This piece is a part of the report the author had made and is intended to share the

knowledge that the author had acquired during the preparation of his Engineering

Report; Design, Installation, Operation and Maintenance of 700 Tons HVAC

System. In partial fulfillment for the requirement of taking the Professional Mechanical

Engineering (PME) Board examination that he had successfully defended.

He wishes to dedicate his works to his two wonderful kids (Amina & Amira) and his

entire family that serve as his inspiration for all of his aspiration and attainment.

With a hope that this will be beneficial for the reader and it may give them an input for

whatever task they may find it might be useful.

Kiram M. Solaiman. PME, AVS, MInstPM

2.5.6 - Expansion Tank (Pressurization of the System)

The expansion tanks are provided in closed hydronics systems to:

1- Accept changes in water volume as water density changes with

temperature to keep system pressure below equipment and piping system

component pressure rating limits.

2- Maintain positive gauge pressure in all parts of the system in order to

prevent air from leaking into the system.

3- Maintain sufficient pressures in all parts of the system in order to prevent

boiling, including cavitations at control valves and similar constrictions.

4- Maintain required net positive suction head (NPSHr) at the suction of

pumps.

Step #1 – Determine the initial precharge pressure; Pi

1. Find the lowest pressure point (LPP) in the system which is the highest

point on the return line just as it drops down to the pump.

2. Determine the minimum pressurization required at the LPP to maintain a

positive gauge. The commonly recommended minimum pressurization is 4

psig (ASHRAE Journal March 2003)

3. The tank will be the smallest and least expensive if located near the LPP.

4. The static pressure rise ∆PS,LPP →tank which is simply the elevation difference

between the LPP and tank that is 36.08 ft. (Actual elevation of for this

design)

Pi = Pi + ∆PS,LPP →tank + ∆Pf,tank →LPP

∆Pf, tank →LPP = 0 when tank is downstream

Pi = 4.0 psig + 36.08 ft/2.31 ft/psig + 0

= 19.62 psig

Step #2 – Determine the maximum pressure; Pma

1. The standard pressure rating of all components in the system will be 125

psig or higher. Hence, Pma is assumed to be at this pressure.

2. Assume the pressure relief will be located near the chiller and the

expansion tank.

3. The static pressure difference ∆Ps, CPP→PRV is zero since they are at the

same elevation. (CPP-Critical Pressure Point; which is the weakest link in

the system, PRV-Pressure relief Valve).

4. The relief valve is downstream of the CPP and is assumed to be equal to

the pump head; that is 108.89 ft (∆Pf, CPP→PRV) = 47.14 psig (108.89 Ft is

the computed pump head for this design)

5. The pressure relief valve set point is:

Prv = Pma + ∆Ps,CPP →PRV - ∆Pf,CPP →PRV

= 125 + 0 - 47.14

= 77.86 psig

Pmax = Prv + ∆Ps,PRV →tank - ∆Pf,CPP →PRV

∆Ps,PRV →tank = 0 (they are both on same elevation)

∆Pf,CPP →PRV = 0 (they are both on same elevation)

= 77.86

The tank minimum volume: (using Table 1, pp.5 ASHRAE Journal Mar 2003)

Va ≥ Ve

≥ Vs [(vh / vc ) – 1]

Where: Vt = Volume of the tank

Va = Tank acceptance volume

Ve = Increase in volume of water as it expands

Vc = specific volume of water @ 40 ºF generally

Used for chiller

= 0.01602

Vh= specific volume of water @ 80 ºF assumed

Maximum temp in the tank

= 0.01608

VS = System volume = 1810.23 gpm

≥ 1743 x [(0.01608/0.01602) – 1]

≥ 6.63 gallons

Vt ≥ Va / [1- (P a + P i)/ (P a + P max)

≥ 6.63 / [1 – (14.7 + 19.62) / (14.7 + 77.86)]

≥ 10.40 gallons

Therefore, the expansion tank must have an acceptable volume of greater than 6.63

gallons and overall volume greater than 10.40 gallons.