8/8/2019 93reverse Return

http://slidepdf.com/reader/full/93reverse-return 1/1

11

Primary Variable Speed Flow 

QIn a constant volume chilled

water system, with two way con-

trol valves in the chilled water

cooling coils, can I use a bypass line

between supply and return piping with a

control valve to maintain constant flow

through the chiller? If so, what should be

the pressure settings of the control valve

and the percentage of flow through the

bypass line?

 A 

The quick answer is “yes”, you

probably could add a by-pass linewith modulating control to main-

tain flow across the chiller. This is the set

up used in Primary Variable Speed/Flow

applications. But there are a lot of cau-

tions to consider before attempting to

retrofit this system, among them the sys-

tem volume, rate of change, pump and

chiller selections and probably the most

important - the control complexity and

operator ability.

Before blessing a system like that, we

would want to know more of the details

concerning the chillers and the total sys-

tem. Depending on the system staging,

the common pipe may have to be sized

either for the largest chiller flow or possi-

bly multiple chiller flows. The

modulating control would be reacting to

differential pressure sensors across each

chiller and would require an algorithm

type controller.

Because of these issues you may want to

consider primary/secondary pumping,

proven over the years as the most simple

and efficient way to control a variable

volume, two-way control valve system.

Variable Flow System 

QIn a variable flow system (one

using variable frequency drive)

should the Triple Duty® Valves

be in the 100% open position? I have an

installation that used VFDs but the Triple

Duty Valves are in the 20% and 40%

open position. It seems to me that energy

savings may increase by letting VFD do

the flow regulating.

 A “Variable flow” systems are those

that control coil operation using

two-way valves. A “variable

speed” system uses an AFD to adjustpump speed to load, and they usually

have two-way valves. If your pump can

meet the load with those Triple Duty

Valves throttling, they are telling you

that the pump is oversized. Check with a

B&G Representative about trimming the

impellers so that you can open up the

Triple Duty Valves. The system will

work better, and at lower cost, that way.

Steam Pressure 

QWhen my condensate return

pumps shut down from a pump

cycle there is a very loud “clunk-

ing” noise in the overhead piping. It

almost sounds like someone inside the

wall hitting the pipe with a hammer or

metal pipe. The piping is 20 yrs. old but

the receiver and pumps are only about

four years old. Is this water hammer?

 A If the condensate goes down-

stream of the elevated section of 

pipe and it flows into a vented

tank, the noise is probably caused by a

type of water hammer as follows: when

the pump is on, the elevated section of 

pipe is full of warm water at some posi-

tive pressure. When the pump stops,

water drains out of the elevated section

into the open tank at a lower level. This

reduces the pressure on the water in the

higher sections of pipe causing some of it

to “flash”, or form steam bubbles. The

collapse of these bubbles as they con-

dense causes the noise. Fix this by

installing a vacuum breaker at the top of 

the piping, or a minimum pressure valve

at the tank to keep the pipe full when the

pump stops.

Pressure Sensor 

QI am designing the HVAC sys-

tem for a 300 bed hospital.

Chiller capacity is 5,000 tons.

The chilled water hydraulic system is of 

the primary chilled water loop type using

a variable speed pumping system. Which

piping system is the best to use in a vari-

able speed pumping system - reverse or

direct return piping?

 A Most designers use the direct

return to avoid the cost of the

extra pipe. If you chose to do it

that way, you will have to insure that thesystem is balanced so that the last coils

will get acceptable flow. Reverse return

piping can ease the balance problem a lit-

tle, assuming that the coils are all within

25% of each other in terms of pressure

drop. Be careful to install the pressure

differential sensors properly.

Pressure Regulator 

QI have a closed-loop chilled

water hydronic system. The

operating temperature is 42 ºF

with a system water temperature rise of 

14 ºF (max temperature of 56 ºF).

I am trying to size an expansion tank for

this system using the Bell & Gossett

ESP-PLUS® selection software, but it

will not let me select a unit for these sys-

tem conditions. Can I conclude that an

expansion tank is not needed? If not, why

not? Does the loop need an expansion

tank to act as a pressure regulator?

 A 

Your chilled water system defi-

nitely needs a compression tank,

but not for the same reason that ahot water system needs one. Normal

“design” temperature changes in your

system will not change the specific vol-

ume very much, but if the system is idle

during a very hot period, you could get

enough net expansion to cause damage,

or at least open the system relief valve.

The compression tank guards against that

problem. ESP-PLUS® selection software

is based on hot water systems. For your

application, use the highest likely ambi-

ent temperature, and a temperature drop

of 1 degree to size the tank.

FREQUENTLY ASKED QUESTIONS

 www.bellgossett.com

To view more FAQ’s, go to

 www.bellgossett.com, click 

on “Knowledge Base”, and

then enter a key word in the

search box.