CFM rating of outdoor unit

[radwin]

What is the cfm rating of an outdoor unit of a 5TR split type aircon unit? If my outdoor unit is inside of a machine room is it necessary to match cfm of outdoor unit with an exhaust fan with a same capacity provided it is the only unit inside the machine room?

 

[willard3]

You can certainly find this out from the manufacturer.
I can't imagine that a condenser is inside, but, if it is, the condenser must be cooled by outside air to work at its rating.

 

[radwin]

yes the design of the machine room has a fresh air intake and the exhaust fan will take the hot air. my fear is if i can not match the the cfm of the outdoor unit with the exhaust fan there will be a draft. is it very significant?

 

[MintJulep]

It's not only necessary to match the cfm, you need to match the design dry bulb air entering temperature (probably about 35C) or preferably lower.

If you don't you won't get the rated capacity.

There is a reason it's referred to as the "outdoor unit", it really should be outdoors.

 

[willard3]

Why do you fear drafts in a machine room? Is it occupied by people? Chillers and boilers won't complain about drafts.

 

[radwin]

the room is only occupied by outdoor units, about four units. my fear is that if the total cfm of the outdoor units did not match with the exhaust fan of the room there will be a draft in the fans of either of the exhaust or the outdoor units.

 

[radwin]

guys what i mean by draft is pressure, I fear that there will be pressure in the exhaust fan if the capacity of exhaust fan is lower than the cfm of outdoor units or there will be pressure in the fans of the outdoor units if the fan of the exhaust is greater in capacity than the outdoor units. we do not have a way to have a ducted exhaust here because of the limitation of the space in the machine room.

 

[MintJulep]

Ah. Ok, thought experiment time.

Imagine a very large space, closed from the outside. Large indoor arena for example.

If there is no air movement at all then the pressure is equal everywhere (we'll ignore pressure difference due to elevation). Measure the pressure difference between inside and outside. Since there is no driving force the pressure difference will be zero.

Now, place a small fan in the center of the floor and turn it on. At the suction side of the fan the local pressure will decrease. At the discharge side of the fan the local pressure will increase. Since we now have a pressure gradient there will be flow established.

Since it's a small fan in a big space if we get far enough away from the fan there will be no flow.

The pressure difference to the outside will still be zero.

Now, let's add an exhaust fan to the building, and a corresponding intake grill. The fan will cause a negative pressure inside the building and this will cause outside air to flow into the building through the grill. Local to the exhaust fan will be a region of low pressure, and local to the grill will be a region of higher pressure. This pressure difference will drive flow within the building. The pressure inside the building is no longer uniform, but there will be an average value.

Turning the small fan on or off won't make any difference in the average internal pressure value.

Since the fan and the grill are the only openings the flow through them must be equal. If the flow is known you can look up the local pressure in the fan or the grill's flow vs. pressure curve.

Now, imagine that the small fan in the center of the floor gets larger and larger, and it is moved close to the intake grill such that the low pressure region impinges on the grill. Now the pressure across the grill increases, and it's flow will increase.

Since flow in must equal flow out the flow through the exhaust fan will also increase, and the local pressure at the exhaust fan inlet will decrease.

So, your condenser fans inside your room can and might affect the performance of any intake or exhaust fan if the room is small enough.

 

[Drazen]

In this case your both intake and exhaust fans present "narrow throats", so as outdoor unit fans are mostly axial fans with low pressure rating, it is likely that your outdoor unit fan will simply find balance on lower cfm, which means you will have no draft but reduced rating of outdoor units.

 

[radwin]

@ Mint Julep, it means that the it is better to design the exhaust capacity cfm bigger than the total cfm of the outdoor units and place the outdoor units a distance from the inlet grill to have a steady flow since you said that it has no effect in the fan of the outdoor units and it has more effect in the fan of the exhaust.

 

[MintJulep]

Radwin,

Given the poor starting point of having condensing units inside I think yes, having a really big exhaust fan is a good idea.