HVAC Equipment and Systems for the Mechanical PE Exam

Pumps


The main focus of the pump section is to help the engineer develop an understanding of the major skills and concepts needed for the PE exam. These concepts include (1) determining total head, (2) determining net positive suction head, (3) reading pump curves, (4) using affinity laws. However, in order to focus on these skills and concepts, the engineer will require a brief introduction on pumps, types of pumps and how they work.

There are three main types of pumps, centrifugal, rotary and reciprocating pumps. Rotary and reciprocating pumps are positive displacement pumps. This document will not cover positive displacement pumps in detail because they are not typically used in the HVAC and Refrigeration field. Centrifugal pumps are the most common type of pumps used in HVAC and Refrigeration. The following information is tailored to centrifugal pumps and should not be applied freely to positive displacement pumps.

Centrifugal pumps operate on the principle of "centrifugal force", which is the conversion of rotational kinetic energy imparted by rotating impellers onto the fluid to produce a flow rate (kinetic energy) at a certain pressure (pressure energy). Fluid enters the pump at the center or eye of the impeller, there the rotating impellers push the fluid to the outer edges, imparting a flow rate and pressure. See Figures 1 and 2 for a diagram of the fluid flow.

There are various types of centrifugal pumps, but there are two main families of centrifugal pumps are (1) end suction pumps [refer to Figure 1] and (2) in-line pumps [refer to Figure 2]. These two families differ on the path the water takes from the inlet to the outlet. In the end-suction pumps, the fluid enters the pump at the impeller and exits the pump at a 90 degree angle from the inlet. The in-line pumps have parallel inlets and outlets.

Within each family are horizontal versus vertical pumps, which are characterized by the orientation of the pump shaft as either horizontal or vertical. In addition, pumps can be further classified by the number of stages that the fluid proceeds through. Finally the last classification is how the pump is connected to the motor. Pumps can be long-coupled where the pump is connected to the motor by a flexible coupling or they can be close-coupled where the connection between the pump and motor is through a rigid coupling.

HVAC Economizer for the Mechanical PE Exam



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