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Bui ld ing Hydronic SystemsPressure Prof i le

For HVAC Project Managers

Fachgesprach 6WTF Institute of Higher Learning

ByMat Ansari PETouchstone Engineering Corporation3838 Cerritos Ave.Los Alamitos, CA 90720

Part -1Get The Theory Out Of The Way

What is this all about?For design-build bidding and estimating, it is useful to know the system pressures at all points in the proposed system for many reasons,

for example:

1. We need to know if any of the proposed piping materials, accessories, sensors, and instruments need to be special

2. Can the boiler be located on the lower floors without exceeding its relief valve setting

3. Most chiller evaporators have a 150 psig standard MAWP – is that OK?

4. If our start-up tech claims that at some location the pressure reading is abnormal - then we need to know enough to agree with, or contradict his opinion.

5. Etc. etc. And so having a good feel for what the pressures are in your system is always helpful

Pressures Expected:Engineers get very upset if pressure gages are omitted from the locations specified – but what good is a pressure reading if you don’t have any clue about the correct value to expect?

What will pressure gage A read with the pump OFF?

Will it increase or decrease when the pump is turned ON?

Where (and when) is the highest pressure that the system will experience?

These are fairly easy questions to answer – once you understand the basics of hydrostatics and the Bernoulli's Equation. So let us take a look at the theory.

A

The Basic Hydrostatic Formula - 1

Density(Kg/m3)

Height(meters)

Accelerationdue to gravity

9.8m/s2

Newton/m2

This is what you learned in freshman Physics or your introductory course in Fluids.

Not a really useful form for American HVAC engineers.

Note: Even if you were to convert this formula directly to IP units, there is a catch:the density is "mass" density and is in "slugs".

Let us move on.

Pressure (Δp) = ρ g h

Where, ∆p is the pressure difference between any 2 points inside a column of fluid having "h" meters of vertical elevation difference.

h


• Here is a more useful version• This is the same formula as in the

previous slide. The units have been changed and the "weight density" (specific weight) of water substituted.

• 2.31 comes from dividing 144/62.4• You may choose to remember 0.433, the

reciprocal of 2.31. Or Both. Your choice.• All it is saying is that every 2.31 feet of a

water column will exert 1 psi pressure at the base.

• You need to remember this Constant!

Δp (psi)h

=2.31

For Water (only)γ = 62.4lbf/ft3

Press. Diff. between any 2 points in a

water column

Vertical distance between the 2 points in Feet

Constant

Note: For hydronic systems using glycol, correct the gamma (γ) value by

multiplying by the specific gravity of the glycol mixture.

h


From Pump

10 psig

23.1 Feet

2.31 Feet = 1 psi

10 psig

The Equivalence Of Pressure And Head

All our pressure gages on hydronic piping read in "psig".

We will modify all our formulae to directly accept "psig".

The Equation That We Should Use Often - [But Do Not]Some Random Notes about Bernoulli's Equation:

1. I have been warned that any discussion of this equation causes sudden attacks of Attention Deficit Disorder in Project Engineers.

2. If you well versed in Dan's equation - then you probably shouldn’t be here

3. BTW it is unfortunate that the way this equation is taught in most of American classrooms is totally useless for applying it to real world HVAC problems in our customary US engineering units.

4. In the following few slides I will present Bernoulli's equation in a more useable format for working with building Hydronic Systems.

5. Our goal is to understand it so well that we never have to use it. By that I mean that we will apply the "principle" but never just plug in numbers.

First - A Flowing Stream Of Fluid Possesses 3 Kinds Of "Mechanical" Energy

Bernoulli's Equation – What is it all about? - 1

1. Imagine a slug of water in a pipe, say inside an equipment room:

• It has a Pressure of P (psf)• It has a Velocity of V (fps) • and the Pipe is located Z (feet) above the FF

or any other convenient Datum

2. Pressure Energy – WTF definition: "the energy

trying to burst the pipe." (Also known as Flow Work or Flow Energy.)

• = Pressure x Volume = P x VOL ft.lbf

3. Kinetic Energy – Energy due to its Velocity.

• = ½ x m x V2 ft.lbf (m is in slugs)

4. Potential Energy – Energy due to Elevation

• = (W x z) or (m x g x z) ft.lbf Where (W is Weight in lbf) and (m is in slugs)

Z (feet)

P (psf) V (fps)

Note the 3 units:Elevation → FeetPressure → Pounds per Square FootVelocity → Feet per Second

Keeping the units straight is half the battle in mastering Bernoulli's Equation!

This body of water has:Weight = W (lbf)And Volume = VOL (ft3)

Using "VOL" so as not to mix up with "V" the velocity.

Arbitrary Datum

Bernoulli claimed that the sum of all 3 Energies: Pressure

Velocity

Elevation

Remains the same as the slug moves from one place to another in the piping – (and we add) UNLESS:


1. Head is loss due to friction and/or dynamic losses2. A pump adds energy to the fluid stream

3. A device like a paddle wheel or turbine takes energy out of the fluid stream

And if that happens, NO BIG DEAL, just adjust for the energy taken out or put in the fluid stream, and everything should add up and balance nicely.

Note: As we all know there are other types of energy that this "slug" can posses, like temperature and internal energy – but this equation is not concerned with that.

Everything that is going to be said about the Bernoulli's Equation in the following slides is strictly from the point of view of an HVAC Project Manager (or Engineer) dealing with ordinary commercial building Hydronic Systems. (Note: "Hydronic" and not "Air" systems.)

And when we finish with it – it will not look anything like the Bernoulli's Equation from your college notes.

• This is the "Head" form of the Bernoul l i 's Equation.

• Where the Head in feet of f luid represents the "Energy Density" or Energy per Unit Weight of f luid.

• This is the form you should use when deal ing with hydronic systems.


+ + =

=

ft.lbf ft.lbf ft.lbf

PotentialEnergy

Constant

P x Vol m x V2 m x g x z Constant

PressureEnergy

kineticEnergy

2

2 x WW WConstant=

P x Vol m x V2 m x g x z

ft.lbf

lbf

ft.lbf

lbf

ft.lbf

lbf

P V2= Constant

γ 2 x g+ + z

1. Let us write down the formulas for these 3 types of mechanical energy. Don’t worry too much if you don’t remember these from high-school Physics. It is not going to make a difference in the end.

2. Note: the units for Mechanical Energy (Work) in the customary US engineering units is ft.lbf

3. These energies are for the total weight and volume of the that particular water slug in the pipe. We would like to normalize it on a unit weight basis so it can be applied to any stream.

Let us divide all 3 terms by W (lbf).

Conversion Notes:𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑊𝑊𝑆𝑆𝑆𝑆𝑊𝑊𝑊𝑊𝑊 (𝛾𝛾) = 𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊

𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑊𝑊𝑉𝑉𝑙𝑙𝑓𝑓𝑓𝑓𝑊𝑊3

∴ 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊

= 1𝛾𝛾

(F = ma) W = mg ∴ 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊

= 1𝑊𝑊

The units for all 3 terms are now ft. lbf/lbf or just "FEET"

• Note carefully the UNITS:• P is in psf, V is in fps, and Z is simply feet.

• γ is in lbf/ft3 for water 62.4 lbf/ft3. (Specific Weight)

• g is in ft/s2 accel. due to gravity 32.2 ft/s2.

Bernoulli's Equation – Whipping It In Shape -1

PressureHead

VelocityHead

ElevationHead

TotalHead

feet feet feet feet

+ + =

= Constantγ 2 x gP + V2 + Z

The Classic "Head" Form of Bernoulli's EquationEnergy Per Pound Weight of Fluid

ft.lbf

lbf

Energy Conservation Statement:1. If the stream containing that slug of water travels from

Point-1 to Point-22. And if there is no Pump between Point-1 and Point-23. And if there is no Head Loss between Point-1 and Point-24. Then:5. The Total Head per Pound Weight at Point-2 will be the

same as the Total Head at Point-16. Even if the Velocity changes or the Elevation changes or

they both change

But if we are only dealing with WATER, which most of the time we are – then why not simplify further?

• Convert Units to what we get in the field:• P is in psf, so multiply by 144 so we can plug in psig

values which we read on our pressure gages.

• γ is in lbf/ft3 for water 62.4 lbf/ft3. (Specific Weight)(If you are running glycol remember to correct this number)

• g is in ft/s2 accel. due to gravity 32.2 ft/s2.

Bernoulli's Equation – Whipping It In Shape -2

Total Headft

Vel. Headft

Elev.Headft

P is in psig V is in ft/sec Z is in ft

= Constant62.4 2 x 32.2

+V2

+ Z = Constant64.4

P x 2.31

P x 144 + V2 + Z

Pr. Headft

+ V22

+ Z264.4

2.31 P1 + V12

+ Z1 =64.42.31 P2

Where:P is in psigV is in ft/secZ is in ft

There are a couple of dozen different ways to write the Bernoulli's Equation, but for water flow this is about as direct and simple as it gets. This is the form you should use for WATER loops (no Pump, or Head Loss included). Before we modify the equation to include Pump Head and Head loss, we want to simplify it even further.

Bernoulli's Equation – Whipping It In Shape - 3

+ V22

+ Z264.4

2.31 P1 + V12

+ Z1 =64.42.31 P2

For Hydronic Loops in Multi-story Buildings - The Velocity Term Is For Decoration Only:• The Z term can be hundreds of feet. The Pressure Head can be hundreds of feet (50 psig = 116 ft. )• What about the Velocity Head?• Say the range of velocities in your water loop is from 8 fps to 4 fps.• If you plug in 8 fps in the left hand side Velocity term and 4 fps in the right hand side velocity term, you get 1 ft and

0.25ft respectively. That is ¾ ft Head difference, which translates to about ⅓ of a psig.• Now if your "Commissioning Ace" has to whack the pressure gage with a screw driver just to make it give him a reading

– obviously this whole Velocity Head business can be ignored (especially for our purpose as stated in the slide-1 Title).

Just so you don’t get the wrong idea: Velocity Head (or Velocity Pressure) is an important concept in building hydronic loops. For example the pipe friction and dynamic losses are all based on Velocity Pressure. We are just ignoring it here because within the narrow range of Velocities experienced in building hydronic loops, the change of Pressure Head due to a change in Velocity is very small. Also, Plumbers when dealing with gravity flow should not ignore the velocity term.

Bernoulli's Equation – The De-Fanged Version

+ Z1 = + Z22.31x P1 2.31x P2Where:P is in psig and Z is in feet above any convenient arbitrary Datum

Almost sad to see the famous Bernoulli's Equation, which has inspired so much fear and awe in so many students - humbled to such a dumbed down relationship.

In words all it is saying now is:Between any 2 points, having no Pump and no Head Loss between them(say in an equipment room)The SUM of the pressure gage readings multiplied by 2.31 and their respective heights above the finished floor, which is the Total Headwill (MUST) be equal!

P1

Z1

1Pipe

P2

Z2

2 Pipe

(2.31 x P1 + Z1) (2.31 x P2 + Z2)

Now I am tired of saying "having no Pump and no Head

Loss between them", so let us now take care of that. It is

really not a big deal at all.

Total Head at

1

Total Head at

1

Total Head at

2

Total Head at

2

Bernoulli's Equation – Pump Head and Head Loss

21

Z Pump Head AddedHpump

Friction + Dynamic Loss

HLossHead Loss

Pump Head Gained

HLoss = Head loss in friction and dynamic losses

HPump = Head added by Pump. The TDH of your Pump selection

Energy Conservation Requires That:

1. Energy In = Energy Out

2. Green arrows represent energy going INTO a control volume boundary

3. Red arrows represent energy LEAVING the control volume boundary

HLOSS+64.4 64.4+ HPUMPZ1 Z2 +2.31 P1 + V12

+ = 2.31 P2 + V22

Bernoulli's Equation – The Complete Picture With Units

Pressurepsig

Pump TDHfeet

Velocityft/sec

Elevationfeet

Friction & Dynamic Loss

feet

Pressure Head (feet)Velocity Head (feet)Elevation Head (feet)

= Pressure Head (feet)Velocity Head (feet)Elevation Head (feet)

PumpHead (feet)

LOSS(feet)

Public Notice:In the slides that follow, I will not stress out about units. Just remember that they are important.

Convertsto feet

Convertsto feet

++ = 2.31 P2 HLOSS+ HPUMPZ1 Z2 +2.31 P1

Bernoulli's Equation – The Final Form We Will Use

Pump TDHfeet

Friction & Dynamic Loss

feet

Pressure Head (feet)Elevation Head (feet) = Pressure Head (feet)

Elevation Head (feet)PumpHead (feet)

LOSS(feet)

Bernoulli's Equation – What is the QUESTION?

There are 6 Variables in the standard Bernoulli's equation, and then Hpump and Hloss. Any one of the eight can be the unknown. For Text Book problems at the end of the Chapter and class room quizzes they can (and will) ask you to solve for any one of the eight variables. Unfortunately the same for EIT exams.

1. But as a Project Manager in the HVAC trade, you should NEVER have to solve for the following:

2. Z1 or Z2: You either look it up on the drawing or measure it at the jobsite. WAC policy is to immediately fire any Project Manager trying to determine pipe elevations using Bernoulli's equation.

3. V1 or V2: As discussed in previous slides, we don’t even use the Velocity term, let alone solve for it. And anyway if you do need the velocity, there are calculators and charts that we all have.

4. Although we could solve for Hpump and Hloss , we usually don’t. (Hpump is frequently used in other industries to size pumps.)

5. So that leaves the Pressure Term. That is our main concern. If water is flowing from point 1 to point 2 we want to know what is the pressure difference between the two points. And that is what the above equation helps us find.

Summary

• The Total Head at any point consists of the sum of:• The Pressure Head• The Velocity Head• The Elevation Head

• This Total Head remains constant at ALL points in the system, except when:• A pump adds Head• Frictional and Dynamic Losses subtract (dissipate) Head

• THIS IS THE PRINCIPLE THAT WE WILL KEEP IN MIND AND IGNORE THE EQUATIONS

100 Feet

KNOWN

Assume 10'Friction & Dynamic LossA

C

B

50

180 Feet

20 Feet

Vel.8 FPS

Vel. 4 FPS

?

Vel. 4 FPS

Warm Up Slide – 1?

May need to make decisions about:

Equipment pressure ratings,pipe material pressure ratings

or relief valves settingsetc. etc.

at points B and C and so

WHAT IS THE PRESSURE?

100 Feet

KNOWN


C

B

50

180 Feet

20 Feet

Elevation Head = 100'

Velocity Head = 1'

Pressure Head = 115.5'

TOTAL HEAD = 216.5'

Vel.8 FPS

Vel. 4 FPS

11

81

Vel. 4 FPS

(ft)Pressure Head(P in psia)

2.31x P=

(ft)Elevation Head(Z in feet)

= Z

(ft)Velocity Head(V in ft/sec)

=V2

64.4

Where "Head" is Energy Density per Pound Weight of Water

Warm Up Slide – 1White patch

shows back of the napkin

calc

100 Feet

KNOWN


C

B

50

180 Feet

20 Feet


Velocity Head = 1'


TOTAL HEAD = 216.5'

Vel.8 FPS

Vel. 4 FPS


Velocity Head = 0.25'

Pressure Head = 26.25' or 11psig

TOTAL HEAD @ B = 206.5'

Head Loss A to B = 10'

TOTAL HEAD @ A = 216.5'

11

81

Vel. 4 FPS


2.31x P=


= Z


=V2

64.4




calcRead all white

boxes from BOTTOM up

100 Feet

KNOWN


C

B

50

180 Feet

20 Feet


Velocity Head = 1'


TOTAL HEAD = 216.5'

Vel.8 FPS

Vel. 4 FPS





Head Loss A to B = 10'


11




TOTAL HEAD @ C = 206.5'

Head Loss A to C = 10'


81

Vel. 4 FPS


2.31x P=


= Z


=V2

64.4


Random Notes:1. We DID use the Bernoulli's equation here, but

maybe not in the conventional equation solving sense.

2. Note how the law of energy conservation dictates that the 216.5' number cannot change.

3. Now you can see why in ordinary Building Hydronic calcs you are wasting time worrying about the velocity term. Also. the energy conservation principle then simply becomes a play between the "Pressure" and the "Elevation" terms. (Multiply or divide by 2.31). Very seldom do you have to write down the formal version of Bernoulli's equation.

4. BTW we started with one stream of 216' head water and ended up with 2 streams of 206' Head water – is that creating energy out of nothing?



calc

46.2 Feet

KNOWN

30

?

Assume 11.6' (5 psi)Friction & Dynamic Loss

A

B

Question: What is the pressure at Point B

1. There is no Pump between Point B and Point A2. There is a 11.6' Head Loss between Point B and

Point A3. Energy conservation then dictates that the Total

Head at Point B equal the Total Head at A plus the Head Loss of 11.6'.

15Warm Up Slide – 2Unknown PR Upstream

Elevation Head = 46.2'



Head Loss B to A = 11.6'

Total Head @ A = 69.3'

Elevation Head = 0'

Velocity Head - ignore



Side Note:Hold it! …WTF? How is water flowing from a low pressure point to a high pressure point in the last 2 slides? Go Back!

HINT:Danny B is snickering in his grave in Switzerland. "Serves you dummies right for not fully writing out my equation!"

It is the TOTAL Head that drives the direction of flow!

Side Note:Pressure – The Conservation Law Abiding Term• It is interesting to note that out of the 3 f lu id energy terms, the

PRESSURE term is the one that makes sure that Conservat ion Law is a lways obeyed.

• The other two have a "couldn ’t care less" , " I am what I am", or "don ’t g ive a S*^#" att itude!

• As an example if you turn a hose f lowing water straight up 20 feet (no pump) ,

• the e l e va t i on term sudden l y s ays I have a 20 feet o f extra Po tent i a l energy ;

• the ve l o c i t y term says that I don ’ t c a re the p i pe d i ameter hasn ’ t changed so my ve l o c i t y and k i net i c energy i s s t i l l t he s ame !

• Since no new energy has been provided to the water stream, it fa l ls on the Pressure Term to reason that things are not r ight – I better back down to keep sanity in the un iverse. So the Pressure goes down by 20 feet head or (20/2.31) = 8 .6 psig . The TOTAL energy then is the same as before.

+ V22

+ Z264.4

2.31 P1 + V12

+ Z1 =64.42.31 P2

KNOWN

170 feet

Question: What is the pressure at the Condenser?

0

Warm Up Slide – 3Sanderson Special?

Cooling Tower

1. Real job condition2. Let Sanderson tell us the story3. We know that the pressure at the open cooling

tower discharge is 0 psig (atmospheric).4. Of course -69 psig is not possible. But it shows

that the whole top side is under deep vacuum and really and the condenser heat transfer is reduced because we have steam instead of water.

5. One way to solve this problem is to have a HUGE and wasteful pressure drop between B and A.

Discharge to atmosphere is

0 gage

Elevation Head = 0'

Velocity Head - ignore

Pressure Head = 0'

TOTAL HEAD @ A = 0'


Pressure Head = -160'

TOTAL HEAD @ B = 10'

Head Loss B to A = 10'

Total Head @ A = 0'

B

A

Be careful when designing vertical down-comers into open tanks.

Assume 10'Friction & Dynamic Loss

- 69

KNOWN

Pump TDH = 69 feet?

A B

Question: What is the pressure at Point B?

1. Pump suction is 10 psig2. The pump adds 69 ft of Head energy –

where will it go?3. The velocity is unchanged at points A

and B, because the pipe diameter is the same, so the velocity Head cannot increase

4. The suction and the discharge pipe elevations are the same, so the Potential energy (Elevation) term cannot increase

5. So once again the Pressure term has to modify itself to keep the energy accounting correct.

6. It must go up by 69 feet (30 psig).

10

Warm Up Slide – 4PUMP Action

40

3 Feet 3 Feet

And Now - Good Bye Till Part – 2

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