Original Deck Flashcards
If asked to find characteristics of power or mass flow rate of a cycle, what should you consider and work?
When asked about cycle, use the net work!
For Braytons factor in compressor input
For Rankine factor in pump work
Don’t just use turbine outputs
Orifice square vs sharp edge?
C for Sharp does not equal C for square; C square is not in the handbook.
Given diameter ratio use this C in the equation
Q=CAosqrt(2/pdP)
What should the difference between “Output” and “Work Required” alarm you to?
Whether or not to use efficiency in the equation or not.
Asked to find rate of heat loss of pipe within a room and given emissivity, what should you remember?
Total rate of heat loss is heat loss from convection + heat loss from radiation.
Key aspect of the NPSH equation?
hz is negative if the liquid is below the pump
Equation for calculating the center of pressure?
Yp = yc + Ic / ycA
Important reminder about junction flow of fluid?
Total flow into junction = total flow out of junction
- How do you find the thermal efficiency of a combined cycle?
- What should always first think of when asking to find efficiency of combined cycles?
- Eff_combined = Wnet_combined / Qin
Wnet_combined = wnet1 + m1/m2*wnet2 - Mass flow rate ratio!!
Dealing with Thermodynamics what is the first thing to do and once that is done what is important to remember about units?
For closed system what is equation for dU?
- Identify if the system is either open or closed.
- Always use absolute temperature and absolute pressure in PV=mRT
- dU = Q- W
1 Knot =
1.68 FT/S
- What is another word for hoop stress?
- Difference between thin and thick wall cylinder?
- When given a cyclinder problem what is the fist thing you should do?
- Circumferential or “Hoop Stress”
- Equation in handbook and radial stress no significant in thin walled but significant in thick walled cylinders
- Determine if it is thin or thick wall
Two sets of steam streams leaving a turbine, what should you remember?
Don’t need both to find efficiency, just use whichever one is being asked for.
What is one equation for finding LHV?
LHV = HHV - m*hfg
What is important to remember about the difference in specific heats Cv and Cp?
Cv is for constant volume processes while
Cp is for constant pressure processes
Using a pitot tube to determine the velocity of air flowing through a circular AC duct, which measurements should you use?
Average of three measurements of mid radius of three equal annular areas and one at the center of an equal central circular area.
If given the Nusselt number what can you calculate?
heat transfer coefficient and thermal conductivity h or k
Nu = hD/k
Finding mean effective pressure / hp of a multi cylinder equation what is misleading about handbook equation?
What should remember about a?
hp = MEP*LAN / 33,000, but this doesn’t include n
hp = PLANn/33000 N = number of power strokes which is rpm/2 for four stroke engines n = number of cylinders
A=total piston area so that means area of one position! If given total volumetric displacement divide by the number of cylinders
What is the equation for the bypass factor in psychrometrics? What is the equation for efficiency of the coil?
BF = (t2 - ADP) / (t1 - ADP) t2 = leaving coil temp t1 = entering coil temp
Coil Efficiency = 1 - BF
What is the quantity of heat produced as a result of complete combustion of a fuel including heat of vaporization?
If combustion is not complete what is in the products (flue gases)?
- HHV
2. Carbon Monoxide
Difference between equation for delta U vs delta H?
Delta U = Cv*delta T
Delta H = Cp*delta T
- Given a Heat Exchanger problem and overall heat transfer coefficient (U), what do you always use for T and what is the equation?
- For multi pass heat exchangers?
- How do you find mass flow rate in a heat exchanger?
- Tlm is always used with heat exchangers and U
Q=UAFTlm - Multi Pass HE - always use counterflow Tlm
- Once you have overall Q from UAFTlm you can use it to find mass flow rate of individual stream by setting equal to Q = m dh
How do you find ADP using psychrometric charts?
- Plot the air entering coil and air exiting coil locations on the chart
- Connect the dots
- Extend the connector line to the saturation curve
Moles vs mass effect on the ideal gas law equation?
Continuity equation difference for gases vs. liquids?
PV = mRT, or PV = nRbarT, or PAV = mdot RT - ideal gases and air in compressors!
Continuity Eq Gases - mdot = pAV - ok to use for steam
Continuity Eq Liquids - Q = AV
Continuity equations used for non ideal gases when Temp isn’t given.
p1A1V1 = p2A2V2 A1V1 = A2V2
When the dry bulb temperature of air increases, what remains constant in psychrometrics?
Humidity Ratio
Now you have two points Tdb2 and W2
What is the thermal efficiency equation for a Brayton Cycle with regeneration?
Describe equation in terms of temperature.
How do you determine regenerator effectiveness?
What piece of equipment supplies Qin?
eff = Wout - Win / Qin eff = (T4-T5) - (T2-T1) / (T4-T3)
Qin = Qin through combustor
Regenerator effectiveness EQ give in handbook
If you’re dealing with multilayer heat transfer what equation and factor should you think of?
Overall Heat Tranfer Coefficient
q=UA(Ta - Tb)
U = 1 / [(1/h1) + (dx/k) + (1/h2)....] U = 1 / Rtot
What is important to remember about Carnot cycle problems?
Units of T are absolute
Equation for calculating fan power and where to find it in the handbook?
Fan BHP or Supply power
P = Qs DPs / 6356 n
Qs = Flow in CFM P = BHP in hp DPs = dSP + dVP
SP = P/.0361 inches H20
VP = (V/4005)^2 inches H20
where V is in FT/MIN
What is important to remember about refrigerant cycle performance?
COP equation for heat pump and COP equation for refrigeration are different equations.
Examples of not knowing when to use Pressure Vs Temp table?
2001 #523 - h3
2001 #536
If given T and P but don’t know if it’s saturated or not, use T
What is the equation for delta P across a pump?
Delta P pump = y*hp
y = specific weight of fluid
hp = head of pump required
What is important to remember about pump power problems?
It is easy to go back and forth between dP across pump and the head produced by the pump if you use gamma.
When you need f to calculate Reynolds number, what should you think of?
Find e/D and Re, use the Stanton moody diagram to find f, plug into Darcys
What is the efficiency of combustion equation?
n = useful heat extracted / hearing value = msteam*(hsteam - hfw) / LHV = LHV - heat losses / LHV
Using steam tables based on given pressure always confirm what…?
It is PSIA not PSIG
Equation for equivalent length in Fluids
Leq = kD/f
What is the SI unit for Force?
1N = 1 kg * 1 m/(s2)
- What is the most important characteristic to use when calculating air properties using air table?
- What can you find that’s important to power calculate from air tables?
- How do you calculate compressor?
- Relative pressure value
- Enthalpy
- Mass flow rate * change in enthalpy - BTU/hr - transfer to Watts
What is the equation for thermal expansion of a metal pipe?
Change in length = alphaLdT
Units - Feet
- Asked to find compressor power required to compress air what is a good equation to use?
- If there are two exit streams from a compressor that is compressing air, what is the equation?
- What Eq is preferred for compressor problems?
- W = mdot (h2-h1), or
W = mdot cp dT —— may mean you don’t need to use the gas tables
Convert units to hp if needed. - W = mdotcp dT
W = mdot2cp(T2-T1) + mdot3cp(T3-T1)
Always factor in efficiency in Work problems
- McdT
How do you determine the psychrometric properties of a mixture?
- Plot outside air and return air based on properties
- Draw line between
- Calculate db temp mixture
4 plot dry bulb mixture temp on connecting line between OA and RA points
What is another equation for R?
What is another equation for k in thermo?
R= Cp - Cv
K=Cp/Cv
What is the key to solving efficiency problems for complex processes?
What should you consider when calculating thermal efficiency of a combined cycle?
- Wnet/Qin
2 mass flow rate*enthalpy; mass flow rate ratio of work to heat will affect the thermal efficiency in a combined cycle
Heat transfer equation?
q=?
Multilayer Conduction?
q=dT/R
Driving Force/Resistance
Overall dT / sun of resistances
Resistance equation differs for Flat wall, cylinder, and sphere
*Heat transfer through each layer is the same as overall heat transfer
Specific Weight (weight/volume) and density of water (mass/volume)?
y = 62.4 lbm/ ft3 p = 1.94 slugs/ ft3
Conversion to US customary for viscosity?
1 lbf *s / ft2 = 479 Poise
What is the equation for the utilization factor in a cogeneration plant?
Eu = (Wt + Qp) / Qin Qp = Process/FW Heat Output Wt = Work of Steam Turbine Output Qin = Boiler Heat Addition
What is the j value conversion in thermo?
When do you see this value used often?
778 ft lbf for 1 BTU
Calculating h/w for a pump using w=vdP
You have air flowing through a compressor and you need to find exit temp of air to use in the efficiency equation, what equations should you consider using?
You can use the constant entropy temp/pressure/v relationships for ideal gases given at the beginning of the thermo section.
When asked to find the amount of air or water circulating through a cooling tower, which two equations should you remember?
How do you find the h of water?
Energy Balance
Mw1hf1 + Maha1 = Mw2hf2 + Maha2
*mass flow rate of water different for inlet and outlet
*combine to get mw2 out of the equation
Mass Balance of Water
Mw2 = Mw1 - (w2 - w1)ma
Sat steam tables - use hf for cooling tower problems
Feed water problems involve similar equations (mass balance and energy balance) but don’t require quality/humidity conversations - just us the open feed water heater equation given in the handbook.
When do you use the NTU method? (Number of Exchange Transfer Units)
When you don’t have both exit temps
- How do you find the enthalpy added from a pump ?
2. What else can you find with this equation?
h = v(P1-P2)144/n778 —- BTU/lbm
It is also the equation for work and power required pump…
W= v(P1-P2)144/n778
P=mdot(h) ——convert to horsepower
***think relationship between pressure volume and enthalpy when calculating pump power required.
When calculating tons of refrigerant required what does this equate to in the physical process of refrigerators?
Heat Absorbed by the Evaporator
What is the molar ratio for air?
What is the molecular weight of carbon, oxygen, nitrogen, and air, hydrogen?
1 mole O2 + 3.76 moles N2 = 4.76 moles air
Oxygen = 32 Carbon = 12 Nitrogen = 28 Air = 29 Hydrogen = 2
Emmisivity or Radiation problem what do you need to remember about A?
What is the equation for heat loss from radiation?
As is SURFACE AREA!
Q = EOAs(T4-T4)
- What curves do you follow in psychrometrics to get enthalpy?
- What curve do you follow in psychrometrics to get dew point?
- Wet bulb temp, you don’t even need dry bulb temp to get the enthalpy number.
- Find intersection, draw horizontal line to left to sat temp.
- What is the primary difference between and Brayton and a Rankine Cycle?
- In a combined cycle what is important to remember about the pressure characteristics in the Rankine cycle?
- When asked to find the mass flow rate ratio in a combined cycle, where should you focus the energy balance equation?
- What type of cycle in the equation handbook is similar to a cogeneration cycle?
- Brayton does not undergo phase change (typically gas and exhaust gases tied into HRSG/rankine). Brayton involves a compressor.
Rankine does go through a phase change (steam to condensed liquid). Rankine involves a pump.
- Pressure is constant through HRSG/Boiler; Pressure is constant through condenser.
- Can determine mass flow ratio by using energy balance equation at the HRSG.
- A cogeneration cycle is similar to a Rankine cycle with regeneration. A Rankine with regeneration sends extraction steam to a feed water heater, where a cogeneration facility sends extraction steam to a process heater.
What is the equation for cooling tower range?
Range = Temp difference between the entering and leaving water
Key to Haden Williams EQ in Handbook?
Hf - loss ft/ft of pipe P - Pressure loss / ft pipe in PSI Q- Flow in GPM D - inches! C - Hazen Williams Coefficient.
What is the equation for stagnation pressure that is not in the handbook?
Stag. pressure = (SG)pgh + (SG)pV^2/2
Force on a submerged plave?
F=yHcA
When asked to find the cooling capacity of an air handling system with coil what equation should you use and remember?
Use the equation for total heat gain
Q = 60.075CFM*dh
Remember dh is across the coil - entering coil temp and leaving coil temp
Convert to tons by dividing by 12,000
How do you find the dew point of flue gas products? Why is the dew point important to monitor?
Find the partial pressure of water in the products, look up in steam tables and find corresponding Tsat the partial pressure of H2O.
Because if temp fall below this value water condenses and combines with products to create a corrosive substance
Given manometer problem and asked to find pressure difference and only thing given is gauge difference, what equation do you use?
Given manometer problem and ask to find feet of water based on gauge difference?
Given nanometer problem and asked to find density given fluid height and dP?
- P1-P2 = h(ym- yw)
- h = hm (SGm/SGw - 1)
- P1-P2/h = density
Total heat gain of a an air supply in a room when given sensible, latent?
Heat gain at the entering coil location:
Qt = Sh + Lh + OA
Qoa = 4.5*CFM*dh dh = hoa - hma Qt = 4.5*CFM *dh dh = h entcoil - h exitcoil
Always confirm if you need to use superheated steam table by…?
Checking to see if T is greater than Tsat based on the pressure table
Flow resistance for what valve below is the least?
Ball
Gate
Globe
Needle
Ball
Typical assumptions for Hazen Williams Coefficient?
C=140 for new water pipe
C=100 for used water pipe
Sludge? C is usually 20-40% lower
- There are two streams exiting a steam turbine and asked to find mass flow rate or output, what equation do you use?
- Where might this type of situation occur other than dedicated turbine problems?
- W = m1(h1 - h2) + m3(h2 - h3)
State 3 is the 1 PSIA state - Cogeneration cycles with process heat or fw heat that uses extraction steam. Same equation above applies to find the work of the stream turbine.
How do you calculate Darcy’s friction factor for laminar flow?
f = 64 /Re
Eq given in the Stanton moody chart
Given sensible and latent heat and asked to calculate required return air volume. What is the most important thing to remember?
CFM is based on the sensible load! Sensible capacity = 1.09cfmdTdb
What is the equation for thermal efficiency of a reheater rankine cycle?
n = (Wt1 + Wt2 - Wp) / (Qb + Qr)
Can easily be translated to enthalpies.
Important characteristic about throttling (pressure reducing)?
It is a constant enthalpy process.
What is the most important habit to do before every problem.
Write out the units required!
What is the heat of vaporization of water at atmospheric pressure? Why is it important to know this when water vapor is released in a room?
Heat of Vaporization of Water = 970 BTU/lbm
Water vapor released into room mass flow*heat of vaporization is equal to the latent heat released.
If f is not given what equation should you consider for calculating head loss due to friction?
Hazen Williams as f is not part of the equation.
Given characteristic curves of pump and system point A, what is minimum pumps in parallel needed to deliver flow?
Given characteristic curves of a pump and asked to calculate supply head or supply flow of x number of pumps?
Find flow of system curve, and divide by flow of one pump at system head.
Draw new pump curves (based on parallel or series) and point of intersection with system curve, find flow and head at intersection point.
Asked to calculate flow rate of a nozzle, what equation should you use?
Flow rate for a Venturi meter which is readily available in the handbook.
When finding efficiency of a combined cycle what is an easy trick when calculating efficiency?
Assume mass flow rate of one cycle is 1, and use ratio to calculate other. Since efficiency is a percentage, these assumptions will work.
Efficiency allows assumption of mass flow rate if you know ratio for combined cycle.
What is the first key to doing power plant combustion problems?
Consider individual reactions…
How do you find partial pressure of sea level air based on osychrometrics?
Find the dew point, find the psat at dew point = partial pressure of air
