How to Size Chilled Water Pipe

How to Size Chilled Water Pipe

Introduction

How to Size Chilled Water PipePresented by: Frank YarizadehDate: August 3, 2021

What is Chilled Water

– Chilled water is a chemically treated water that is cooled (chilled) down to 42-45 deg F and supplied to a heat exchanger (cooling coil) in order to result in cooled and dehumidified air for cooling a building.

What is a Chilled Water Cooling Coil?

– Chilled or hydronic cooling coils are used to cool or cool and dehumidify an air stream as it passes through the coil.

– Cooling coil typically has copper tubes and aluminum fins.

How much heat is transferred during the cooling process-Airside ?

• Amount of heat transferred from the air stream to the chilled water coil is a mix of sensible and latent cooling effects, and is referred to as total heat transfer:Q (sensible)= (cfm)x(1.085)x(DT)Q (latent)= (cfm)x (4840)x(DW)Q(total)= Q (sensible) + Q(latent)= (cfm)x(4.5)x(DH)Where:Cfm= airflow rate in cubic feet per minute DT= change in dry bulb temperature of air across the coilDW= change in humidity ratio of air in grains of moisture per pound of dry air across the coilDH= change in enthalpy of air in BTU per pound of dry air across the coil

How much heat is transferred during the Cooling Process- Waterside?

• Amount of heat transferred to the chilled water from the air stream:Q = (M)x(C)x(DT)Reworking the above equation based on properties of waterQ= (GPM)x(500)x(DT)Where:M= mass flow rate of waterC= specific heat of water GPM= chilled water flow rate in Gallons Per Minute DT= change in chilled water temperature (chilled water return temp minus chilled water supply temp)

Example

• Airside:– Airflow= 10,000 cfm – Entering air temp.= 80 deg F (DB), 67 deg F (WB)– Leaving air temp= 51 deg F (DB), 50.9 deg F (WB)

• Waterside:– Chilled water supply temp= 45 deg F – Chilled water return temp= 55 deg F

• Question is:– What is the total heat transfer from air stream– How much chilled water flow (GPM) is required

Example

• The amount of heat transferred from an air stream to the chilled water:Q(total)= (cfm)x(4.5)x(DH)H1= entering air enthalpy= 31.448 btu/lbH2= leaving air enthalpy= 20.77 btu/lbQ(total)= (10000)x(4.5)x(31.448-20.77)= 481,000 btuh

• Chilled water flow rate required is:Q= (GPM)x(500)x(DT)Q= 481,000 btuhT1= entering chilled water supply temperature= 45 deg FT2= leaving chilled water temperature= 55 deg F481,000 btuh= (GPM)x(500)x(55-45)Flow= 96.2 GPM

• TIP: CHW flow required based on 10 DT= 2.4 GPM/TON

Pipe sizing rules

• Golden rules:Size 2” pipe and smaller based on maximum 4 fps velocity.Size 2-1/2” pipe and larger based on maximum pressure drop of 4 ft/100 ft of pipe.

Pipe sizing tools

Pipe sizing charts- steel pipe

Pipe sizing charts- copper pipe

Pipe sizing charts- plastic pipe

Pipe sizing examples- Steel Pipe

– 5, 10 20, 40, 60, 80 GPM

Pipe sizing examples- Copper Pipe

– 5, 10 20, 40, 60, 80 GPM

• Questions & Comments

Questions & Comments