Steps for Analyzing Cooling Requirement for Server Room 7

[reslife]

All,

I’m a recent grad in ME, so my experience with real world problems is limited. I have been tasked with analyzing the HVAC of a server room with an area of about 500 sq. ft. and a raised floor. I don’t have any experience with HVAC analysis, but here is my approach:


1. Determine through manufacturing documents the power load in watts of all the equipment: Computers, servers, uninterruptible power supplies (UPS) and anything else using power in the room.

2. Convert each power to Btu/hr if the heat dissipation is not specified in the manufacturing documents.

3. Convert this total Btu/hr into tons of cooling and then look in some catalogs for systems that can handle my cooling ton requirement.


I assume that the power load from the equipment in the room will be the biggest contributor to the heat, and thus the cooling requirement, so I am focusing on that. Please tell me if this is the standard approach for HVAC analysis. Here are some questions that I still have about my approach:


-I feel like I’m not considering scalability in this approach because I’m only considering what equipment is currently going in the room, and not what might be put in there in the future. Should I consider the situation of having the maximum amount of server racks filled with the most power drawing equipment and calculate the cooling required from this?

-The room is lit with fluorescent bulbs. Since this is a smaller room, can I assume that the heat gain from the light is negligible?

-I was told that there wont be any people in this room, so is it safe to assume heat gain from occupancy is negligible, or is there a standard that I should assume a certain amount of people might be present?

-The room is located in the middle of a larger building with other offices/people. Do I have to factor in heat transfer through the walls/ceiling/floor? (Because the server room doesn’t have any walls that are exposed to the outside weather, I was thinking no).

-There are no windows in this room, but I was wondering how important figuring out the heat transfer through the entry door is.

-If I just add up all the power loads and convert this to tons of cooling, I’m not really considering temperatures even though I have a feeling I should. Am I missing something here?

-I feel like I’m forgetting about the “V” in HVAC. How do I determine whether I need to spec for ventilation or not?


I feel like I might be overanalyzing this problem. I know this is a long post full of many questions, but I do appreciate any help or insight any of you can give a young engineer. Thanks!

 

[hepa99]

1. some manufacturer have heat dissipation (BTUH) available, request for that information.

2. if you dont get heat load requirement of Server, etc...convert watts to BTUH with multiplying factor (not 100%).

3. no. you need to consider room condition (temp, humidity, ventilation requirment) and what type of HVAC system you will design before you select HVAC equipment.

- ask the owner if they will put more server in the future. if they will not give you information, just provide some allowances on your load computation to accomodate future additional server.

- ask the owner if they will turn ON the light 24/7.....if they dont know the answer, consider the lighting load.

- if they told you that theres no people in the room, you dont need to consider the occupancy load..

- you need to know what's the required temperature/humidity to maintain the server room. if there's a temp differnce between in/out of server room, YES you need to consider the heat transfer on your cooling load

- you need to know the specs of the door and do the cooling load calculation.

- yes you need to consider the room ventilation, some server have exhaust requirement so you need to know if there's exhaust requirement. You need to know the humidity requirement of the equipment (server, etc.) or ask the manufacturer what is the required humidity you need to maintain inside the server room.

 

[initialdonald]

reslife,

If you intend to consider for future capacity of the server room, please consider how the HVAC system will operate when future capacity has not been met. Basically you would like to avoid short cycling of any of the equipment.

When selecting the equipment find out what type of staging options are available. For example multi-staged compressors or hot gas bypass. Also the use of multiple AC units to handle the sum of the future load is always a good but expensive option.

I am not sure if you are responsible for the ductwork or not, but if you are think about where the intake and exhaust sides of the server rack are sometimes referred to as the hot aisle/cold aisle.

One more thing when designing for a server room. Consider the overall HVAC system's redundancy. If you were to install one AC unit for that server room and that system decides to fail at some point, well guess what your server room is offline which can mean that your company will be offline too. Time is money.

Hope this helps.

 

[imok2]

Server rooms make significant demands on a building's cooling capacity and usually have their own independent cooling systems as a result. When you are shopping around for server room air conditioning quotes, you need a ballpark estimate of your cooling needs.

The calculations below are a rough guide only. They are adequate for price comparison but before getting a cooling system installed, installer should evaluate the room to give you an exact figure.

Step 1: Standard Cooling Needs

Server room air conditioning is measured in British Thermal Units (BTUs). Calculation is merely a matter of evaluating how much heat is generated in each of several categories, adding up the numbers, and matching it with at least that much cooling.

Room BTU: Any room requires cooling based on its dimensions to reflect normal heat coming in from other areas. Multiply the area of the room in square feet by 31 BTUs.

Equipment BTU: Add up the wattage of all equipment in the room that will be on all the time, including servers and routers. Multiply this by 3.5 to get the BTUs needed. To be fair, the wattage rating of equipment is the maximum wattage and the equipment generally draws less power than this, but it is better to overestimate than underestimate.

Lights BTU: Lights are less efficient users of energy than equipment and put out more heat. Add up the wattage of all lights in the room, but multiply by 4.25 to get the BTU requirements

 

[AbbyNormal]

Step 3, they always add more computer equipment

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Clymber]

You may also want to visit

http://www.apc.com

and browse the section entitled, "White Papers for Cooling Solutions". They have lots of good information.

HTH,
Brad

 

[AbbyNormal]

Take into consideration that some IT guys like the rooms on the cold side and the cooling capacity of 'nominally sized' equipment drops right off.

I usually have a high airflow as the load is mostly sensible

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[reslife]

AbbyNormal,

You state that you usually have a high airflow because the load is mostly sensible. How do I calculate what my airflow should be?

 

[AbbyNormal]

sensible cooling load, divided by the specific heat of the flowing air, divided by the temperature differential between the air coming out of the diffusers and the temperature you want to maintain the room at.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[hepa99]

abby, you forgot to divide sensible cooling load by density of flowing air (p).

V =Qs/(p x Cp x AT)

 

[AbbyNormal]

volume flow, mass flow, I figured a graduate engineer could take it from there

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[reslife]

Yes, I picked up on the density part =) Thanks!

 

[hepa99]

abby, even senior engineer will question your post....unless they're rule of thumb fanatic.

 

[AbbyNormal]

well what can I say hepa, you are just too sharp for me

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[KiwiMace]

The layout of the HVAC units and racks in the room will contribute about as much to the actual load as the servers. The equipment is finiky about how it gets it's air. Look up the ASHRAE TC9.9 series of books. They produce a handbook which is very useful.

I work full time in critical facilities and 90% of the operational improvements I see are preventing bypass air from flowing straight past the servers and back to the return side.

 

[chicopee]

Scanning throughout the various replies, I have not seen anything about air infiltration thru cracks and under the door; also you shoud define what temperature and relative humidity conditions that you want to maintain in the server room.
If adjacent rooms,hallways, upper and lower floors have nearly the same temp. as the server room discount them in H.T. thru walls and floors.
Also check the plenum of the server room to be sure that it is sealed off from the rest of the floor otherwise a heat source may show up.
An enthalpy balance of the air and moisture under steady state conditions is fairly easy in order to define your CFM and BTU/hr (12,000 btu/hr=1 ton) cooling requirement.

 

[AbbyNormal]

I would have the air exfiltrating out of the room, only latent load would be the outside air brought in for positive pressure.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[reslife]

Thank you all for your replies.

I have found the heat load in the room and asked the customer the temperature he wishes the room to be, which is 72 degrees F. I'm now contacting the manufacturer of the unit to determine what temperature the air coming into the room will be, so that I can calculate a temperature differential, which will help me calculate a flow rate.

One more question: When I determine my flow rate, is this something a unit is specced for, or can the flow rate of a system be changed by the user?

 

[hepa99]

you don't need to ask the manufacture, plot it on psycrometric chart and you decide.

 

[imok2]

The final flow rates are determined by you in the air balance requirements. This is as simple as diffuser adjustments if your total heat load calculations are correct.