Heat exchangers in commercial dairy processing:

• Double pipe heat exchangers have low surface area-to-volume ratios and are used for small duties or viscous pastes.
• Scraped surface heat exchangers (SSHE) are designed for highly viscous, sticky, or crystallizing products like ice cream and ghee.
• U-tube heat exchangers are shell-and-tube configurations primarily used for clean steam or high-pressure chemical duties.
• Plate Heat Exchangers (PHE) are the universal industry standard for continuous milk pasteurization (HTST) due to high heat transfer coefficients ($U = 3000\text{--}5000\text{ W/m}^2\cdot\text{K}$), compact design, easy CIP cleanability, and high thermal regeneration efficiencies exceeding $90\text{--}95\%$.

Pressure requirements in Ultra-High Temperature (UHT) sterilization systems:

• $6\text{--}8\text{ psi}$ ($0.4\text{--}0.55\text{ bar}$) is below atmospheric steam saturation pressure and milk would instantly boil.
• $2\text{--}4\text{ bar}$ corresponds to saturation pressures at $120\text{--}143^\circ\text{C}$, but lacks sufficient operating overpressure margin to suppress boiling, prevent cavitation, and overcome piping hydraulic resistance.
• In continuous UHT processing systems (operating at $135\text{--}150^\circ\text{C}$), the product must be maintained under substantial internal overpressure ($6\text{--}8\text{ bar}$) throughout the heating and holding sections to prevent boiling, suppress flash evaporation, overcome pressure drops across plate/tubular geometries, and maintain sterile forward flow through the back-pressure valve.
• $2\text{--}4\text{ psi}$ is far below minimum requirements.

Regeneration efficiency in an HTST plate pasteurizer is defined as the ratio of temperature increase achieved in the regenerator to the maximum possible temperature increase required for pasteurization:

$$\%\,\text{Regeneration Efficiency} = \frac{T_{\text{regen}} - T_{\text{initial}}}{T_{\text{pasteurization}} - T_{\text{initial}}} \times 100$$

Given:
$T_{\text{initial}} = 4^\circ\text{C}$, $T_{\text{regen}} = 67^\circ\text{C}$, $T_{\text{pasteurization}} = 74^\circ\text{C}$

$$\%\,\text{Regeneration Efficiency} = \frac{67 - 4}{74 - 4} \times 100 = \frac{63}{70} \times 100 = 0.90 \times 100 = 90\%$$

Option analysis:

• $80\%$: Incorrect calculation.
• $75\%$: Incorrect calculation.
• $90\%$: Correct, exact regeneration efficiency.
• $85\%$: Incorrect calculation.

Option analysis:
(A) In continuous HTST regenerator systems, the raw milk booster pump is located in the raw regenerator circuit to deliver milk through filters, while a positive differential pressure is maintained on the pasteurized side by the timing pump and backpressure control valve.