List the Units of Measurement Used in Heating and Cooling

AgTM 330Week 14

Heating and Cooling

Objectives:

- List the units of measurement used in heating and cooling.

- List the three methods of heating spaces, animals and humans.

- Name at least four types of electric heating.

- Describe the types of thermostats used for heating and cooling equipment control.

- List the components of an oil or gas heater.

- Describe the installation of an electric forced-air heating system, which utilizes

Available heat in the building.

- Explain at least two farm uses of heating cable installed in concrete or asphalt floors

and slabs.

- Describe the installation of electric heating cable in floors and slabs.

- Size conductors and electrical components for an air conditioner.

- Explain the function of a heat pump.

Heating Units of Measurement

Temperature is the flow of heat as voltage difference between two wires

Resistance of material to the flow of heat is R-value

Heat is measured in several units

BTUBritish Thermal Units

kCalkilocalories

kWhkilowatt-hours

kJkilojouleSI unit of heat

Electric Heaters are rated in kilowatts (kW)

ie, baseboard heaters (4 ft.=1000 watts, 6 ft = 1500 watts, 8ft.=2000 watts)

Fuel-fired furnaces are rated in Btu per hour(Btu/hr)

Heating and Cooling load calculation are generally made in (Btu/hr)

Air-conditioning units are rated in (Btu/hr) for smaller sizes and

For larger sizes in tons

Def. 1 ton of refrigeration is the amount of heat absorbed by 1-ton block of ice in 24-hour period.

Stopped on 4-20-11

Therefore heating is rated in heat per unit of time.

Trend toward

Kilowatt (kW)

Kilojoule(kJ) joule is a small unit of measure of heat

Megajoule(MJ)1000 kJ

Conversion factor for heating and cooling units.

Multiply / By / To Find
BTU
KCal
KWh
KWh
KW
Tons
Tons
KW
Btu/hr
KCal/hr / 1.055
4.187
3600
3.6
3413
12000
3.516
3600
1.055
4.187 / Kj
Kj
Kj
Mj
Btu/hr
Btu/hr
KW
Kj/hr
Kj/hr
Kj/hr

Principles of Transferring Heat

1. Convection

• Moving heat through a fluid such as air
• Main method of moving heat for most heating and cooling systems

(forced air throughout the space to be heated)

• Natural convection, baseboard heaters.
• Convection ovens

2. Conduction

• moving of heat from one object to another object in touches
• heat moves through walls, floors, windows and ceilings
• Animals cool them selves by conduction by lying down on the ground
• Heating cables in the floor for animals

3. Radiation

Process of emission, transmission and absorption of radiant energy

Heat is emitted from an object and travels in the form of electromagnetic radiation

heat can be moved directly from the source to a person, animal or object

Heat from the sun reaches earth by radiation through emptiness of space

Heating with electricity

Heating is accomplished by passing current through a wire containing a desired

amount of resistance. Baseboard, toasters, hairdryers, curling irons

Infrared heat lamps and quartz infrared tubes use tungsten filaments.

Electric resistance heaters use nichrome wire.

Example;

A 2-kW electric resistance heater designed to operate at 235 V has a resistance of 27.7 ohms and produces 2 kWh of heat in 1 hour.

Equivalent to 6826 Btu/ht or 7.2 MJ

Using equation 2.7 where Heat = (I2) ( R)(t)

Remember heat is in Wh.

Types of Electric Heat

1. Baseboard heater

resistance style, good for clean areas, isolated

ie. Notice items above the heater

2. Self-contained heater

wall or ceiling mounting with fan to force air over heating elements

3. Electric furnace

Self-contained unit with resistance heating elements and a blower

force the warm air through a duct system

good where they can extract heat from some heat source.

Milk, cooling towers, methane

4. Infrared heater

Temporary heat source for newborn animals, work areas

5. Ceiling cable

radiant electric heat where resistance cable is embedded in ceiling plaster

which heats objects in the room

6. Floor cable

buried in floor and steps

common in vegetable grading, packing sheds, logging shops, animal passageways, and baby piglets.

7. Thermostats

Switch operated by temperature change

Two types Heating or cooling

Heating type closes, a switch that turns a heater on when temperature falls and opens when opens when the temperature rises.

Cooling thermostat closes when the temperature rises and turns on an air conditioner, refrigeration unit or ventilation fan and then opens when the temperature falls

Styles

• Low voltage 24 v
• Line voltage i.e. Baseboard

8. Sizing Circuit wires and over current protection

branch-circuit wire supplying an individual electric heating device must have a rating not less than 1.25% times the ampere rating of the heater, plus any motors

Example, single-phase electric unit heat rated at 7.5 kW, 240 v and has fan motor that draws 1 amp at 240 V.

heater draws 31.3 amps plus 1 amp for a total of 32.3 amp

(32.3amp)(125%) = 40Amp breaker

Minimum wire size is found in NEC Table 310-16

Copper wire the minimum size is no. 8 AWG THW

Example: A farm office is heated with two baseboard heaters on the same circuit.

Heater 1 is rated at 8.3 amp2000W

Heater 2 is rated at 6.3 amps, 1500 W

Both are rated for 240- Voltage

Total of heaters is 14.6 A, therefore based on wire size of 125% of this value = 18.3 amp

Minimum size wire is #1-12 AWG copper and protected with a 20-amp breaker.

Most line voltage thermostats are rated at no more than 22 amps and many have lower

Ratings, check the box prior to installation.

Def. Heating contactor is an electrically operated switch.

High-wattage heaters are usually supplied with a contactor.

Line voltage thermostats are directly tapped into the power side of the line to heater.

Only voltage would be possible be the coil with only 1 amp.

Drawback, is of shorted the line thermostat will remain on.

This is called a Class 1, remote controlled – result is a fire hazard.

Remote-controlled line voltage thermostat must be in EMT, IMC, or rigid

Nonmetallic.

Problem 15-1

A single-phase electric unit heater is rated at 7.5 kW at 235 V, and has a full-lead current rating of 32 Amp. The heater is controlled with a contactor operated with a line-voltage thermostat. The heater circuit wire is # 8 AWG copper THW, and the over-current device is rated at 40Amp. Determine the minimum size TW copper wire permitted for the thermostat if there are no fuses in the heater to protect the thermostat wire.

Solution.

The heater circuit over-current device is not permitted to exceed three times the thermostat wire ampere rating. a # 14 AWG TW copper wire has a rating of 20 Amp. No. 14 AWG copper TW wire is permitted for the thermostat.

Problem 15-2

Two 3200W, 240 Volt quartz infrared heaters are installed in a barn and supplied by one circuit. The heaters are controlled with a contactor operated by a line-voltage thermostat. Determine the following.

• Minimum heater circuit wire size.
• Minimum size circuit breaker for the heater circuit
• Minimum size remote-control thermostat wire size not requiring separate over current protection.

All of the wire is THWN copper in conduit

Solution.

The ampere rating of each infrared heater was not given in the problem, but it can be calculated easily.

Current = 3200 Watt/240volt = 13.3 amp, plus there are two heaters in the circuit. For a total of 26.6 amp

Minimum branch wire = 1.25 x 26.6 amp = 33.3 amp

NEC Table 310-16, the minimum size wire is no. 8 AWG copper THWN

Sizing the circuit breaker for 33.3 amp, use a 40-amp breaker

Thermostat wire ampere rating = 40 amp/3 = 13.3 amp so

THWN wire with an ampere rating of at least 13.3 amp = No. 14 AWG is adequate.

Disconnect

Electric heater required to have a disconnect that will open any ungrounded wire to the heater.

Must be with in sight of the heater unit

Used on electric heating equipment of less than 1/8 hp

A switch on the electric heating equipment with a marked off position that opens all ungrounded wire may serve as the disconnect

Circuit breaker or fusible switch protecting the heater circuit does not need to be insight from the heater, but it must be accessible.

A line-voltage thermostat may be temperature operated only, or it may also have a manual off position. Where the thermostat is turned to the off position, it cannot be turned on by a change in temperature.

A thermostat with a manual off position is permitted to serve as a disconnect for electric heating equipment, provided it opens all ungrounded (hot) wires

Single – pole thermostat, can serve if off position and only 120 voltage

Double-pole thermostat is required for 240 voltage

Single –pole thermostat for 240-voltage does not serve as a disconnect.

High-Temperature Limit Switch

Space heating contains high-temperature limit switch

Furnace blowers that opens the circuit and shuts off the heat before danger of fire

Switches are wired in series with the electric heating element.

If the high-temperature switch fails and becomes permanently open, heater will not work and can be tested with a continuity tester.

Installing Electric Heating Equipment

• Protected from physical abuse
• Resistant to corrosion and moisture
• Contactors installed in damp areas should be periodically treated with suitable

Lubricant to prevent rust and operating freely

• Kept a safe distance from combustible materials
• Grounding is extremely important in ag application

Fuel-Fired Heaters

Space heaters fired with gas or oil.

Air handling systems, ducts, and ventilation systems

Water radiators, baseboard units, which requires an electric circulating pump.

Divided into sub-zones

Usually runs with a low-voltage thermostat

Disconnect in sight of the furnace

Time-delay fuses sized at 1.15 times the motor nameplate current

Electric Heating Cable in Floors and Slabs

Ready to install mats either 10W for supplemental heating in northern climates.

20W per square foot are usually adequate to provide total heating for most inside

areas

Heat mats for swine are as high as 40Watts per square foot

Can go as high as 60 Watts per square foot

Heat Sources in Agricultural Buildings

Animal, product and natural heat produce significant quantities to augment a space heating systems

Mature dairy cow can produce up to 3000Btu of heat per hour

People at normal work produce 1000 Btu per hour.

Fruit and vegetables products produce heat while in storage

Heat of respiration, which must be removed by refrigeration system

Natural heat – sun

Heating cable may not be needed if building orientation or color

Example of room remodel in bedroom

Darrell’s water heater

Window arrangement winter vs. summer

Heat from lights and machinery contribute to heating system

Definition of plenum is an enclosed space in which various sources of natural and mechanical heat are utilized together to augment the heating system

Animals and humans give off moisture into a room as well as heat

Ventilation is required to eliminate excess moisture

Orange peel in room and roofs

Pipe Heating Cables

Usually plug-in type

Wrapped around the pipe in a spiral pattern and never should cross itself or overheating will result

Heat tapes for pipelines are available with thermostat control located close to the desired heating activities.

Energy Guide

Air conditioners and household refrigeration appliances have an energy guide that gives an estimated cost

Air conditioners contains an energy efficiency rating (EER)

Refrigeration Principles

Refrigeration removes heat from a room or object, because ice absorbs 144 BTU of heat per hour for every pound of ice that melts. The heat absorbed to simply melt the ice is called latent heat.

Refrigeration system compressor simply pumps the refrigerant throughout the system.

The refrigerant repeatedly changes back and forth from a liquid to gas so the liquid evaporates, it absorbs heat and when it condenses, it gives off heat.

The compressor pumps refrigerant vapor into the condenser where it changes to a liquid and gives off heat.

The liquid then travels to the expansion valve.

The pressure suddenly drops at the expansion valve, and the liquid begins to evaporate.

The liquid refrigerant is completely evaporated by the time it leaves the evaporator.

Heat is absorbed by the refrigerant in the evaporator.

The refrigerant vapor returns to the compressor where it begins anther cycle.

The refrigerant absorbs heat at the evaporator and gets rid of heat at the condenser.

Common Refrigerants

Today, there are three specific types of refrigerants used in refrigeration and air-conditioning systems:

1. Chlorofluorocarbons or CFCs, such as R-11, R-12, and R-114
2. Hydrochlorofluorocarbons or HCFCs, such as R-22 or R-123
3. Hydrofluorocarbons or HFCs, such as R-134a. All these refrigerants are "halogenated," which means they contain chlorine, fluorine, bromine, astatine, or iodine.

Refrigerants, such as Dichlorodifluoromethane (R-12), Monochlorodifluoromethane (R-22), and Refrigerant 502 (R-502), are called PRIMARY REFRIGERANTS because each one changes its state upon the application or absorption of heat, and, in this act of change, absorbs and extracts heat from the area or substance.

The primary refrigerant is so termed because it acts directly upon the area or substance, although it may be enclosed within a system. For a primary refrigerant to cool, it must be placed in a closed system in which it can be controlled by the pressure imposed upon it. The refrigerant can then absorb at the temperature ranges desired. If a primary refrigerant were used without being controlled, it would absorb heat from most perishables and freeze them solid.

SECONDARY REFRIGERANTS are substances, such as air, water, or brine. Though hot refrigerants in themselves, they have been cooled by the primary refrigeration system; they pass over and around the areas and substances to be cooled; and they are returned with their heat load to the primary refrigeration system. Secondary refrigerants pay off where the cooling effect must be moved over a long distance and gastight lines cost too much.

Refrigerants are classified into groups. The National Refrigeration Safety Code catalogs all refrigerants into three groups:

• Group I – safest of the refrigerants, such as R-12, R-22, and R-502
• Group II – toxic and somewhat flammable, such as R-40 (Methyl chloride) and R-764 (Sulfur dioxide)
• Group III – flammable refrigerants, such as R-170 (Ethane) and R-290 (Propane).

R-12 DICHLORODIFLUOROMETHANE (CC12 F2 ) Dichlorodifluoromethane, commonly referred to as R-12, is colorless and odorless in concentrations of less than 20 percent by volume in air. In higher concentrations, its odor resembles that of carbon tetrachloride. It is nontoxic, noncorrosive, nonflammable, and has a boiling point of -21.7°F (-29°C) at atmospheric pressure.

WARNING
Because of its low-boiling point at atmospheric pressure, it prevents liquid R12 from contacting the eyes because of the possibility of freezing.

One hazard of R-12 as a refrigerant is the health risk should leakage of the vapor come into contact with an open flame of high temperature (about 1022°F) and be decomposed into phosgene gas, which is highly toxic. R-12 has a relatively low latent heat value, and, in smaller refrigerating machines, this is an advantage.

R-12 is a stable compound capable of undergoing the physical changes without decomposition to which it is 6-20.commonly subjected in service.

The cylinder code color for R-12 is white.

R-22 MONOCHLORODIFLUOROME-THANE (CHCIF2 ) Monochlorodifluoromethane, normally called R-22, is a synthetic refrigerant developed for refrigeration systems that need a low-evaporating temperature, which explains its extensive use in household refrigerators and window air conditioners. R-22 is nontoxic, noncorrosive, nonflammable, and has a boiling point of -41°F at atmospheric pressure.

R-22 can be used with reciprocating or centrifugal compressors. Water mixes readily with R-22, so larger amounts of desiccant are needed in the filter-driers to dry the refrigerant.

The cylinder code color for R-22 is green.

R-502 REFRIGERANT (CHCIF2 /CCIF2 CF3 ) R-502 is an azeotropic mixture of 48.8 percent R-22 and 51.2 percent R-115. Azeotropic refrigerants are liquid mixtures of refrigerants that exhibit a constant maximum and minimum boiling point. These mixtures act as a single refrigerant. R-502 is noncorrosive, nonflammable, practically nontoxic, and has a boiling point of -50°F at atmospheric pressure. This refrigerant can only be used with reciprocating compressors. It is most often used in refrigeration applications for commercial frozen food equipment, such as frozen food walk-in refrigerators, frozen food display cases, and frozen food processing plants.

The cylinder color code for R-502 is orchid.

R-134a TETRAFLUOROETHANE (CH2 FCF3 ) R-134a, tetrafluoroethane, is very similar to R-12, the major difference is that R-134a has no harmful influence on the ozone layer of the earth's atmosphere and is a replacement for R-12 applications. Noncorrosive, nonflammable, and nontoxic, it has a boiling point of -15°F at atmospheric pressure. Used for medium-temperature applications, such as air conditioning and commercial refrigeration, this refrigerant is now used in automobile air-conditioners.

The cylinder color code for R-134a is light (sky) blue.

Additional Refrigerants

In addition to the previously mentioned refrigerants, other less common refrigerants are used in a variety of applications.

R-717 Ammonia (NH3 ) Ammonia, R-717, is commonly used in industrial systems. It has a boiling point of -28°F at atmospheric pressure. This property makes it possible to have refrigeration at temperatures considerably below zero without using pressure below atmospheric in the evaporator. Normally it is a colorless gas, is slightly flammable, and, with proper portions of air, it can form an explosive mixture, but accidents are rare.

The cylinder color code for R-717 is silver.

R-125 Pentafluoroethane (CHCF5 ) Pentafluoroethane, R-125, is a blend component used in low- and medium-temperature applications. With a boiling point of -55.3°F at atmospheric pressure, R-125 is nontoxic, nonflammable, and noncorrosive. R-125 is one replacement refrigerant for R-502.

All refrigerants have their own characteristics. It is extremely important to charge a system with the refrigerant specified. Use of an incorrect refrigerant can lead to reduced efficiency, mechanical problems, and dangerous conditions.

Vapor-compression cycle

The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. Figure 1 provides a schematic diagram of the components of a typical vapor-compression refrigeration system.