System Sizing & Design Flashcards
7 cards from real CWS practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 System Sizing & Design flashcards as text
A reverse osmosis unit has a feed pressure of 60 psi, a permeate pressure of 0 psi, and an osmotic pressure of 15 psi. What is the net driving pressure (NDP)?
Answer: 45 psi
NDP = feed pressure − permeate pressure − osmotic pressure = 60 − 0 − 15 = 45 psi.
When selecting tank dimensions for a residential softener, a tall narrow tank versus a short wide tank of equal volume will produce what design difference?
Answer: Better service flow rates in the tall narrow tank due to longer contact time and lower upflow velocity
A taller, narrower tank provides longer contact time and lower linear velocity during service flow, improving ion exchange efficiency.
Which condition would require INCREASING the size of a planned water treatment system beyond the initial design calculation?
Answer: Future plans for additional bathrooms or irrigation expansion
Anticipated future demand increases, such as adding bathrooms or irrigation, must be factored into design to avoid premature capacity exhaustion.
For a UV disinfection system, the 'dose' delivered to the water is a function of which two parameters?
Answer: UV intensity and exposure time (flow rate)
UV dose (mJ/cm²) = UV intensity (mW/cm²) × exposure time (seconds), where exposure time is determined by flow rate through the chamber.
A well pump delivers water at 10 gpm. A pressure tank is being sized to minimize pump short-cycling. If the minimum drawdown volume required is 5 gallons per cycle, what tank size is most appropriate using a 20/40 psi pressure switch?
Answer: 20-gallon tank
At 20/40 psi with an air pre-charge at 18 psi, a 20-gallon tank provides approximately 5-7 gallons of drawdown, meeting the minimum cycle requirement.
An acid neutralizer using calcite media is sized for a 10 gpm service flow. To prevent calcite loss during backwash, what maximum backwash rate is typically specified?
Answer: 5 gpm per square foot of bed area
Calcite backwash rates are typically kept at or below 5 gpm/ft² to achieve bed expansion without washing calcite particles out of the tank.
When sizing a chemical feed pump for chlorination, the pump output must be matched to which primary variable?
Answer: The chlorine demand of the water multiplied by the flow rate
Chemical feed rate (gpd or ml/min) must equal the chlorine demand (mg/L) × flow rate to achieve the target residual concentration.