1) The latent load of a building is needed to calculate __________.

A) Heating only

B) Cooling only

C) Heating and cooling

Admin (HW5)

• Cooling is sized in Tons• 12,000 BTU/hr = 1 ton = cooling capacity of 1 ton

of ice• 1 W = 3.413 BTU/hr• 1 BTU is the amount of energy it takes to raise 1 lb

of water 1 °F

• Heating systems are sized in kBTU/hr or W

Internal gains

• What contributes to internal gains?

• How much?

• What about latent internal gains?

• ASHRAE Fundamentals ch. 29• Table 1 – people

• Table 2 – lighting, Table 3 – motors

• Table 5 – cooking appliances

• Table 6 -10 Medical, laboratory, office

Internal Gains - Computers

• NYT article on Intel’s newest chip and strategy

• Power users want efficiency, not speed

• “It turns out, Dr. Schmidt told the audience, that what matters most to the computer designers at Google is not speed but power — low power, because data centers can consume as much electricity as a city.”

Putting it all together

• Conduction through walls, roof, floor, windows• Infiltration and ventilation• Slab edge (ground contact)

• Multiply “UA” by design ΔT

• Cooling load (different design ΔT)• Add solar gain through opaque surfaces and through glazing• Add sensible internal gains

• Latent cooling load (using cooling t and t* to get outdoor conditions)• Latent loads due to infiltration and due to latent internal gains

2)What do you need to know to calculate sensible internal gains for a

computer?

A. Tables in ASHRAE ch. 29

B. The make and model number of the computer

C. The latent internal gains

D. A power meter

E. A and D

Conclusions

• Conduction and convection principles can be used to calculate heat loss for individual components

• Convection principles used to account for infiltration and ventilation

• Radiation for solar gain and increased conduction

• Include both sensible and latent internal gains

Objectives

• Choose a heating system

• Describe required auxiliary equipment

• Calculate pipe sizes for water and steam systems

Choosing a Heating System

• What is it going to burn?

• What is it going to heat?

• How much is it going to heat it?

• What type of equipment?

• Where are you going to put it?

• What else do you need to make it work?

Choosing a Fuel Type

• Availability• Emergencies, back-up power, peak demand

• Storage• Space requirements, aesthetic impacts, safety

• Cost• Capital, operating, maintenance

• Code restrictions• Safety, emissions

Selecting a Heat Transfer Medium

• Steam• Necessary for steam loads, little/no pumping• But: lower heat transfer, condensate return, bigger

pipes

• Water• Better heat transfer, smaller pipes, simpler• But: requires pumps, lower velocities, can require

complex systems

Choosing Water Temperature

• Low temperature water (180 °F – 240 °F)• single buildings, simple

• Medium and high temperature (over 350 °F)• Campuses where steam isn’t viable/needed• Requires high temperature and pressure equipment

• Nitrogen system to prevent steam formation

Choosing Steam Pressure

• Low pressure (<15 psig)• No pumping for steam• Requires pumping/gravity for condensate

• Medium and high-pressure systems• Often used for steam loads

Steam Tables•ASHRAE HVAC Systems and Equipment (2000), Chapter 10

•ASHRAE Fundamentals (2001), Chapter 6, Table 3

Pipe Sizing Example

• 1 inch pipe (ID)• Medium-temperature water (250°F) at 2.5 fps

• Return water at 120 °F• q=M×C×ΔT• q=ρ×Q×C×ΔT

• q = 375 kBTU/hr

F130F lb

BTU1

in144

ftin

2

1

hr

s3600

s

ft5.2

ft .017

lb2

22

2

q

Pipe Sizing Example

• 1 inch pipe ID• Medium-pressure steam (50 psig) at 60 fps

• Return water at 180°F• q=M × hfg+ M × C×ΔT• q=ρ×Q×( hfg + C×ΔT)

• q = 164 kBTU/hr

F120

F lb

BTU1

lb

BTU 912

in144

ftin

2

1

hr

s3600

s

ft60

ft 6.7

lb2

22

2q

Answer for water = 375 kBTU/hr

Conclusions

• Steam needs bigger pipes for same heat transfer• Water is more dense and has better heat transfer

properties

• You can use steam tables and water properties to calculate heat transfer• Vary design parameters

3) Given the following formulas, which of the following is generally true?

A) qwater > qsteam because ρwater > ρsteam

B) qsteam > qwater because of the added hfg term

C) qwater = qsteam because water’s higher density offsets the added hfg term in steam

D) None of the above are true.

Water: q=ρ×Q×C×ΔT

Steam: q=ρ×Q×( hfg + C×ΔT)

4) Which of the following is an advantage of using steam versus water as a heat transfer

medium?

A) Needs smaller pipes than water

B) More efficient (higher heat transfer) than water

C) Does not necessarily require pumping

D) All are advantages of steam

What About Air?

• Really bad heat transfer medium• Very low density and specific heat• Requires electricity for fans to move air• Excessive space requirements for ducts

• But• Can be combined with cooling• Lowest maintenance• Very simple equipment

• Still need a heat exchanger

Equipment

• Load demand, load profile• Amount and type of heat• Response time

• Efficiency• 80 – 85 % is typical• Electricity is ~100 %

• Combustion air supply• Flue gas discharge (stack height)

5) Electric resistance heating is preferred to natural gas heating because it is more efficient

(~100% vs. ~80%).

A) True

B) False

6) Which of the following is an advantage of a natural gas heating system?

A) More efficient than electric resistance heating.

B) Does not require mechanical ventilation.

C) Cost is lower than electric resistance heating when natural gas is readily available.

D) Requires venting of exhaust fumes to outside

Choosing a Boiler

• Fuel source• Transfer medium• Operating temperatures/pressures• Equipment

• Type• Water tube

• Fire tube

• Electric resistance/electrode

• Space requirements

• Auxiliary systems

Boilers

• Water Tube Boiler• Water in tubes, hot combustion gasses in shell

• Quickly respond to changes in loads

• Fire Tube Boiler• Hot combustion gasses in tubes, water in shell

• Slower to respond to changes in loads

Electric

• Resistance• Resistor gets hot• Typically slow response time (demand issues)

• Electrode• Use water as heat conducting medium• Bigger systems

• Cheap to buy, very expensive to run

• Clean, no local emissions

7) Does a boiler always need to be vented to the outside?

A. Yes

B. No

Auxiliary

• Burner type (atmospheric or power vented)

• Feedwater systems• Returns steam condensate (including accumulator)• Adds water to account for blowdown and leaks• Preheats the water• Removes dissolved gasses

• Blowdown system• Periodically drain and cool water

Auxiliary

• Water treatment• Dissolved minerals and gasses cause:

• Reduced heat transfer

• Reduced flow (increased pressure drop)

• Corrosion

• Treatment options• Chemical (add bases, add ions, add inhibitor)• Temperature (heat to remove oxygen)

8) Under which circumstances would you consider using air as a heat transfer medium?

A) Ductwork is already in place for a cooling system.

B) The building in question has a sporadic maintenance schedule.

C) The building has a very brief heating season.

D) All of the above.

Location

• Depends on type• Aesthetics• Stack height• Integration with cooling systems

Cooling Processes

• q=ρ×Q×( hfg + C×ΔT)

• Steam condenses – adds latent heat of vaporization to the air

• Thought experiment• Air at 220°F• Evaporating water in a pipe• What would happen to air temperature?

• Refrigerant for vapor compression cycle

Cooling Intro

• 1 inch pipe (ID)• Water (45°F) at 2.5 fps

• Return water at 60 °F• q=M×C×ΔT• q=ρ×Q×C×ΔT

• q = 43 kBTU/hr

F15F lb

BTU1

in144

ftin

2

1

hr

s3600

s

ft5.2

ft .017

lb2

22

3

q

Heating = 375 kBTU/hr