HVAC definitions of loss

[Sigurður Bjarni Gíslason]

Hi you all,

I need to know how to define the pressure loss through an inlet and outlet of a compartment.

I have a 30 Pa resistance through a nozzle into the compartment and 50 Pa out of the compartment.

In "nodes" connecting to HVAC VENTS it's possible to define forward and backward loss, what units are used and can I use Pa as a unit there?

Can anybody also elaborate on the directions of loss defined.

Sigurdur Bjarni Gislason

[Kevin]

First, locate the section of the User's Guide where this is discussed. Then tell us the section that is not clear. We want the User's Guide to explain things clearly. If it does not, let us know so that we can make it more clear.

[dr_jfloyd]

Also read the Technical Manual so you know what equations the HVAC model is solving.  

[Sigurður Bjarni Gíslason]

This topic is discussed in 9.2.3 under LOSS. 

There it says the following:

"For a terminal node (e.g., a node connected to the ambient or to a VENT) the LOSS is entered as a pair of numbers representing loss for flow entering the HVAC system and for flow exiting the HVAC system."

Where is the unit described there?

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[dr_jfloyd]

All inputs are shown with their units in the tables in the back of the User's Guide.  The table of HVAC inputs (17.9) in the User's Guide states it is unit less, but I can see your point that it might be helpful to state in the definition of the LOSS input that it is dimensionless.  I will add that to the guide.

[Sigurður Bjarni Gíslason]

Could you or anyone here help me to form this unitless number from the knowledge that I have 30 Pa resistance?

Where is the connection?

[dr_jfloyd]

Read the Technical Manual. Look at the HVAV momentum equation. Solve for K.

[Sigurður Bjarni Gíslason]

We solved for K, there is no variable there that has to do with pressure.

Does anyone see the need for allowing users to put in pressure drop in Pa in these air inlet/out nozzles in FDS to be able to model the reality more accurately?

Sigurdur Gislason

On Thursday, September 25, 2014 4:51:06 PM UTC, Sigurður Bjarni Gíslason wrote:

[dr_jfloyd]

This is 1st semester fluid mechanics. You may wish to review a text on modeling pipe and/or HVAC flows:

Eq 10.3 in the Technical Guide:

rho L du/dt = delta P + buoyancy head + fan pressure - loss

Your measurement of delta P was at steady state so du/dt is zero.  buoyancy head you can compute from the physical dimensions of your system.  With no fan, the fan term is zero.

pressure loss is 1/2 K rho u^2 (do a unit analysis and you will see that rho u^2 has units of pressure)

pressure drop you measured is unique to a specific flow speed so you know u.

solve for K.

[yko]

Hi ,

I had the same confusion, and I checked the verification guide where you can find some example cases ( HVAC_flow_loss.fds)showing the use of loss coefficient and flow loss.

This case will answer your question.

By the way,

The user guide and the verification guide use the term 'flow loss', 'flow loss coefficient', but I think technically it should be " pressure loss" and 'pressure loss coefficient' .  Am I wrong?

And there is one more thing that I found in the example case ( HVAC_flow_loss.fds) shown in the verification guide.

In  HVAC_flow_loss.fds, 

&HVAC TYPE_ID='DUCT',NODE_ID='TEE','OUTLETA',ID='DUCT2B',AREA=0.1,LOSS=16/
&HVAC TYPE_ID='DUCT',NODE_ID='TEE','OUTLETB',ID='DUCT2C',AREA=0.1,LOSS=4/

 

tow ducts are assigned loss coefficients of 4 and 16  (a single number ) .

However , the user guide (p92) says

" LOSS is a pair of real numbers giving the forward and reverse dimensionless loss coefficient in the duct."

Running the case, I had no problem, yet this gives me some trouble understanding the user guide and example cases.

Thanks

yko

[dr_jfloyd]

There isn't a consistent terminology for what to call K.  Depending on the reference you might see flow loss, pressure loss, form loss, fitting loss, head loss, resistance loss, ...  I don't have any real preference on the matter, so I will throw it out to any other users reading this, is there a term that makes more sense to you?

In terms of the examples, in this case LOSS=16 is equivalent to LOSS(1)=16, i.e. only the 1st array element is being set.  In this example the flow is only forward so the reverse value isn't needed.  However, we should strive to have our examples be clear when compared to how we describe inputs, so I will modify the inputs to 16,16 and 4,4.

[Ed]

From texts I've seen, pressure loss coefficient seems to be the more common term.  However, at least for me, the texts accompanying Equations 10.3 and 10.8 made it clear what K is referring to.