GATE CH common mistakes
GATE CH · Understand the traps and how to avoid them.
Writing the answer from a formula sheet without checking dimensions and units of the final number, so a mass balance result is reported in kmol when mol was asked, or a heat duty in kW when kJ/h was required.
GATE CH numerical answer type (NAT) questions accept only a number; candidates rush the arithmetic and skip the sanity check because no options exist to reveal an order-of-magnitude error.
What to do instead
Before entering a NAT value, carry units through every term symbolically, then confirm the final unit matches the question. Do a quick order-of-magnitude estimate (e.g., typical heat duties, typical Re ranges) and reject answers off by factors of 10, 1000, or 3600.
Treating a CSTR and a PFR as interchangeable for a given conversion when the reaction order is not first, then using the wrong volume or residence time.
The design equations look similar and many candidates memorise only the first-order case, where CSTR and PFR volumes differ by a factor but the algebra is simple; for order n they apply the first-order shortcut.
What to do instead
Always start from the mole balance: V = F_A0 X/(-r_A) for CSTR and V = F_A0 ∫dX/(-r_A) for PFR. Substitute the actual rate law, integrate analytically for simple orders, and check that for n>0 the CSTR volume exceeds the PFR volume at the same conversion.
Ignoring the mechanical energy balance friction term or applying Bernoulli with friction but forgetting the pump head, leading to a negative or unrealistic pressure drop in pipe-flow problems.
Bernoulli is over-learned from physics and candidates drop the h_f and pump-work terms when the problem mentions a pump or a long pipe, or they add friction with the wrong sign.
What to do instead
Write the full mechanical energy balance between the two points with every term labelled (pressure, elevation, velocity, pump head, friction head, minor losses). Compute friction factor from the correct regime (64/Re laminar, Colebrook or Moody turbulent) and include minor losses as ΣK v²/2g.
Reading a McCabe-Thiele or psychrometric-style construction from memory and misplacing the operating line or q-line, giving a wrong reflux ratio or number of stages.
These graphical methods are taught once and candidates reconstruct them from vague memory rather than deriving slopes and intercepts from material balances.
What to do instead
Derive the rectifying line slope L/V and intercept from the reflux ratio and distillate composition, derive the stripping line from the boil-up ratio and bottoms composition, and locate the q-line intersection from feed quality q. Compute stages by stepping between the equilibrium curve and the operating line, counting the reboiler correctly.
Confusing heat-transfer coefficients in series: adding convective resistances without the area ratio, or forgetting the wall conduction resistance and fouling terms in an overall U calculation.
The formula 1/U = 1/h_i + 1/h_o is memorised for a plain thin wall and applied to a thick-walled tube or a fouled exchanger, dropping the r_o/r_i factors.
What to do instead
Write the overall resistance per unit length or per unit outside area explicitly: 1/(U_o A_o) = 1/(h_i A_i) + ln(r_o/r_i)/(2πkL) + R_f,i/A_i + R_f,o/A_o + 1/(h_o A_o). Check which area U is based on before substituting numbers.
Using the wrong log-mean temperature difference, e.g., plugging arithmetic mean temperatures or forgetting the correction factor F for a multipass or cross-flow exchanger.
LMTD is a single formula and candidates apply it blindly even when the flow arrangement requires an F factor, or they subtract temperatures in the wrong order and get a negative log argument.
What to do instead
Compute ΔT1 and ΔT2 at the two ends consistently, take the LMTD, and multiply by F from the appropriate chart (F=1 only for true countercurrent). If the two end differences are nearly equal, use the arithmetic mean as the limiting case.
Mixing up molar and mass basis in material and energy balances, especially converting between mole fraction and mass fraction for a multi-component stream.
Problems state compositions in one basis and ask for flows in another; candidates multiply mole fraction by total mass flow directly.
What to do instead
Set up a small conversion table: component, mole fraction, molecular weight, mass fraction, mass flow, molar flow. Convert via average molecular weight M = Σ y_i M_i, and verify Σ mass fractions = 1 and Σ mole fractions = 1 before proceeding.
Applying the wrong thermodynamic relation, such as using ideal-gas enthalpy for a real gas or using Cp instead of Cv in a closed-system energy balance.
The ideal-gas relations are the ones most practised, and candidates do not check whether the problem specifies a real fluid, a saturated mixture, or a constant-volume process.
What to do instead
Identify the system (open/closed), the process (isochoric, isobaric, adiabatic, isothermal), and the fluid model before writing the first law. Use dU = Cv dT and dH = Cp dT only for ideal gases; for steam use tables or quality-based values, and for real gases use residual properties or departure functions.
Skipping the units in reaction kinetics and using k in the wrong order-dependent units, so a second-order rate constant is entered with first-order units and the resulting volume is wrong.
Rate constants are often quoted numerically without units in textbooks, and candidates plug them in without checking consistency with the concentration units used in the rate law.
What to do instead
Write the units of k from the rate law: for -r_A = k C_A^n, k has units of (mol/volume)^(1-n)/time. Convert all concentrations to the same basis (mol/L or mol/m³) and all times to the same unit before evaluating the design equation.
In the General Aptitude and Engineering Mathematics sections, spending too long on one multi-step numerical question and leaving easy 1-mark questions unattempted or guessed blindly.
The paper is scored per question with negative marking on MCQ items, and candidates over-invest in hard numericals early, running out of time for high-yield quick items.
What to do instead
Do a first pass answering all quick, high-confidence questions, flagging the rest. Attempt NAT questions since there is no negative marking for wrong NAT answers on most GATE papers, and only guess MCQs when at least one option can be eliminated. Budget roughly 1.5 minutes per mark and reserve the last 20 minutes for flagged items.