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16/10/2026

6 process engineer interview formulas, and the follow-up question that trips people up 🎯

Interviewers rarely stop at the formula, they ask the follow-up. Here are 6 chemical engineering interview questions in the order you meet them in a plant: flow, pressure, heat, pumps, separation, gas.

1️⃣ Reynolds number: Re = ρvD/μ
Follow-up: what are the laminar and turbulent limits? Below about 2,300 the flow is laminar, above about 4,000 it is turbulent, with a transitional zone in between.

2️⃣ Bernoulli equation: P + ½ρv² + ρgz = constant
Follow-up: when does it fail? With friction, pumps or turbines, and compressible flow. Example: in a venturi from 100 mm to 50 mm at 2 m/s, the throat velocity is 8 m/s and pressure drops 30 kPa (ideal, no friction).

3️⃣ LMTD formula: ΔT_lm = (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂)
Follow-up: what if ΔT₁ = ΔT₂? The formula gives 0/0, but the limit is LMTD = ΔT.

4️⃣ NPSH available: NPSHa = (P_s − P_v)/ρg + z − h_f
Follow-up: hot liquid, does NPSHa go up or down? Down, because vapour pressure rises. With 2 m of suction lift and 0.5 m of losses, water falls from 7.6 m at 20 °C to 3.2 m at 80 °C.

5️⃣ Raoult’s law: P_i = x_i · P_i^sat
Follow-up: when does it fail? In non-ideal mixtures, where you add the activity coefficient: yP = xγP^sat.

6️⃣ Ideal gas law: PV = nRT
Follow-up: molar volume at 0 °C and 1 atm? 22.4 L/mol (about 22.7 L at 1 bar, because STP definitions differ).

💬 Which follow-up would you have missed? Tell us below 👇
📌 Save this before your next interview
📤 Send this to your placement group
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🔎 Tags: process engineer interview questions, chemical engineering interview questions, Reynolds number laminar turbulent, Bernoulli equation assumptions, LMTD formula, NPSH available, Raoult’s law deviations, ideal gas law molar volume, venturi, process engineering, chemical engineering, campus placement preparation

12/10/2026

NTU–Effectiveness Method explained in 16 seconds. Heat exchanger design, Part 2.

Outlet temperatures unknown? Plain LMTD turns into trial and error. The NTU effectiveness method skips that.

Worked example for a counterflow heat exchanger: hot stream 120 °C, cold stream 30 °C, Ch = 2 kW/K, Cc = 4 kW/K, UA = 2 kW/K.

Capacity ratio Cr = 0.5
NTU = UA / Cmin = 1.0
Heat exchanger effectiveness ε = 0.565
Heat duty Q = 101.65 kW out of 180 kW maximum

Save this for your next heat exchanger design or rating problem. Send it to the process engineer who still guesses outlet temperatures.

Follow CHEMKLUB for more process engineering concepts: heat exchanger design, pump selection, control valve sizing, Aspen HYSYS and Aspen Plus simulation.

🌸 Happy Navratri from ChemKlub! 🙏Wishing you and your loved ones happiness, positivity, strength, and prosperity. ✨
11/10/2026

🌸 Happy Navratri from ChemKlub! 🙏

Wishing you and your loved ones happiness, positivity, strength, and prosperity. ✨

09/10/2026

Because low suction pressure means less NPSH available. When NPSH available (NPSHa) drops towards NPSH required (NPSHr), the liquid at the impeller eye turns into vapour bubbles.

The bubbles collapse as pressure rises inside the impeller. That is cavitation.

What it does to a centrifugal pump:

• Flow and head fall
• Noise and vibration go up
• Impeller and mechanical seal get damaged

That is why plants add trips: a low suction pressure switch (PSL), low flow, low motor amps and high vibration. The trip is the pump protecting itself.
Common reasons suction pressure drops:

• Low level in the suction tank
• Choked strainer
• Suction valve partly closed
• Hot liquid, so vapour pressure rises
• Blocked tank vent, so vacuum builds

The rule: keep NPSHa above NPSHr, with a margin. NPSHr also rises with flow, so a pump running far out on its curve needs more.

Two safety points:

• A pressure switch is one layer of protection. Many plants also use a low-low level switch on the vessel.
• Trip settings come from your plant’s own NPSH calculation and HAZOP. Never bypass a trip. Find the cause first.

This post is for learning, not a site-specific design.

Want to learn NPSH, cavitation, pump curves and pump sizing step by step? Join our live webinar: Pump Design using Aspen HYSYS. 31 Oct 2026, 8–10 PM.

Your pump can pass the datasheet and still cavitate. 🏭Before you trust a pump simulation, check it in three steps:First,...
08/10/2026

Your pump can pass the datasheet and still cavitate. 🏭

Before you trust a pump simulation, check it in three steps:

First, work out the head from the pressure rise: H = ΔP / (ρg). Then get the shaft power from the head and flow, using P = ρgHQ / η.

Second, calculate NPSH available for your pump design: (P_suction − P_vapour) / (ρg) + static head − friction losses in the suction line.

Third, compare it with the NPSH required from the vendor pump curve, and keep a safe margin above it.

Worked example (example values, not real pump data): hot water at 90 °C gives NPSH available of 3.6 m against 3.0 m required.

That leaves only 0.6 m of margin, short of the 1 m used in this example, so the pump needs a fix. Swipe through the carousel for the steps, diagrams and calculation.

💾 Save this for your next centrifugal pump review.

📤 Share it with a colleague who sizes pumps.

💬 What NPSH margin does your company use? Tell us below.

🎓 Live webinar: Pump Design using Aspen HYSYS
📅 31 Oct 2026, 8:00–10:00 PM IST
Covers pump fundamentals, NPSH and cavitation, pump and system curves, pump selection and sizing, and pump simulation in Aspen HYSYS.
Join the broadcast for the link.

Follow ChemKlub for more process-engineering basics.

pump design, NPSH, cavitation, centrifugal pump, pump curve, system curve, pump sizing, Aspen HYSYS, chemical engineering, process engineering, ChemKlub

07/10/2026

🔴 LIVE webinar on Pump Design using Aspen HYSYS. In one session you’ll go from the basics to a working simulation:

01 ▸ Pump Fundamentals: design, classification and application
02 ▸ NPSH & Cavitation
03 ▸ Pump Curves & System Curve
04 ▸ Pump Selection & Sizing
05 ▸ Pump Simulation in Aspen HYSYS

📅 Saturday, 31 October 2026
⏰ 8 PM – 10 PM (IST)

👉 Comment “PUMP” and we’ll send you the registration link.

Save this post and share it with a friend who works with pumps. 🔁

Our Aspen Plus V14 course on Udemy takes you from the basics of process simulation to unit operations like heat exchange...
06/10/2026

Our Aspen Plus V14 course on Udemy takes you from the basics of process simulation to unit operations like heat exchangers, reactors and distillation columns.

Then you learn to read your results and use process optimization to improve plant performance, with real examples from petrochemicals, ammonia synthesis and polymerization.

✅ 6 hours 24 minutes of hands-on video
✅ Made for students, graduates, working engineers and researchers
✅ No prior simulation experience needed

All of this is yours for just ₹399 with redeem code CHEMKLUB2027.

Want the link? Comment "interested" and we'll send it to you. 💬

05/10/2026

Every control valve sizing job starts with one question: how much flow does this valve have to pass, and can it still control it? These formulas answer it in order.

1️⃣ Flow coefficient (Cv): Cv = Q · √(SG/ΔP)
This is the core control valve sizing formula, in US units (Q in gpm, ΔP in psi, for liquids). Example: 100 gpm of water across 25 psi gives Cv = 20.

2️⃣ Kv from Cv: Kv = 0.865 · Cv
Metric datasheets use Kv (m³/h, bar), so the same example gives Kv = 17.3.

3️⃣ Valve authority: β = ΔP_valve / ΔP_system
This tells you how much of the total pressure drop the valve takes. As a rule of thumb, aim for roughly 0.25–0.5 or higher. Too low, and the valve loses control.

4️⃣ Valve characteristic: linear vs equal-percentage

A linear valve gives Q/Qmax = x.
An equal-percentage valve follows Cv = Cv,max · R^(x−1), so at 50% open it passes only 14.1% of Cv,max when R = 50.
Low authority distorts both. At β = 0.5, the linear valve passes 63% of maximum flow at half open.

5️⃣ Rangeability: R = Cv,max / Cv,min
This is how wide a flow range the valve can control. For an equal-percentage valve, R = 50 is typical.

The sequence matters: size the valve (Cv), convert the units (Kv), check authority (β), choose the characteristic, then confirm the range (R).

⚠️ Real valves are usually sized with margin, so they sit part-open at design flow, which lowers authority. Always check β using the actual fully-open ΔP.

💬 Linear or equal-percentage: which do you specify more often, and why? Tell us below 👇

📌 Save this for your next control valve selection

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🔎 Tags: control valve sizing, control valve selection, valve sizing calculation, Cv formula, flow coefficient, Kv conversion, valve authority, equal percentage valve, linear valve characteristic, installed flow characteristic, valve rangeability, process control, instrumentation and control, process engineering, chemical engineering, process plant design

03/10/2026

Every shell and tube heat exchanger design starts with one question: how much heat has to move? These six formulas answer it in order.

1️⃣ Heat duty: Q = ṁ · Cp · ΔT
The heat the hot or cold stream must gain or lose.

2️⃣ LMTD (log mean temperature difference)
The gap between the hot and cold streams changes along the exchanger, so we use the LMTD formula, not a simple average.

3️⃣ Heat transfer rate: Q = U · A · ΔT_lm
Put the duty and LMTD in, and you get the heat transfer area you need: A = Q / (U · ΔT_lm).

4️⃣ Overall heat transfer coefficient (U)
Film, fouling and wall resistances add up in series. The largest resistance dominates, and fouling lowers U.

5️⃣ Reynolds number: Re = ρvD/μ
Flow is laminar below 2300 and turbulent above 4000. Turbulent flow raises the film coefficients, which improves U.

6️⃣ Pressure drop: ΔP = f · (L/D) · (ρv²/2)
This is the Darcy–Weisbach equation. Higher velocity improves heat transfer, but pressure drop grows with v², so you balance U against pumping cost.

💬 Which is harder for you: calculating U or estimating pressure drop? Tell us below 👇

📌 Save this for your next heat exchanger sizing calculation

➕ Follow ChemKlub for more process engineering and chemical engineering concepts

Topics covered: heat exchanger design, heat exchanger sizing, shell and tube heat exchanger, LMTD formula, log mean temperature difference, overall heat transfer coefficient, heat transfer rate formula, fouling resistance, Reynolds number laminar turbulent, Darcy–Weisbach pressure drop, process engineering, chemical engineering

Today, we honour the values of truth, peace, and non-violence that continue to inspire generations. May Mahatma Gandhi’s...
02/10/2026

Today, we honour the values of truth, peace, and non-violence that continue to inspire generations. May Mahatma Gandhi’s timeless ideals guide us towards a more peaceful, responsible, and better world.

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