What is the meaning of lbm/min/KW

[snydergj]

I have a question? I'm working on a proposal for an aircraft product and the manufacturer says they can supply a air flow rate of XX Lbm/min/KW. Why is there a KW included with this number. I thought that it was just lbm/min. They are only supplying the air.
Thanks.

Gabe

 

[MintJulep]

Without more information on the context, can't say.

Possible guesses:

Air flow is variable, as a function of some load measured in KW.

Power consumtion of the device that provides the air varies with the flow.

 

[snydergj]

They have supplied a graph that shows flow rate vs temperature of the air at different altitudes. Flow rate and Temperature increase as the altidude decreases. They have the flow rate units as(LBM/MIN/KW). My understanding is that I would tell them that I need as certain flow rate per KW to cool my enclosure and that is where the LBM/MIN/KW makes sense. The aircraft has a cooling system the pumps air to electronic racks and it has no relationship to the watts my unit produces.

 

[MintJulep]

Is the temperature an absolute, or a delta?

 

[snydergj]

The graph shows an area. At any given temperature there is a min and max flow rate. So basically the graph looks like a trapezoid. So to answer your question I guess the temperature is absolute at a given altitude but the flow rate can vary.

 

[MintJulep]

I'm at a loss then.

Cooling effect could be measured in KW, and assuming that we are dealing with sensible heat only, would be proportional to mass flow rate and a temperature difference.

 

[CESSNA1]

SNYDERDJ: My interpetation would be that the supplier will provide a device that either:

1. Will provide XX Lb/min of air for each kw of power it is supplied.

2. Will provide XX lb/min of air for each kw of power or heat in the device into which the air is being put, presumably for cooling.

Alternatively why do need more air if cooling air is already being supplied?

More detail and information is required to resolve this issue.

Regards
Dave

 

[25362]

Snydergj is apparently right. Somewhere I've seen that fan-cooled avionics have a standard air flow rate of 8.28 lb/min/kW, while the maximum air flow rate for the same purpose is 4.2 lb/min.

 

[friartuck]

In the UK we have to evaluate the efficiency of fans by working out the airflow in relation to the kW used.(specific power)
An inefficient fan obviously uses a lot of power to generate the required airflow. In Avionics, I guess that this is just as important because every kW used reduces the overall aircraft fuel consumption/range etc

Friar Tuck of Sherwood

 

[TD2K]

I would agree with Snydergj, it's analogous IMO to a lb/hr per kW for fuel consumption by an engine or lb/hr per kW for NOx emissions or so on.

 

[TrevorP]

Maybe a stupid question from me - why don't you just ask the manufacturer what they mean?

 

[smckennz]

Hi Trevor P

that is indeed a stupid question. What would we have to talk about if we all did things like ask people what they mean?
However it sounds like a bid evaluation, and evaluators can look bad if they ask bidders stupid questions. I do it all the time.
The lb/min/kW sounds like a generic requirement from a vendor who sells gear that needs air supplied by others. Each kW that their gear gear emits or generates requires X lb/min. I suspect the air supply vendor has cut and pasted the demand. Who knows? Maybe its part of a larger system and the vendor hasnt worked it out yet. Perhaps it is the power required for their unit. Maybe it is cooling capacity.

In this case, I would ask the stupid question.

Cheers

Steve

 

[snydergj]

Steve is correct, this is a bid proposal and yes I don't want to look stupid. I have just never seen those units put together in that way and thought that I would tell them I needed that much airflow per KW in my product instead of them telling me that they supply airflow per KW. Thanks for all of the replys.

Gabe