Small Tonnage Chiller (STC)

Small Tonnage Chillers (STCs) are one part of an overall heating, ventilation, and air conditioning (HVAC) system.

STCs produce chilled water (or glycol) by cooling the chilled water loop.

All STCs need a chilled water pump to move chilled water through their pipes. For some models, manufacturers offer a chilled water pump kit as part of the total package so that the customer need not buy
one separately.

STCs chill their water (or glycol) and reject the heat removed via one of two methods…

• Air cooled condensers
• Water cooled condensers

Although it will have a higher install cost, a chilled water system can often produce a healthier, more pleasant indoor environment than a packaged rooftop unit or DX split system because it offers…

• Better humidity control
• Better temperature control
• Better air quality
• Less ducted noise in the conditioned space

Outdoor Units

The outdoor unit transfers the heat from inside the building to the outside air.

To do this, the outdoor unit contains a...

1. Condenser- transfers the heat absorbed by the refrigerant in the evaporator to the outside air. As the condenser rejects heat, the refrigerant vapor condenses back to a warm temperature liquid before entering the expansion device and evaporator.
2. Compressor- circulates the refrigerant through the refrigeration circuit. It draws low-pressure refrigerant through the connecting pipes from the evaporator inside the building, then discharges it as high-pressure refrigerant to the outdoor condenser. The outdoor unit can serve two conventional air-handling units, or may be tied in with a full featured air-handling unit.
3. Fan- pulls air through the condenser coil, transferring heat from the condenser coil to the outside air. The use of a fan increases the efficiency of the coil without increasing its size.
4. Connecting pipes- two pipes connect the outdoor refrigeration unit to the indoor unit. The suction pipe is larger and insulated. The liquid pipe is smaller.

"Splitable" Packaged Units

Although most units ship as a factory charged, unitized package, 4-15 ton, horizontal, air cooled units can be field split. The condenser section can be placed close to the outside wall and the evaporator and blower installed near the conditioning zone. Installation is possible through standard height doorways and elevators. Refrigerant piping above the ceiling takes up less space than ductwork.

• 4-10 ton units have valves to shut off the refrigerant so that the unit can be separated.
• 12 and 15 ton units ship in two pieces.
• 5-15 ton, horizontal, water cooled units also can be separated to facilitate
  moving the units through doorways and up into the ceiling location.
• Premium units can be split due to their modular construction.

Inside an Indoor package unit

Indoor packaged units offer a low profile indoor design that eliminates the need for any unsightly exterior equipment.

Floor-by-floor installation provides independent zone and temperature control, eliminating many of the complications encountered with rooftop equipment.

Basic components inside an indoor package include…

1. Cabinet
2. Compressors
3. Condenser coil
4. Evaporator coil
5. Fans
6. Filter
7. Controls

Heat Pumps

Climates with moderate heating needs often turn to heat pumps as an energy efficient option for indoor packaged unit cooling.

Heat pumps use electricity and extract heat from the cool outdoor air, moving it into the indoor space to make it warmer.

The refrigeration circuit becomes a heat pump by using a crossover reversing valve.

1. Outdoor Air Coil- Outdoor Air coil transfers refrigerant heat to the outside air during cooling

3. Crossover reversing valve- When the heating is required, the heat pump's crossover reversing valve switches the refrigerant flow. Thus, the roles of the evap and condenser are reversed.

4. Indoor Coil- extracts heat and transfers it to the refrigerant during cooling.

2. Outdoor Air Coil- extracts heat from the outside air transferring it to the refrigerant during heating

3. Crossover reversing valve- When the heating is required, the heat pump's crossover reversing valve switches the refrigerant flow. Thus, the roles of the evap and condenser are reversed.

5. Indoor Coil- transfers refrigerant heat to indoor air during heating

The Refrigeration Cycle

Air conditioners use a vapor compression, refrigeration cycle that transfers heat from a lower temperature source to a higher temperature heat sink.

Heat naturally flows from hot to cold, so work is required to move heat from cold to hot. The work is performed by a compressor driven by an electric motor.

The compressor changes pressure between two compartments in the air conditioner by pumping a refrigerant through a connecting loop.

A package refrigeration loop or circuit consists of a...

1. Compressor
2. Air cooled condenser coil
3. Thermal expansion valve
4. Evaporator coil
5. Return loop

Montreal Protocol

The Montreal Protocol established an international agreement to make a concerted effort to reduce emissions that deplete the ozone layer. Since many refrigerants have an effect on the ozone layer, this was a key piece of legislation for the HVAC community.

It was agreed that several refirgerants will be banned and others phased out. The goal was to move from chloroflorocarbons (CFCs) to hydrochoroflourocarbons (HCFCs) and ultimately Hydroflourocarbons (HFCs).

CFCs are no longer sold and cannot be recharged into systems. HCFCs are being phased out with the goal of having them gone by the year 2030.