Numeric Parameters:

 

Rooftop Handling Units can in numerous different sizes, and therefore have varying numeric parameters depending on the specific system. Ranges are given for majority of the parameters, where appropriate. One of the few standards which are consistent throughout all system sizes is the fan motor which normally runs at 1800 rpm.

 

Energy Efficiency:

• Seasonal Energy Efficiency Ratio (SEER): measures air conditioning and heat pump cooling efficiency, calculated by the cooling output for a typical cooling season divided by the total electric energy input during the same time frame. A higher SEER rating means a greater energy efficiency. Also, the higher the SEER rating, the higher the annual savings cost as seen in Figure 1 below. The annual cooling cost at SEER 19.5 is about half of the cost at SEER 10.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1: SEER Rating vs. Annual Cooling Costs

 

 

• Fan Efficiency: The efficiency of the overall system depends greatly on the blower/fan efficiency. The Fan efficiency is the ratio between power transferred to the airflow and the power used by the fan. The chart below (Figure 2) shows the power consumption by fans (kW) per air volume (m3/h) to extrapolate total pressure (Pa).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2: Fan power consumption based on air volume and total pressure
http://docs.engineeringtoolbox.com/documents/197/VentilationFanPowerConsumption.gif

 

 

• Heat Recovery Efficiency: Heat recovery efficiency depends on the heat recovery device inside the unit. A run around coil transfers heat through water that is circulated in air to a liquid heat exchanger coil with efficiency of 50%. A heat pipe transfers heat through coils with efficiency up to 65% using refrigerant that evaporates on one side and condenses on the other. A cross plate heat exchanger transfers heat across metal plates with efficiency up to 70%. The highest efficiency of up to 85% comes from a thermal wheel which is a slowly rotation matrix of corrugated metal that operates in both opposing airstreams.

 

Dimensions:

• Dimensions vary based on the size of the model. Refer to the table below for the approximate minimum, maximum and typical dimensions.

 

 

 

 

 

Weight:

• The weight of a RTU varies based on the size of the model, but is a super critical component when determining the supports needed for the unit.

 

Temperature Control Capabilities:

• Air handling units in general can create comfortable indoor environments as determined by the ASHRAE comfort zone on the psychometric chart (Figure 3). The supply air from an AHU is typically about 90-95 °F for heating and about 48-55 °F for cooling. The supply air temperature allows for a distribution of heating or cooling throughout the space to reach a desired room temperature.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3: ASHRAE comfort zone

http://www.wbdg.org/images/env_sustainability_8.jpg 

 

Outdoor Air Temperature Suitability:

• All AHUs, including RTUs, are suitable for a wide range of outdoor temperatures. They can be sized to meet a variety of load requirements as long as the outdoor temperatures are not below the frost point, when the outside air dry bulb temperature will cause frost to form based on the return air bulb temperature and humidity. Frost can cause damage to air handler parts and prevent optimum operation of the unit. This can only be prevented by pre-heating the outside air right as it enters the RTU. The Psychometric charts below (Figure 4) show that pre-heating the outside air prevents frost. The frost point is located where the line from the return air condition to outside air condition is tangent to the saturation line. Pre-heating the outside air places the supply air above the frost point line.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4: Psychometric charts depicting frost point and frost prevention
http://www.trane.com/Commercial/Uploads/Pdf/1071/clch-prb012-en%200801%20energy%20wheel%20eb.pdf

 

 

Heating and Cooling Maximum:

• Nominal cooling capacities can range from 3 tons for a small unit to 150 tons for a large unit. The nominal airflow of the system can range from about 6,000 CFM for a small unit and 50,000 CFM for a large unit.