pe · pdf file · 2013-10-25mechanical: hvac and refrigeration pe principles and...

sample questions + solutions

mechanical: HVAC and REFRIGERATION

PEprinciples and practiceof engineering

racticprinciples and pr ceof engineering

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Copyright ©2011 by NCEES. All rights reserved. All NCEES sample questions and solutions are copyrighted under the laws of the United States. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means without the prior written permission of NCEES. Requests for permissions should be addressed in writing to [email protected]. NCEES PO Box 1686 Clemson, SC 29633 800-250-3196 www.ncees.org ISBN 978-1-932613-57-5 Printed in the United States of America November 2010 First printing April 2013 Second printing


iii

CONTENTS

Introduction to NCEES Exams ......................................................... 1

About NCEES

Updates on exam content and procedures

Exam-day schedule

Admission to the exam site

Candidate Agreement

Scoring and reporting Exam Specifications .......................................................................... 3

Sample Questions and Solutions

Morning Sample Questions ................................................................. 9

Afternoon Sample Questions ............................................................. 27

Morning Solutions .............................................................................. 53

Afternoon Solutions ........................................................................... 69

Psychrometric Chart ....................................................................... 83


1

About NCEES NCEES is a national nonprofit organization dedicated to advancing professional licensure for engineers and surveyors. It develops, administers, and scores the examinations used for engineering and surveying licensure in the United States. It also facilitates professional mobility and promotes uniformity of the U.S. licensure processes through services for its member licensing boards and licensees. These services include the records program, study materials, credentials evaluations, exam administration, and more. NCEES is composed of member licensing boards from the United States and its territories. Updates on exam content and procedures NCEES.org is our home on the Web. Visit us there for updates on everything exam-related, including specifications, exam-day policies, scoring, and corrections to published study materials. This is also where you will register for the exam and find the additional steps you should follow in your state to be approved for the exam. Exam-day schedule Be sure to arrive at the exam site on time. Late-arriving examinees will not be allowed into the exam room once the proctor has begun to read the exam script. The report time for the exam will be printed on your Exam Authorization. Normally, you will be given 1 hour between morning and afternoon sessions. Admission to the exam site To be admitted to the exam, you must bring two items: (1) your Exam Authorization and (2) a current, signed, government-issued photo identification. Candidate Agreement The NCEES Candidate Agreement describes references and other personal materials allowed in the exam room. It also describes other exam policies designed to protect exam content. This document will be printed in your exam book. It is also available for download at NCEES.org. If you register to take an NCEES exam, you are strongly encouraged to review this document prior to exam day, as you will be asked to affirm that you abide by the policies and procedures it describes. Scoring and reporting NCEES typically releases exam results to its member licensing boards 8–10 weeks after the exam. Depending on your state, you will be notified of your exam result online through your My NCEES account or via postal mail from your state licensing board. Detailed information on the scoring process can be found at NCEES.org.


9

MECHANICAL

MORNING SAMPLE QUESTIONS

A psychrometric chart (normal temperature at sea level) is provided on page 83 for your possible use. Unless

stated otherwise, assume sea level conditions.


MECHANICAL MORNING SAMPLE QUESTIONS

Copyright 2011 by NCEES 10 GO ON TO THE NEXT PAGE

101. Poisson's ratio is a term that arises in:

(A) heat transfer

(B) HVAC

(C) mechanical behavior

(D) vapor power cycles 102. Which of the following is used as a measure of energy efficiency of refrigeration systems?

(A) COP

(B) Hydraulic head

(C) Latent heat

(D) SHR 103. A system with an initial cost of $90,000 has an operating cost of $2.50/hr and is operated

16 hr/day for 300 days/year. After 10 years, the system has a salvage value of $10,000. Assuming the interest rate is constant at 10%, the total annualized cost over the 10-year life is most nearly:

(A) $12,000 (B) $14,000 (C) $20,000 (D) $26,000

104. You manage a manufacturing production department that operates 80 hr/week. The process has a

production rate of 60 units/hr, with a scrap rate of 1 out of 120 units. With a 10% lost production time allowance, the number of good units that can be made per week is most nearly:

(A) 4,284 (B) 4,320 (C) 4,760 (D) 4,800


MECHANICAL MORNING SAMPLE QUESTIONS


2,500 lbf

2

1

3

4

119. Which of the following factors is least likely to affect the fatigue life of a steel spring?

(A) Alternating stresses

(B) Cycle period

(C) Primary stresses

(D) Surface roughness 120. The 2,500-lbf load shown in the figure is supported by a beam that is secured by six 3/4-in.-

diameter bolts in a hexagonal array with neighboring bolts 3 in. apart. The shear stress is greatest in which bolt?

(A) 1 (B) 2 (C) 3 (D) 4


27

HVAC AND REFRIGERATION

AFTERNOON SAMPLE QUESTIONS

A psychrometric chart (normal temperature at sea level) is provided on page 83 for your possible use. Unless

stated otherwise, assume sea level conditions.


HVAC AND REFRIGERATION AFTERNOON SAMPLE QUESTIONS


95°F WATER OUT

85°F WATER IN

RECEIVER

CONDENSER

COMPRESSOR

BRINE COOLER

SURGE

DRUMFLOAT

VALVE

20°F BRINE OUT

25°F BRINE IN

OIL

SEPARATOR

SUCTION

38.5 psia

WITH 20°F

SUPERHEAT

DISCHARGE

PRESSURE

229 psia

501. An ammonia (R-717) refrigerant brine chiller with a flooded evaporator is shown below. Properties of saturated ammonia are presented on page 29, and a pressure-enthalpy diagram for ammonia appears on page 30. Assuming no pressure loss through components, the coefficient of performance (COP) of the system shown, with a refrigerant flow rate of 22 lb/min, is most nearly:

(A) 3.0 (B) 3.2 (C) 3.9 (D) 4.1

NOTE: Refrigerant entering the compressor is superheated from external sources.




4,000 cfm

T1 = 35°F

5,000 cfm

T4 = 75°F

T3 = 61.2°F

T2 = ?

45°F

55°F

15-gpm PUMP

LEGEND:

O/A = OUTSIDE AIR M/A = MIXED AIR

E/A = EXHAUST AIR S/A = SUPPLY AIR

T/A = TEMPERED AIR R/A = ROOM AIR

ENTHALPY WHEELAIR HANDLER WITH

COOLING COIL

T/A M/A

S/A

6,000 cfm

R/A

75°F db/50% rh

6,000 cfm

O/A

95°F db/78°F wb

1,500 cfm

E/A

1,500 cfm

510. The system shown has an enthalpy wheel that is 80% effective in both sensible and latent. The cooling coil has a capacity of 17.1 tons with a sensible heat ratio of 0.70. The dry bulb temperature of the tempered air (T/A) leaving the wheel is most nearly:

(A) 75.9F (B) 79.0F (C) 85.0F (D) 91.0F

511. A water (sp gr = 1.0) run-around cycle is shown in the figure below. Assume the coil bypass

factor is negligible and there is no heat gain or loss in the piping system. The conditions are such that no condensing occurs at the coils. The water loop operates at a 10F temperature difference, as shown. For the conditions given, the temperature T2 of the air leaving the coil is most nearly:

(A) 39F (B) 49F (C) 52F (D) 55F




BYPASS

CONTROL VALVE (TYP.)

AHU-3

AHU-2

AHU-1

CHILLER

PUMP

75 ft

535. A direct-pumped variable-flow chilled water system, serving three air handlers in a building, is shown in the flow schematic. AHU-3 and the system bypass are located 75 ft above the main mechanical room housing the chiller and pump. Chiller selected for:

500 gpm, 10 T 20-ft-head evaporator pressure drop. Chiller rated for 50% minimum flow

Each OAHU is selected for: 12 T, 150 gpm, 10-ft-head pressure drop.

The control valves are selected for a maximum 5-psi pressure drop at the design flow rate. For proper system operation, the minimum required differential pressure (psi) between the chilled water supply and return headers measured at the chilled water bypass is most nearly:

(A) 80 (B) 45 (C) 35 (D) 10


53

MECHANICAL

MORNING SOLUTIONS


MECHANICAL MORNING SOLUTIONS

55

CONSECUTIVE DAYS TASK AND DURATION PLANNED

REVISED-ACTUAL

A (2) B (2) C (7)

D (11) E (2)

E (2)

0 2 4

13 14 15

16

101. Poisson's ratio is a measure of how a material deforms when acted upon by a force. It is applied in situations where the mechanical behavior of a machinery component is important.

THE CORRECT ANSWER IS: (C)

102. The coefficient of performance (COP) is a measure of energy efficiency of refrigeration systems.

THE CORRECT ANSWER IS: (A)

103. Annualized purchase price over 10 years $90,000 (A/P, 10%, 10) = 0.1627

Annualized salvage value after 10 years $10,000 (A/F, 10%, 10) = 0.0627

Annual operating cost = (300 days/yr)(16 hr/day)($2.50/hr) = $12,000

Total annual cost = $90,000(0.1627) – $10,000(0.0627) + $12,000 = $26,016

THE CORRECT ANSWER IS: (D) 104. 80 hours – 10% lost production time = 72 hours

72 hours × 60 units/hour = 4,320 units 1 scrap/120 units × 4,320 units = 36 scrap 4,320 – 36 units = 4,284 units

THE CORRECT ANSWER IS: (A)

105. If Task C is delayed by 3 days, but has 2 days to spare while waiting on Task D, then the total

compensation delay will be 1 day of delivery time.

THE CORRECT ANSWER IS: (B)


69

HVAC AND REFRIGERATION

AFTERNOON SOLUTIONS


HVAC AND REFRIGERATION AFTERNOON SOLUTIONS

71

psia

104°F

10°F

610 740ENTHALPY, Btu/lb

s = 1.32

s = c

501. ASHRAE Handbook–Fundamentals

COP = 1 4

2 1

h hh h

= 627.0 161.1 = 3.9745.0 627.0

where: h1 = entering compressor 38.5 psia, 20F superheat h2 = leaving compressor, 229 psia, constant entropy from h1 to h2 h3 = saturated liquid leaving condenser at 229 psia h4 = entering evaporator constant enthalpy h3 to h4


502. From P-H diagram at 38.5 psia, saturation temperature is 10°F. Actual suction temperature is

10°F + 20°F (superheat) = 30°F

3 = 7.8 ft3/lb

THE CORRECT ANSWER IS: (D) 503. Using chart, for Btu/lbm:

h at 10F saturated vapor is 610 Btu/lb, s = 1.32 h at 104F and s = 1.32 is 740 Btu/lb Work of compression = 740 – 610 = 130 Btu/lb THE CORRECT ANSWER IS: (B)


HVAC AND REFRIGERATION AFTERNOON SOLUTIONS

72

504. Refrigeration load:

Supply air at 58°F db / 56°F wb v = 13.25 ft3/lb (from psychrometric chart or tables)

Leaving air = lb/ft25.13

hrmin/60cfm000,203

= 90,566 lb/hr

Air mix entering cooling coil is 25% outside air and 75% return air and

hmix = 32.8 Btu/lb hsupply = 23.8 Btu/lb (from psychrometric chart or tables)

Coil load = 90,566 lb/hr (32.8 – 23.8) =

ton

hr/Btu000,12

hr/Btu094,815 = 67.9 tons refrigeration

OR

Coil load = 4.5 cfm h = 4.5 20,000 (32.8 – 23.8)

= 810,000 Btu/hr =

ton

hr/Btu000,12

hr/Btu000,810 = 67.5 tons


505. Air flowing through the solution with no bypass will leave at the solution temperature.

Since 5% is bypassed at 80F, t2 = (0.95) (100) + (0.05) (80) = 99F

THE CORRECT ANSWER IS: (D) 506. LAT db is given as 55F.

LAT wb is calculated.

EAT is determined; MAT

cfm600,3

cfm700F92cfm900,2F78 = 80.7F db

From psychrometric chart MAT wb = 66.2F; h = 30.99 Btu/lb

QT = cfm (4.5) ( h); cfm = 3,600 h = hma – hla, QT = SH + LH + OA = 163, 264 Btu/hr

h = 600,35.4

264,163 = 10.08 Btu/lb

hma = 30.99 – 10.08 = 20.91 hla = 51.1°F wb EAT = 80.7°F db / 66.2°F wb; LAT = 55°F db / 51.1°F wb

THE CORRECT ANSWER IS: (B)


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