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Heat Transfer - Engineering

Q1:

In a forward feed multiple effect evaporator unit

A viscosity of liquid is highest in first effect.

B transfer from effect to effect is done by pumps.

C no pump is required to withdraw the product from the last effect.

D none of these.

ANS:D - none of these.

  1. "Viscosity of liquid is highest in the first effect":
In a multiple effect evaporator system, the feed liquid is heated and evaporated in stages, typically consisting of several evaporator units called effects. As the liquid moves from one effect to another, its viscosity may change due to concentration changes, temperature changes, or both. In many cases, the liquid being fed into the first effect is more concentrated and therefore has a higher viscosity compared to subsequent effects where the concentration decreases. This higher viscosity in the first effect can affect the heat transfer efficiency and overall performance of the evaporator system.
  1. "Transfer from effect to effect is done by pumps":
In a multiple effect evaporator system, transfer of liquid from one effect to another is necessary to maintain the continuous process of evaporation. This transfer is typically accomplished using pumps. Pumps are used to overcome pressure drops, friction losses, and other hydraulic resistances encountered as the liquid moves through the various stages of the evaporator system. The pumps ensure a steady flow of liquid from one effect to the next, allowing for efficient heat transfer and evaporation.
  1. "No pump is required to withdraw the product from the last effect":
In a forward feed multiple effect evaporator system, the pressure decreases as the liquid progresses through each effect. The pressure in the last effect is typically lower compared to preceding effects. As a result, when the liquid reaches the last effect and has been concentrated to the desired level, the product can be withdrawn without the need for a pump. The lower pressure in the last effect allows for the product to flow out under gravity or with minimal assistance, such as using a simple outlet valve or gravity-driven discharge system. This eliminates the need for additional pumping equipment and reduces operational complexity and cost. So, in summary:
  • The viscosity of the liquid is highest in the first effect due to its initial concentration.
  • Transfer from effect to effect is typically done by pumps to maintain continuous flow and overcome hydraulic resistances.
  • No pump is required to withdraw the product from the last effect due to the lower pressure, allowing for gravity or minimal assistance in product discharge.