they have to draw heat from a room which is already cold to bring it to a place which is already hot. Is this factor irrelevant?

It seems relevant to me: I have tried to put plastic sheets so to drive hot air from the servers to the input of the conditioners, preventing it to mix to the cold air output from the conditioners. This gave hotter air to the conditioners (might have been about 10 celsius hotter, I am not sure). They appeared to work much more efficiently, like 50% more efficiently, but I have not tested this really well to be so sure.

If everything was not running, how would you test this? Like bringing the temperature down then looking at the speed at which the temperature increases?

It seems like the current conditioners cannot keep the temperature stable to the temperature which has been preset to them. But I haven't really tested well.

Yes they are working full time, no cycle. It might be possible for us to stop all equipment for a while.

Thanks a lot

Great! Really helpful, thanks a lot!

OF COURSE, stupid me! I used google calc badly. Also see my reply to the other post...

| I thought that the biggest problem was the conditioning system working | less efficiently if they have to draw heat from a room which is already | cold to bring it to a place which is already hot. | Is this factor irrelevant?

It's what A/C systems are designed to do. They "compress" heat so it is actually coming out of the precipitator at a higher temperature than it entered the evaporator. Stick your hand in the air flow out from the A/C outside and you will see it is hot.

| It seems relevant to me: | I have tried to put plastic sheets so to drive hot air from the servers | to the input of the conditioners, preventing it to mix to the cold air | output from the conditioners. This gave hotter air to the conditioners | (might have been about 10 celsius hotter, I am not sure). They appeared | to work much more efficiently, like 50% more efficiently, but I have not | tested this really well to be so sure.

That would help if you don't mind the rest of the room operating at a higher temperature.

|> This depends on so many factors that you pretty much just have to test it |> and see. It's hard to test this with everything running, which you most |> likely must do. One reason is the test varies depending on what the |> outside temperature is. | | If everything was not running, how would you test this? | Like bringing the temperature down then looking at the speed at which | the temperature increases?

The timing of temperature rise under varying external conditions would give some clue as to the rate the heat comes into the building from outside. If you can do it with all computers off, that's more accurate. But I guess you don't have that option.

|> Do the existing A/C systems run continuously or do they cycle on and off |> in a normal manner? Is the temperature stable now, or does it go up and |> down rather extreme? Is it just too hot in there and the A/C you have now |> just can't cool it any better? | | It seems like the current conditioners cannot keep the temperature | stable to the temperature which has been preset to them. But I haven't | really tested well. | | Yes they are working full time, no cycle.

You should also look at whether they have the correct blower sizing, and that they are operating with all other conditions in good shape. Make sure the precipitator baffles are clear. Make sure the freon is in good state. If you have air in the freon line, that can reduce efficiency. If you hear the compressor working really hard right when it kicks on, there may be air in there (which cannot be compressed to a liquid).

| It might be possible for us to stop all equipment for a while.

Good thermal testing might take a couple days. I doubt you can do this with things turned off.

Someone with experience in A/C systems and building insulation might be able to give you a good estimate of your building efficiency, and adjust your new A/C sizing appropriately. But my guess is you will need 50% to

100% more than what you have now.

I would also figure in redundancy at this time, too. In one place I used to work, we made a conservative heating estimate and figured 2 units to cool it, but installed 3 units anyway for backup purpose so we could take one unit out of service at any time. I've heard of some places going to

250% to 300% for high redundancy (they were uptime critical).

I understand that it's what they are supposed to do, and approximately how they work, but are you sure that the efficiency of the AC is not influenced by the temperature difference between the room and the outside?

I kinda understand the steps involved in determining the heat flow through the walls due to the temp difference, what I don't know how to keep into account is the decrease in efficiency in the AC system if the room where to draw heat from is 8 celsius below the outside temp

Thanks for your help

Yes, it is. Amperage draw of the compressor increases with head pressure, which increases with the temperature of the condenser.

While calculations can be done to put you in the ballpark, the final choice of capacity is an educated guess. There's too many variables involved and any unit will only be ideal under one set of conditions.

8 c is not that big a delta inside to outside unless you are using full fresh air makup. That is still mostly affected by the building insulation. The high side of the freon loop will still be significantly hotter than the ambient.

On Sat, 29 Mar 2008 00:29:26 +0100 kunt wrote: | snipped-for-privacy@ipal.net wrote: |> On Fri, 28 Mar 2008 15:16:43 +0100 kunt wrote: |> |> | I thought that the biggest problem was the conditioning system working |> | less efficiently if they have to draw heat from a room which is already |> | cold to bring it to a place which is already hot. |> | Is this factor irrelevant? |> |> It's what A/C systems are designed to do. They "compress" heat so it is |> actually coming out of the precipitator at a higher temperature than it |> entered the evaporator. Stick your hand in the air flow out from the A/C |> outside and you will see it is hot. | | I understand that it's what they are supposed to do, and approximately | how they work, but are you sure that the efficiency of the AC is not | influenced by the temperature difference between the room and the outside?

That depends on what kind of efficiency you mean. Sure, if the outside is hotter on one day than another, the A/C has to work harder on that hot day. This is due both to the greater heat flow coming in, as well as the lesser temperature difference at the precipitator. But this is a working condition and not a part of the equipment efficiency. But maybe what you are wanting is a working efficiency level.

| I kinda understand the steps involved in determining the heat flow | through the walls due to the temp difference, what I don't know how to | keep into account is the decrease in efficiency in the AC system if the | room where to draw heat from is 8 celsius below the outside temp

If the inside room temperature equals the outside, there is no heat to move. Heat the room up and it flows outside. The flow will be in proportion to the temperature difference. The details of the building insulation itself would give you some absolute figures (if you want to spend all the time to model every aspects of the building construction). Normally one would want a computer room to be at some specific temperature, such as 18C. Then you have to consider the extra heat coming in, and opposing the precipitator, at the highest outside temperature you need to handle. The heat released by the computers usually well exceeds the heat ingress through the building in a typical computer room installation. You need enough A/C to keep the room cold if it didn't have computers, plus enough A/C to remove the heat released by the computers, plus a level of redundancy to handle taking a unit or two offline for maintenance or failure.

What is your highest outside temperature? Are you in Troms? Norway?

On Fri, 28 Mar 2008 19:52:15 -0500 snipped-for-privacy@aol.com wrote: | On Sat, 29 Mar 2008 00:29:26 +0100, kunt wrote: | |>I kinda understand the steps involved in determining the heat flow |>through the walls due to the temp difference, what I don't know how to |>keep into account is the decrease in efficiency in the AC system if the |>room where to draw heat from is 8 celsius below the outside temp | | 8 c is not that big a delta inside to outside unless you are using | full fresh air makup. That is still mostly affected by the building | insulation. The high side of the freon loop will still be | significantly hotter than the ambient.

Based on his email address, he appears to be located in Norway. Just how far north is the next question. A/C experts in Lappland might be hard to find.

No I am sorry, my email is a bogus fake address, I hate being traced so I morph frequently.

Latitude is that of Lyon, France. Temperature max reaches 40 Celsius in summer, we want to keep it below 32 C.

Thanks everybody

On Sat, 29 Mar 2008 19:01:07 +0100 kunt wrote: | snipped-for-privacy@ipal.net wrote: |> |> What is your highest outside temperature? Are you in Troms? Norway? | | No I am sorry, my email is a bogus fake address, I hate being traced so | I morph frequently. | | Latitude is that of Lyon, France. Temperature max reaches 40 Celsius in | summer, we want to keep it below 32 C.

So I wonder who is getting a bunch of questions in their mailbox.

If you want to morph you own email to avoid spammers, you can do that. But one should not just use a valid address that could be used by someone else. Either use a non-existant top level domain or "example.com".