MEBS 6008

1
MEBS 6008 Heat Pumps
2
What Is a Heat Pump?

• A heat pump is a self-contained, packaged
  cooling-and-heating unit with a reversible
  refrigeration cycle.
• A heat pump is basically a device that transfers
  heat from one substance to another substance.
• It has these same basic refrigeration components
  compressor, condenser, evaporator, and expansion
  device.
• The difference is that it can also reverse the
  refrigeration cycle to perform heating, as well
  as cooling, by reversing the functions of the two
  heat exchangers.
• The operation of the refrigeration cycle changes
  depending on whether the unit is in cooling or
  heating mode.
• Heat pump is generally reserved for equipment
  that heats for beneficial purposes, rather than
  that which removes heat for cooling only.

3
What Is a Heat Pump?

• Dual-mode heat pumps alternately provide heating
  or cooling.
• Heat reclaim heat pumps provide heating only, or
  simultaneous heating and cooling.
•  An applied heat pump requires competent field
  engineering for the specific application, in
  contrast to the use of a manufacturer-designed
  unitary product.
• Built-up heat pumps (field- or custom-assembled
  from components) and industrial process heat
  pumps are two types.

4
Heat Pump Cycles

• Most modern heat pumps use a vapor compression
  (modified Rankine) cycle or an absorption cycle.
•  
• Although most heat pump compressors are powered
  by electric motors, limited use is also made of
  engine and turbine drives.
•  
• Applied heat pump systems are most commonly used
  for heating and cooling buildings, but they are
  gaining popularity for efficient domestic and
  service water heating, pool heating, and
  industrial process heating.
•  

5
Introduction of heat source and heat pump system

• Heat sources include the ground, well water,
  surface water, gray water, solar energy, the air,
  and internal building heat.
• Frequently, heating and cooling are supplied
  simultaneously to separate zones.
• Decentralized systems with water loop heat pumps
  are common, using multiple water-source heat
  pumps connected to a common circulating water
  loop.
• They can also include ground coupling, heat
  rejecters (cooling towers and dry coolers),
  supplementary heaters (boilers and steam heat
  exchangers), loop reclaim heat pumps, solar
  collection devices, and thermal storage.

6
Review of a Typical Vapour Compression Cycle

• Refrigerant enters the evaporator in the form of
  a cool, low-pressure mixture of liquid and vapor
  (I).
• Heat is transferred to the refrigerant from the
  relatively warm air or water to be cooled,
  causing the liquid refrigerant to boil.
• The resulting vapor (II) is then pumped from the
  evaporator by the compressor, which increases the
  pressure and temperature of the refrigerant
  vapor.
• The resulting hot, high-pressure refrigerant
  vapor (III) enters the condenser where heat is
  transferred to ambient air or water, which is at
  a lower temperature.
• Inside the condenser, the refrigerant condenses
  into a liquid.

7
Review of a Typical Vapour Compression Cycle

• This liquid refrigerant (IV) then flows from the
  condenser to the expansion device.
• The expansion device creates a pressure drop that
  reduces the pressure of the refrigerant to that
  of the evaporator.
• At this low pressure, a small portion of the
  refrigerant boils (or flashes), cooling the
  remaining liquid refrigerant to the desired
  evaporator temperature.
• The cool mixture of liquid and vapor refrigerant
  (I) travels to the evaporator to repeat the cycle.

8
Heat Pump Cycle 
A heat pump cycle comprises the same processes
and sequencing order as a refrigeration cycle
except that the refrigeration effect q14 or qrf,
and the heat pump effect q23 ,both in J/kg, are
the useful effects.
where   h4 h1 enthalpy of refrigerant
entering and leaving evaporator, respectively, J
/kg Win work input, J/kg   The coefficient
of performance of the heating effect in a heat
pump system COPhp is 
9
Basic types of heat pump cycles

• Closed vapor compression cycle
• This is the most common type used in both HVAC
  and industrial processes.
• It employs a conventional, separate refrigeration
  cycle that may be single-stage, compound,
  multistage, or cascade.

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Basic types of heat pump cycles
Mechanical vapor recompression cycle with heat
exchanger

• Process vapor is compressed to a temperature and
  pressure sufficient for reuse directly in a
  process.
• Energy consumption is minimal, because
  temperatures are optimum for the process.
• Typical applications for this cycle include
  evaporators (concentrators) and distillation
  columns.

11
Basic types of heat pump cycles
Open vapor recompression cycle

• A typical application for this cycle is in an
  industrial plant with a series of steam pressure
  levels and an excess of steam at a
  lower-than-desired pressure.
• The heat is pumped to a higher pressure by
  compressing the lower pressure steam.

12
Basic types of heat pump cycles

• Heat-driven Rankine cycle
• This cycle is useful where large quantities of
  heat are wasted and where energy costs are high.
• The heat pump portion of the cycle may be either
  open or closed, but the Rankine cycle is usually
  closed.

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HEAT SOURCES AND SINKS
Air

• Outdoor air is a universal heat-source and
  heat-sink medium for heat pumps and is widely
  used in residential and light commercial systems.
• Extended-surface, forced-convection heat transfer
  coils transfer heat between the air and the
  refrigerant.
•  
• Typically, the surface area of outdoor coils is
  50 to 100 larger than that of indoor coils.
•  
• The volume of outdoor air handled is also greater
  than the volume of indoor air handled by about
  the same percentage.
•  
• During heating, the temperature of the
  evaporating refrigerant is generally 6 to 11 K
  less than the outdoor air temperature.

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HEAT SOURCES AND SINKS
Air

• When selecting or designing an air-source heat
  pump, the outdoor air temperature in the given
  locality and frost formation in particular must
  be considered.
• As the outdoor temperature decreases, the heating
  capacity of an air-source heat pump decreases.
•  
• This makes equipment selection for a given
  outdoor heating design temperature more critical
  for an air source heat pump than for a fuel-fired
  system.
•  
• The equipment must be sized for as low a balance
  point as is practical for heating without having
  excessive and unnecessary cooling capacity during
  the summer.

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HEAT SOURCES AND SINKS
Air

• When the surface temperature of an outdoor air
  coil is 0C or less, with a corresponding outside
  air dry-bulb temperature 2 to 5.5 K higher, frost
  may form on the coil surface.
• If allowed to accumulate, the frost inhibits heat
  transfer therefore, the outdoor coil must be
  defrosted periodically.
•  
• The number of defrosting operations is influenced
  by the climate, air-coil design, and the hours of
  operation.
•  
• It was found that little defrosting is required
  when outdoor air conditions are below -10C and
  60 rh (confirmed by psychrometric analysis).
•  

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HEAT SOURCES AND SINKS
Air

• Under very humid conditions, when small suspended
  water droplets are present in the air, the rate
  of frost deposit may be about three times as
  great as predicted from psychrometric analysis.
•  
• The heat pump may require defrosting after only
  20 min of operation.
• The loss of available heating capacity due to
  frosting should be taken into account when sizing
  an air source heat pump.
•  
• Early designs of air source heat pumps had
  relatively wide fin spacing of 5 to 6 mm, based
  on the theory that this would minimize the
  frequency of defrosting.
•  
• With effective hot-gas defrosting a much closer
  fin spacing is permitted that reduce size and
  bulk of the system.
•  
• In current practice, fin spacing of 1.3 to 2.5 mm
  are widely used.

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HEAT SOURCES AND SINKS
Water

• City water is seldom used because of cost and
  municipal restrictions.
• Groundwater (well water) is particularly
  attractive as a heat source because of its
  relatively high and nearly constant temperature.
•  
• The water temperature is a function of source
  depth and climate (Any information on water
  temperature of HKs situation ?).
•  
• Frequently, sufficient water is available from
  wells for which the water can be re-injected into
  the aquifer.
•  
• The use is non consumptive and, with proper
  design, only the water temperature changes.

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HEAT SOURCES AND SINKS
Water

• The water quality should be analyzed, and the
  possibility of scale formation and corrosion
  should be considered.
•  
• In some instances, it may be necessary to
  separate the well fluid from the equipment with
  an additional heat exchanger.
•  
• Special consideration must also be given to
  filtering and settling ponds for specific fluids.
  
•  
• Other considerations are the costs of drilling,
  piping, pumping, and a means for disposal of used
  water.
•  
• Information on well water availability,
  temperature, and chemical and physical analysis
  is available from U.S. Geological Survey offices
  in many major cities (Again, Hong Kongs
  situation?)

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HEAT SOURCES AND SINKS
Water

• Heat exchangers may also be submerged in open
  ponds, lakes, or streams.
•  
• When surface or stream water is used as a source,
  the temperature drop across the evaporator in
  winter may need to be limited to prevent
  freeze-up.
•  
• In industrial applications, waste process water
  (e.g., spent warm water in laundries, plant
  effluent, and warm condenser water) may be a heat
  source for heat pump operation.
•  
• Sewage, which often has temperatures higher than
  that of surface or groundwater, may be an
  acceptable heat source.
•  
• Secondary effluent (treated sewage) is usually
  preferred, but untreated sewage may used
  successfully with proper heat exchanger design.

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HEAT SOURCES AND SINKS
Ground

• The ground is used extensively as a heat source
  and sink, with heat transfer through buried
  coils.
•  
• Soil composition, which varies widely from wet
  clay to sandy soil, has a predominant effect on
  thermal properties and expected overall
  performance. The heat transfer process in soil
  depends on transient heat flow.
•  
• Thermal diffusivity is a dominant factor and is
  difficult to determine without local soil data.
•  
• Thermal diffusivity is the ratio of thermal
  conductivity to the product of density and
  specific heat.
•  
• The soil moisture content influences its thermal
  conductivity.

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HEAT SOURCES AND SINKS
Solar Energy

• Solar energy may be used either as the primary
  heat source or in combination with other sources.
•  
• Air, surface water, shallow groundwater, and
  shallow ground-source systems all use solar
  energy indirectly.
•  
• Using solar energy directly as a heat source for
  heat pumps can provide heat at a higher
  temperature than the indirect sources, resulting
  in an increase in the heating coefficient of
  performance.
•  
• Compared to solar heating without a heat pump,
  the collector efficiency and capacity are
  increased because a lower collector temperature
  is required.

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HEAT SOURCES AND SINKS
Solar Energy
There are two basic types of solar-source heat
pumps systems direct and indirect.  
Direct

• The direct system places refrigerant evaporator
  tubes in a solar collector, usually a flat-plate
  type. A collector without glass cover plates can
  also extract heat from the outdoor air.
• The same surface may then serve as a condenser
  using outdoor air as a heat sink for cooling.

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HEAT SOURCES AND SINKS
Solar Energy

• Indirect system
• An indirect system circulates either water or air
  through the solar collector.
• When air is used, the collector may be controlled
  in such a way that
• The collector can serve as an outdoor air
  preheater,
• The outdoor air loop can be closed so that all
  source heat is derived from the sun, or
• The collector can be disconnected from the
  outdoor air serving as the source or sink.

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AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)

• In an air-source heat pump system, outdoor air
  acts as a heat source from which heat is
  extracted during heating, and as a heat sink to
  which heat is rejected during cooling.
•  
• Since air is readily available everywhere,
  air-source heat pumps are the most widely used
  heat pumps in residential and many commercial
  buildings.
•  
• The cooling capacity of most air-source heat
  pumps is between 1 and 30 tons (3.5 and 105 kW).
•  
• Air-source heat pumps can be classified as
  individual room heat pumps and packaged heat
  pumps.
•  
• Individual room heat pumps serve only one room
  without ductwork.
•  
• Packaged heat pumps can be subdivided into
  rooftop heat pumps and split heat pumps.

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AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Split System Heat Pump
Roof top package unit
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AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Most air-source heat pumps consist of

• Coils through which air is conditioned,
• Outdoor Single or multiple compressors,
• Indoor coils where heat is extracted from or
  rejected to the outdoor air,
• Expansion valve
• Reversing valves that change the heating
  operation to a cooling operation and vice versa,
• An accumulator to store liquid refrigerant, and
  other accessories.

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AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Indoor Coil

• In an air-source heat pump, the indoor coil is
  not necessarily located inside the building.
• The indoor coil in a rooftop packaged heat pump
  is mounted on the rooftop.
• But, an indoor coil always heats and cools the
  indoor supply air.
• During cooling operation, the indoor coil acts as
  an evaporator.
• It provides the refrigeration effect to cool the
  mixture of outdoor and re-circulating air when
  the heat pump is operating in the re-circulating
  mode.
• During heating operation, the indoor coil acts as
  a condenser.
• The heat rejected from the condenser raises the
  temperature of the conditioned supply air.
• For heat pumps using halocarbon refrigerants, the
  indoor coil is usually made from copper tubing
  and corrugated aluminum fins.

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AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Outdoor Coil

• The outdoor coil acts as a condenser during
  cooling and as an evaporator to extract heat from
  the outdoor atmosphere during heating.
• When an outdoor coil is used as a condenser, a
  series-connected subcooling coil often subcools
  the refrigerant for better system performance.
• An outdoor coil always deals with outdoor air,
  whether it acts as a condenser or an evaporator.
• Like the indoor coil, an outdoor coil is usually
  made of copper tubing and aluminum fins for
  halocarbon refrigerants.
• Plate or spine fins are often used instead of
  corrugated fins to avoid clogging by dust and
  foreign matter.

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AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Reversing Valve

• Reversing valves are used to guide the direction
  of refrigerant flow when cooling operation is
  changed over to heating operation or vice versa.
•  
• The rearrangement of the connections between four
  ways of flowcompressor suction, compressor
  discharge, evaporator outlet, and condenser
  inletcauses the functions of the indoor and
  outdoor coils to reverse. It is also called a
  four-way reversing valve.
•  
• The efficiency losses altogether with leakage,
  heat transfer, and the pressure drop across the
  reversing valve cause a decrease of 4 to 7
  percent in heat pump performance.
•  
• Other accessories include filter dryer, sight
  glass, strainer, liquid level indicator, solenoid
  valves, and manual shutoff valves.

Compressor.

• Reciprocating and scroll compressors are widely
  used in heat pumps.

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AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Expansion Device

• A variety of expansion devices may be used in
  heat pumps.
• The most common types are thermal expansion
  valves (TXV), electronic expansion valves, and
  capillary tubes.
• All of these devices reduce the pressure and
  temperature of the refrigerant within the cycle.
• Expansion valves, such as the TXV, have the added
  capability of metering the quantity of
  refrigerant flowing through the cycle in order to
  match the load to enhance the efficiency of the
  cycle.
• TXVs used in heat pumps may be bi-directional
  (that is, refrigerant flows in one direction
  when in cooling mode and in the opposite
  direction when in heating mode).
• Another way is to design the refrigerant piping
  inside the heat pump to ensure that refrigerant
  flow through the valve is in the same direction
  in either mode.

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AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Cooling Mode

• When the discharge air temperature sensor detects
  an increase in the air temperature above a
  predetermined limit at the exit of the indoor
  coil, cooling is required in the air-source heat
  pump.
•  
• The indoor coil now acts as an evaporator and
  extracts heat from the conditioned air flowing
  through the indoor coil.
•  
• After evaporation, vapor refrigerant from the
  indoor coil passes through the sliding connector
  of the slide and flows to the suction line.
• Hot gas discharged from the compressor is led to
  the outdoor coil, which now acts as a condenser.
•  
• An economizer cycle can be used when an outdoor
  air sensor detects the outdoor temperature
  dropping below a specific limit during cooling
  mode.

32
AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Heating Mode

• When the discharge air sensor detects a drop in
  air temperature below a predetermined limit at
  the exit of the indoor coil, heating is required.
  
• The outdoor coil now acts as an evaporator.
• When the discharge air temperature sensor detects
  a drop in air temperature further below preset
  limits, the electric heater (that is
  supplementary heater) would be energized in steps
  to maintain the required discharge air
  temperature.

33
AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Heating Mode

• Supplementary heating is energized only when the
  space heating load cannot be offset by the
  heating effect of the heat pump.
•  ASHRAE/IESNA Standard 90.1-1999 stipulates heat
  pumps equipped with internal electrical
  resistance heaters shall have controls to prevent
  supplemental heater operation when the heating
  load can be met by the heat pump alone during
  heating or setback recovery.

34
AIR-SOURCE HEAT PUMP SYSTEMS (Air-to-Air Heat
Pumps)
Cycling Loss and Degradation Factor

• For split packaged air-source heat pumps, indoor
  coils are located inside the building and outdoor
  coils are mounted outdoors.
• When an on/off control is used for the
  compressor, during the off period, refrigerant
  tends to migrate from the warmer outdoor coil to
  the cooler indoor coil in summer and from the
  warmer indoor coil to the cooler outdoor coil
  during winter.
• When the compressor starts again, the transient
  state performance shows that a 2- to 5-min
  operating period of reduced capacity is required
  before the heat pump can operate at full
  capacity.
• Such a loss due to cycling of the compressor is
  called cycling loss.

35
Water-to-Air Heat Pumps
These heat pumps rely on water as the heat source
and sink, and use air to transmit heat to, or
from, the conditioned space. They include the
following   1) Groundwater heat pumps  2)
Surface water heat pumps
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Water-to-Air Heat Pumps
Groundwater heat pumps

• They use groundwater from wells as a heat source
  and/or sink.
• These systems can either circulate the source
  water directly to the heat pump or use an
  intermediate fluid in a closed loop, similar to
  the ground-coupled heat pump. 

Surface water heat pumps

• They use surface water from either a lake, pond,
  or stream as a heat source or sink.
• Similar to the ground-coupled and groundwater
  heat pumps, these systems can either circulate
  the source water directly to the heat pump or use
  an intermediate fluid in a closed loop.

37
Water-to-Air Heat Pumps
 Internal-source heat pumps

• They use the high internal cooling load generated
  in modern buildings either directly or with
  storage.
• These include water loop heat pumps. 

Solar-assisted heat pumps

• They rely on low-temperature solar heat as the
  heat source.
• Solar heat pumps may resemble water-to air, or
  other types, depending on the form of solar heat
  collector and the type of heating and cooling
  distribution system.
•  

38
Water-to-Air Heat Pumps
Wastewater-source heat pumps

• They use sanitary waste heat or laundry waste
  heat as a heat source.
• The waste fluid can be introduced directly into
  the heat pump evaporator after waste filtration,
  or it can be taken from a storage tank, depending
  on the application.
• An intermediate loop may also be used for heat
  transfer between the evaporator and the waste
  heat source.

39
Water-to-Water Heat Pumps

• These heat pumps use water as the heat source and
  sink for cooling and heating.
• Heating-cooling changeover can be done in the
  refrigerant circuit, but it is often more
  convenient to perform the switching in the water
  circuits.
• Direct admittance of the water source to the
  evaporator is one approach.
• Alternatively, applying the water source
  indirectly through a heat exchanger (or
  double-wall evaporator) to avoid contaminating
  the closed chilled water system, which is
  normally treated may be necessary.

40
Ground-Coupled Heat Pumps.

• These use the ground as a heat source and sink.
• A heat pump may have a refrigerant-to-water heat
  exchanger or may be of the direct-expansion (DX)
  type.
• In systems with refrigerant-to-water heat
  exchangers, a water or antifreeze solution is
  pumped through horizontal, vertical, or coiled
  pipes embedded in the ground. 

41
Ground-Coupled Heat Pumps
Direct expansion ground-coupled heat pumps use
refrigerant in direct expansion, or flooded
evaporator circuits for the ground pipe coils.
42
Ground-Coupled Heat Pumps

• Soil type,moisture content, composition, density,
  and uniformity close to the surrounding field
  areas affect the success of this method of heat
  exchange.
• With some piping materials, the material of
  construction for the pipe and the corrosiveness
  of the local soil and underground water may
  affect the heat transfer and service life.
• In a variation of this cycle, all or part of the
  heat from the evaporator plus the heat of
  compression are transferred to a water-cooled
  condenser.
• This condenser heat is then available for uses
  such as heating air or domestic hot water.

43
Refrigerant-to-water heat exchanger

• It may be a tube-in-tube, tube-in-shell, or
  brazed-plate design.
• The example shown here is a tube-in-tube, or
  coaxial, heat exchanger.
• It is constructed as a small tube running inside
  another larger tube.
• The water flows through the inner tube and
  refrigerant flows through the outer tube.
• In the cooling mode, the refrigerant-to-water
  heat exchanger acts as the condenser.
• The water flowing through the inner tube absorbs
  heat from the refrigerant flowing through the
  outer tube.
• In the heating mode, it acts as the evaporator
  and the refrigerant absorbs heat from the water.

44
Benefits of using water-source heat pump

• In the heat recovery mode gt saves energy by
  reducing the operating time of the cooling tower
  and boiler.
• Allowing different space temperature in many
  spaces with dissimilar cooling and heating
  requirements (each independently controlled space
  is served by its own heat pump and own
  thermostat).
• The same piece of equipment is used to provide
  both cooling and heating to the space. Even
  though a separate cooling tower and boiler may be
  included in the system, only one set of water
  pipes is required. This can reduce the system
  installation cost.
• A water-source heat pump system typically
  requires less mechanical floor space than
  centralized systems. This increases the rentable
  space and revenue in tenant-occupied buildings.
• If one heat pump fails and must be replaced, it
  does not affect the operation of the rest of the
  system.

45
Key issues associated with water source heat-pump
system.

• Outdoor air for ventilation may bring a few
  challenges. Most commercial buildings have a
  separate, ducted ventilation system.
• Next, because a heat pump is located in, or very
  close to, the occupied space and contains both a
  compressor and a fan, the resulting noise level
  in the space must be considered during system
  design.
• Proper maintenance of the heat pumps requires
  that they be located in accessible areas. Units
  that make access as easy as possible increases
  the chance that the equipment will be properly
  maintained.

46
Water-source heat pumps Configurations
Configurations available to suit various building
types.
Horizontal units

• Horizontal units are designed for installation in
  ceiling plenums, especially for spaces where
  floor space is at a premium.
• Typical applications include offices and schools.

47
Water-source heat pumps
Vertical units

• Vertical units are designed to be installed in
  separate spaces such as closets or maintenance
  rooms.
• Common applications for small vertical units
  include schools, apartments, condominiums, and
  retirement homes.
• Larger vertical units are generally used in
  spaces that are more open, such as cafeterias and
  gymnasiums, or used as a dedicated ventilation
  system to condition the outdoor air brought into
  the building.

48
Water-source heat pumps
Console units

• They are designed for installation under windows,
  in perimeter spaces or in entryways, where ducted
  systems cannot be used and floor space is not a
  constraint.
• Typical applications include offices, apartment
  buildings, motels, and dormitories.
• Because of their rugged design, they are
  typically used in schools.

49
Water-source heat pumps
Vertical-stack units

• They are designed for corner installation in
  multistory buildings such as hotels, apartments,
  condominiums, and retirement centers, where a
  minimum amount of floor space is available.
• They are designed to be stacked above each other
  to minimize piping and electrical installation
  costs.

50
Heat adder rejecter
Water-source heat pumps
Use of water to water heat pump
Ground loop
51
Water-source heat pumps
Operating strategy
Warm weather

• Water-source heat pumps can run in either heating
  or cooling mode.
• During warm weather, when all the heat pumps are
  operating in cooling mode, heat removed from the
  air is transferred to the water loop.
• This causes the temperature of the water in the
  loop to rise, making it necessary to remove heat
  from the water.
• A cooling tower or evaporative water cooler
  rejects this heat to the outdoor air, maintaining
  a leaving-water temperature of approximately
  32ºC.

52
Water-source heat pumps
Operating strategy
Cold weather

• During cold weather, when most of the heat pumps
  are operating in heating mode, heat is removed
  from the water loop and transferred to the air.
• This causes the temperature of the water in the
  loop to drop, making it necessary to add heat to
  the water loop.
• A boiler or water heater adds heat to the water
  loop, maintaining a leaving-water temperature of
  approximately 16ºC.

53
Water-source heat pumps
Operating strategy
Mild Weather

• During mild weather, such as spring and fall, the
  heat pumps serving the sunny side and interior of
  the building operate in cooling mode and reject
  heat into the water loop.
• The heat pumps serving the shady side of the
  building operate in heating mode and absorb heat
  from the water loop.
• Heat rejected by the units operating in cooling
  mode can be used to offset the heat absorbed by
  the units in heating mode.
• If the water temperature stays between 16ºC and
  32ºC, neither the boiler nor the cooling tower
  need to operate.
• Under this situation, a water-source heat pump
  system provides a form of heat recovery and an
  opportunity to save energy.
• In case heat generated by lights, people, and
  office equipment may require year-round cooling
  in the interior spaces, this heat recovery
  further reduces boiler operation during the
  winter months.

54
Ground-Source Heat Pump Systems

• A ground-source heat pump uses the earth as the
  heat rejecter and heat adder.
• These systems take advantage of the earths
  relatively constant temperature, and use the
  ground or surface water as the heat rejecter and
  heat adder.
• Ground-source heat pump systems dont actually
  get rid of heatthey store it in the ground for
  use at a different time.
• During the summer, the heat pumps absorb heat
  from the building and store it in the ground.
• When the building requires heating, this stored
  heat can be recaptured from the ground.
• In a perfectly balanced system, the amount of
  heat stored over a given period of time would
  equal the amount of heat retrieved.

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Ground-Source Heat Pump Systems

• In a properly designed ground-source heat pump
  system, neither cooling tower nor boiler may be
  necessary that saves initial cost and floor
  space.
• Ground-source heat pump systems offer the
  potential for operating-cost savings when
  compared to the traditional cooling-tower-and-boil
  er system.
• However, a significant amount is on the
  installation cost of the ground heat exchanger.
• Installation requires excavation, trenching, or
  boring, and in some areas there are very few
  qualified contractors for installing the ground
  heat exchanger.

56
Ground-Source Heat Pump Systems
There are several types of ground-source
systems Ground-coupled system

• This system uses a closed system of special,
  high-density polyethylene pipes that are buried
  in the ground at a depth that takes advantage of
  the earths natural heat sink capabilities.
• When the building cooling load causes the
  temperature of the water loop to rise, heat is
  transferred from the water, flowing through the
  buried pipes, to the cooler earth.
• Conversely, when the temperature of the water
  loop begins to fall, the water flowing through
  the buried pipes absorbs heat from the earth.
• In a properly designed, ground-coupled system,
  operating and maintenance costs are low because a
  cooling tower and boiler are not required in the
  system.

57
Ground-Source Heat Pump Systems
Pipe pattern

• The pipes that make up the ground heat exchanger
  can be oriented in a vertical or horizontal
  pattern.
• The choice depends on available land, soil
  conditions, and excavation costs.

Vertical loops

• Vertical loops are the most common in commercial
  applications due to the limited land generally
  available.
• Vertical bore holes are drilled to depths of 60
  to 150 m, with a diameter of 10 to 20 cm each.

58
Ground-Source Heat Pump Systems
Horizontal loops
Horizontal loops are often considered when
adequate land is available. Historically,
horizontal loops consisted of a single layer of
pipe buried in the ground using a trencher.
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Ground-Source Heat Pump Systems
Multiple-layer horizontal loops

• With the limited of land for installation,
  multiple-layer horizontal loops have been
  adopted.
• While less land and trenching is required, more
  total length of piping is required compared to a
  single layer loop.
• The pipes are placed in trenches, typically 1.8 m
  deep and spaced 1.8 to 4.6 m apart.
• Trench length can range from 8.7 to 34.7 m/kW.

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Water-to-water Heat Pump Unit Selection Procedure
Determine the system design conditions for
source and load-side(s) of the equipment
Entering liquid temperatures for the source-side
can be-1.1oC to 49oC Entering liquid temperatures
for the load-side 7oC to 49oC Define the
selection parameters. Entering water
temperature, Fluid flow rate, and Fluid
pressure drop. Determine unit requirements.
Total cooling capacity/total heating Staging of
capacity to satisfy cooling requirements.
Pressure drop reduction through the load-side of
multiple units, even when a single unit might
meet capacity. Antifreeze will be required in the
fluid loop if source-side leaving water
temperature falls below 1oC.