Forms of Head Pressure (Capacity Control)

Air side control

•  fan cycling

•  fan speed

•  modulating dampers (air shutters)

Refrigerant Side Control

•  condenser coil flooding valves

•  ORD – (hot gas bypass to receiver): opens on a rise of inlet

•  ORI – (condenser back pressure regulator): opens on a rise of differential

•  OROA – (ori & ord in one valve)

•  ORI will throttle closed to increase head pressure

•  When ORI is on it restricts the flow of the condenser which causes the liquid to back up or flood the condenser

•  When there is a 20 PSIG difference of either side of the ORD it will open to let hot gas to enter the receiver

•  The hot gas will boil off in the receiver

•  When the unit is off the receiver is saturated

•  When its running most receiver will be subcooled

•  Only place in the system where you can have a subcooled liquid and vapor because the vapor cannot escape

•  When hot gas is sent into receiver it boils off and creates more pressure to help send the liquid through the system

Example diagram:

Advantages

1. air readily available

2. lower first cost & installation cost

3. relatively simple maintenance

4. no freeze up problems

Disadvantages

1. larger volume of air may be required

2. noise concerns

3. operating cost can be higher

4. large variations in discharge press due to ambient temp. swing daily and seasonal

Water Cooled Condensers

3 Types:

-Shell and coil

•  refrigerant and the shell side

•  also acts as a receiver

-Shell and tube

-Tube in tube

•  counter flows

•  ref. is on the outside

-SDT is controller by a water Regulating Valve

-Which is set between 85 and 110 F

Location of Water Regulating Valve

•  preferred location is at the Outlet of the condenser to prevent air entering and increasing the rate of oxidation used the condenser

•  Back flow valve required (Plumbing Code Requirement)

•  Waste Water Systems must be open drain and trapped

•  more water lowering head pressure, less water higher head pressure

Advantages:

1. smaller size

2. constant SDT

3. less maintenance if water quality is good

4. no requirements for capacity control during periods of low outside air temps
   
   

Disadvantages

1. not suitable for outdoor use

2. water may be costly

3. bylaws may require an expensive recirculation system (cooling tower)

Evaporative Condensers

•  uses water and air as the condensing medium

•  as water evaporates mineral deposits accumulate, a float system is used to replace water

•  water sump must be emptied periodically to flush mineral deposits

•  used on larger systems

•  if used in cold climates sump must be heated or located indoors

Simple Low Ambient Control

•  turn of water

•  turn off fans

•  reduce fan speeds

•  discharge dampers

•  air bypass