How to find head for closed and open circuits?

BASICALLY THERE ARE TWO TYPES OF HEAD

STATIC HEAD AND DYNAMIC HEAD

Static Head is the difference of heights which shall be considered in open circuits and shall not be considered in closed circuits like chilled water closed loop circuits

Dynamic height includes pressure loss by friction, in fitting bends, tees, valves, in equipment and shall be considered in both open and closed circuits

Carrier and ASHRAE has many reference how to take equivalent length for pressure losses

* Open loop  calculation:

 Pipe length friction losses + fitting friction losses + hieght difference (highest point elevation - lowest point elevation ) + operating pressure of the fixture unite.

* Closed loop calculation:

 Pipe length friction losses + fitting frictional losses + pressure drop of farthest equipment (FCU or AHU)

* Practically there are two ways to find out the friction losses :

1- equivalent length ---> Carrier handbook

2- K-factor ---> ASHRAE fundamental

* The main friction equations are Colbrook White & Hazen Williams 

* The most used softwere is H-sym , Pipe flow expert, Trane pipe design

Calculating friction losses in piping network, the expression term is head and the unit is meters or feet.

Procedures are similar for open and close circuits except few variations:

In open circuit the total pipe length to be considered is from the pump discharge upto to the last discharge outlet.

The height to where it pumps is the gravity or static head in meters or feet should be considered separately, from the storage water level , then to the pump and to the highest discharge point.

In closed circuit the total pipe length to be considered is from the pump discharge upto the last discharge outlet and then the main piping end which returns back and connected to the pump suction. The entire loop to be considered.

The gravity head in meters here shall be zero for all closed circuits, because the pumps lifts the medium volume upto a certain highest point and the same volume returns back to the pump( suction) installed at lowest point through the piping network. Therefore a natural siphon effect takes place due to gravity and hence there is no need of external energy to lift the liquid medium.

Now for manual friction loss calculation of the total pipe length of different sizes, flow and velocities use the friction loss graph, to match the corresponding parameters we shall get the friciton loss in m or ft /100 ft or m of pipe length. Then by interpolating with the system length the sectional values shall be computed and summed up.

Fitting losses are dynamic losses, the values in m or ft can be obtained from data bases or text book reference which includes valves and accessories. All individual values determined and summed up.

Now we have to sum up the friction losses as below:

Friction losses of total pipe length.

Gravity or static head.

Dynamic losses of fittings, valves, accessories.

All units are in meters or feet.

Grand sum of all the above shall be the total friction loss in terms of head in meter or feet.

While proceeding one should take care of Reynolds number, which determines whether the flow in the piping system is laminar or turbulent.

First of all open loop system is generally employed in the case of expansion tank which is diaphragm type or open to air type this system is not employed any more in out services any more ,

another important application of open type system is used in cooling towers where the condenser pump delivers the water to cooling tower.

Here very important parameter to keep in mind NPSH Net positive suction head is very important in case of open loop system.

Closed loop system is used in the most common expansion tank which is closed type .

NPSH Net positive suction head is not important in case of close loop system.

Ashrae ,carrier design manual and many books like hvac handbook.

can  brief you deeply into the matter