HVAC and REFRIGERATION

Transcription

1 This is a preview. Some pages have been omitted. PE principles and practice prracticce of engineering mechanical: HVAC and REFRIGERATION sample questions + solutions

2 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 permissions@ncees.org. NCEES PO Box 1686 Clemson, SC ISBN Printed in the United States of America November 2010 First printing April 2013 Second printing

3 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 Morning Solutions Afternoon Solutions Psychrometric Chart iii

4 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. 1

5 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. 9

6 MECHANICAL MORNING SAMPLE QUESTIONS 101. Poisson's ratio is a term that arises in: (A) (B) (C) (D) heat transfer HVAC mechanical behavior vapor power cycles 102. Which of the following is used as a measure of energy efficiency of refrigeration systems? (A) (B) (C) (D) COP Hydraulic head Latent heat 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, 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 Copyright 2011 by NCEES 10 GO ON TO THE NEXT PAGE

7 MECHANICAL MORNING SAMPLE QUESTIONS 119. Which of the following factors is least likely to affect the fatigue life of a steel spring? (A) (B) (C) (D) Alternating stresses Cycle period Primary stresses 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) ,500 lbf Copyright 2011 by NCEES 17 GO ON TO THE NEXT PAGE

8 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. 27

9 HVAC AND REFRIGERATION AFTERNOON SAMPLE QUESTIONS 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 This is a preview. Some pages have been omitted. SURGE DRUM FLOAT VALVE OIL SEPARATOR 20 F BRINE OUT 25 F BRINE IN BRINE COOLER SUCTION 38.5 psia WITH 20 F SUPERHEAT DISCHARGE PRESSURE 229 psia NOTE: Refrigerant entering the compressor is superheated from external sources. COMPRESSOR CONDENSER 95 F WATER OUT 85 F WATER IN RECEIVER Copyright 2011 by NCEES 28 GO ON TO THE NEXT PAGE

10 HVAC AND REFRIGERATION AFTERNOON SAMPLE QUESTIONS 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 The dry bulb temperature of the tempered air (T/A) leaving the wheel is most nearly: (A) (B) (C) (D) 75.9 F 79.0 F 85.0 F 91.0 F ENTHALPY WHEEL AIR HANDLER WITH COOLING COIL O/A 95 F db/78 F wb 1,500 cfm T/A M/A E/A 1,500 cfm S/A 6,000 cfm 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 R/A 75 F db/50% rh 6,000 cfm 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 10 F temperature difference, as shown. For the conditions given, the temperature T 2 of the air leaving the coil is most nearly: (A) (B) (C) (D) 39 F 49 F 52 F 55 F 4,000 cfm T 1 = 35 F 45 F 15-gpm PUMP T 3 = 61.2 F 55 F T 2 =? 5,000 cfm T 4 = 75 F Copyright 2011 by NCEES 37 GO ON TO THE NEXT PAGE

11 HVAC AND REFRIGERATION AFTERNOON SAMPLE QUESTIONS 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 BYPASS CONTROL VALVE (TYP.) AHU-3 75 ft AHU-2 AHU-1 CHILLER PUMP Copyright 2011 by NCEES 49 GO ON TO THE NEXT PAGE

12 MECHANICAL MORNING SOLUTIONS 53

13 MECHANICAL MORNING SOLUTIONS 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) = Annualized salvage value after 10 years $10,000 (A/F, 10%, 10) = 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) 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, 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. CONSECUTIVE DAYS TASK AND DURATION PLANNED A (2) B (2) C (7) REVISED-ACTUAL D (11) E (2) E (2) 16 THE CORRECT ANSWER IS: (B) 55

14 HVAC AND REFRIGERATION AFTERNOON SOLUTIONS 69

15 HVAC AND REFRIGERATION AFTERNOON SOLUTIONS 501. ASHRAE Handbook Fundamentals h COP = 1 h4 = =3.9 h h where: h 1 = entering compressor 38.5 psia, 20 F superheat h 2 = leaving compressor, 229 psia, constant entropy from h 1 to h 2 h 3 = saturated liquid leaving condenser at 229 psia h 4 = entering evaporator constant enthalpy h 3 to h 4 THE CORRECT ANSWER IS: (C) 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 = 7.8 ft 3 /lb 3 THE CORRECT ANSWER IS: (D) 503. Using chart, for Btu/lbm: h at 10 F saturated vapor is 610 Btu/lb, s = 1.32 h at 104 F and s = 1.32 is 740 Btu/lb Work of compression = = 130 Btu/lb psia 10 F 104 F s = c s = 1.32 THE CORRECT ANSWER IS: (B) ENTHALPY, Btu/lb 71

16 504. Refrigeration load: This is a preview. Some pages have been omitted. HVAC AND REFRIGERATION AFTERNOON SOLUTIONS Supply air at 58 F db / 56 F wb v = ft 3 /lb (from psychrometric chart or tables) 20,000 cfm 60 min/hr Leaving air = = 90,566 lb/hr ft /lb Air mix entering cooling coil is 25% outside air and 75% return air and h mix = 32.8 Btu/lb h supply = 23.8 Btu/lb (from psychrometric chart or tables) 815,094 Btu / hr Coil load = 90,566 lb/hr ( ) = = 67.9 tons refrigeration Btu/hr 12,000 ton OR Coil load = 4.5 cfm h = ,000 ( ) 810,000 Btu/hr = 810,000 Btu/hr = = 67.5 tons Btu/hr 12,000 ton THE CORRECT ANSWER IS: (C) 505. Air flowing through the solution with no bypass will leave at the solution temperature. Since 5% is bypassed at 80 F, t 2 = (0.95) (100) + (0.05) (80) = 99 F THE CORRECT ANSWER IS: (D) 506. LAT db is given as 55 F. LAT wb is calculated. 78 F 2,900 cfm 92 F 700 cfm EAT is determined; MAT = 80.7 F db 3,600 cfm From psychrometric chart MAT wb = 66.2 F; h = Btu/lb Q T = cfm (4.5) ( h); cfm = 3,600 h = h ma h la, Q T = SH + LH + OA = 163, 264 Btu/hr 163,264 h = = Btu/lb 4.5 3,600 h ma = = h la = 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) 72

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