HVAC question

[KZ001]

Hi,

I am designing a HVAC system for an equipment room that houses a large electrical apparatus. The load in the room can vary from 10,000Btu/h to 85000Btu/h. The room area is 1345ft^2. Should I use CAV or VAV box?

 

[Drazen]

whar design temperature is allowed? should you not use heat rejection?

 

[KZ001]

The design temperatures are 72F and 76F for heating and cooling respectively respectively. Not familiar with heat rejection system. There is a cooling system in the equipment rooms that utilizes a water as cooling fluid.

 

[Drazen]

heat rejection is a concept, meaning you replace indoor air with enough outdoor air to reject heat, and save lot of energy that would be consumed by cooling.

can you be more specific about what equipment you have? ordinary electrical equipment can sustain much larger temperature range, while server rooms are something completely different.

it is rarely the case that common all-space cooling will be acceptable for electrical rooms - you either need heat rejection by ventilation or close control system, or at least separate cooling system as it is quite possible that cooling might be needed when other spaces are in heating mode.

 

[ChasBean1]

VAV (not CAV) for any electrical or IT equipment space is a must if you have a system that can do it. You require variable cooling and little (if any) ventilation. You probably want about 4,000 cfm of 55-60°F cooling air given your load.

Have the min VAV setting as low as that VAV device can take. If it can do a 10:1 turn-down, ~400 cfm minimum. You can use 0 as a minimum, but understand that there are control limitations (meaning large fluctuations) with large VAV devices controlling in low ranges...

 

[SAK9]

As Drazen pointed out,you need to ascertain what is the highest temperature the equipment can withstand.For eg if it can function at 45C, mechanical ventilation may be adequate.10000 to 85000 is a large range so multiple units with some variable capacity can be considered.Also examine whether the load variation is happening at a single location or spread across few locations.

 

[ChasBean1]

The question was whether this is a VAV or CAV application. VAV or CAV has the same cooling capability.

 

[cry22]

Chas

VAV does not make sense in such an application. Variable load or not. The load is just too small to warrant the cost of VSD's, VAV boxes and Controls associated with it.

The space is 1400 SF housing large electrical equipment (one would understand swith gear or transformer). Using two small AC units CV (split system or CHW) along with an exhaust fan for winter air-side economizer will do just fine.

 

[KZ001]

Thank you everyone for your help! I am sorry I should’ve clarified that I am using WSHP to condition the room. The WSHP is connected to a closed loop cooling system. The cooling system is designed to carry heat away from the electrical equipment. That energy can be utilized to heat the space during winter. Also, I said that the load vary from 85000btu/h to 10000btu/h, however the load will drop from 85mbh to 10mbh and rise from 10mbh to 85mbh. It expected drastic load change. How would CAV perform in that case?

 

[cdxx139]

Why use conditioned air for electrical room?

Is outdoor air out of the question? Fan sized for 9000 CFM w 9 deg delta. Turn fan on, when T above 90 degF, turn off when below 80 degF.

If not, is the WSHP recirculating and sized for 85 MBH? Then your all set.

Are you getting ari from another air ahndling unit? And that is why you ask for CAV vs VAV? If the rest oif the system is VAV, keep VAV. But it seems like a waste of energy for an electrical room.

knowledge is power

 

[KZ001]

Outdoor air is not acceptable, there is a coal plant close to the building.

 

[317069]

You say: "I am using WSHP to condition the room. The WSHP is connected to a closed loop cooling system. The cooling system is designed to carry heat away from the electrical equipment"
can you explain about the cooling system you mentioned above in details.
- does this cooling system serves only the electrical room or entire building,
- how come you can not use outside air because of a coal plant close to the room, there should be a clearances between buildings, what about people who works in the coal plant.
- are you going to use WSHP or it is already there.
- is there a people in the electrical room or just machines
- is it homework

 

[ChasBean1]

A CAV box would be an energy pig. Look, a CAV box and a VAV box are the exact same device. A CAV box you program to deliver (in this case) 4,000 cfm during full cooling and 4,000 cfm during full heating. A VAV box (what you need here) you program to deliver 4,000 cfm during full cooling and maybe 400 cfm during full heating. I’m not sure if I missed something but your application is very simple.