Mechanical Systems Existing Conditions Evaluation

Diane McArdle
Mechanical Option
Skye Laboratories, Inc. Building E
Fort Worth, Texas
November 11, 2002

Mechanical Systems Existing Conditions Evaluation

Executive Summary

Skye Laboratories, Inc. Building E is an energy intensive animal testing research facility. During the design process, special consideration for the well being of the animals was a central focus. Through redundancy the mechanical system was reinforced so that in the case of extreme weather conditions the system would operate efficiently and valuable research would not be lost. The building is divided into 6 zones. The largest zone is the animal holding area that is a constant volume system served by six air handling units with energy recovery pipes. The surgery zone is supplied by air from the air handling units from zone 1 and reheated. Two of the six zones house the mechanical equipment and are conditioned to ensure the equipment operates properly. The office zone is a variable air volume system with a dedicated air handling unit. The cage wash area is a control volume system and is supplied 100% outdoor air.

This report is organized to present an outline of the mechanical system that is graphically demonstrated through the attached drawings. Schematics are provided for the chilled water loop, condenser loop, and diagrams illustrating the chilled water and hot water service to the air handling units. Since the varying design temperatures and operations vary throughout the zones, airflow diagrams are attached for each air handling unit map out the summer and winter conditions. The attached schedules describe the major components of the system. Since steam is provided by an existing boiler not located on the site, there is limited knowledge on the hot water control system.

Design Objectives and Requirements

Skye Laboratories, Inc. Building E is animal testing research facility currently in the process of construction located in Fort Worth, Texas. The building is a part of a series of facilities on the Skye Laboratories, Inc. Campus. Building E contains the fourth chiller plant on the grounds. The plant was designed with the capability of future integration with the chilled water campus loop. The new plant and campus chilled water loop provide 44 °F chilled water. Since 44 °F chilled water was not sufficient to cool the animal holding areas, post-cooling coils were piped to a small glycol chiller loop with 40 °F water. The chiller plant could be run with 40 °F chilled water but the chillers operate more efficiently supplying 44 °F water. The post-cooling coils also provide and extra factor of safety at a low cost. Redundancy of the system was a priority to the owner since millions of dollars in research would be lost if the mechanical system did not operate at design temperatures.

The building is divided into six zones according to their indoor design conditions. The most critical area of the building is Zone 1 which is animal holding rooms that are maintained at a constant year round temperature of 66 °F/55 RH with a RSHR of 0.77. Six handling units with heat pipe energy recovery service this area with 175,000 cfm of 100% outside air. Zone 2 is a surgery area located within zone 1. It requires 68°F/45% RH supply air that is provided by the air handling units for zone 1 and reheated. The remaining area of the first floor is a cage wash area and offices. The cage wash area is supplied 100% outside air at 72°F/50% RH in summer and 35% RH in winter. Zone 4 is the office area. It is supplied 35,000 cfm at 75°F/50% RH in summer and 35% RH in winter. The office area complies with ASHRAE 90’s outdoor air requirements and is a variable air volume system. Zone 5 is the chiller plant. It a constant volume system that is supplied 7000 cfm at 65°F in winter and 80°F/55 RH in summer. The chiller plant’s maximum outside air ventilation was determined by the code for the amount of refrigerant present. The minimum outside air is 0.5 cfm/sq. ft. at normal conditions and maximum outside air in the event of a refrigeration leak. The second floor of the building iszone 6 which is primarily the mechanical mezzanine. It is supplied 30,000 cfm at 80 °F/55 RH in summer and 65 °F in winter. The mechanical mezzanine and chiller plant are conditioned because the climate is so hot in Texas. Both spaces are provided enough air to keep the system operating properly.

Energy Sources and Rates

Skye Laboratories, Inc. Building E has a total energy consumption of 3200 kW/year. The estimated energy consumption for the HVAC equipment is 2250 kW/year. There are no available rebates that affected design decisions. The rate for gas costs $4.0/ MMBTU and electricity runs about $0.055/ kWh.

Design Heating and Cooling Loads

Heating and cooling loads vary throughout the building. The table below illustrates the summer and winter cooling loads for each zone.

System Operation

Chilled Water Control

Chilled water is pumped from the four chillers located in the new plant on the North side of the facility. These chillers are sized to deliver 44 °F water to all the air handling units in Building E. Additional glycol chillers deliver 40°F water to the post cooling coils in the air handling units designated for zone 1. The glycol chillers are necessary to maintain the required 66 °F temperature in the animal holding areas.

Chillers operate on a primary-secondary pumping system as shown in the attached chilled water diagram. The chillers are energized as the building load increases. When a chiller is energized, its associated primary water pump is energized and the valve will open. Then the DDC system energizes a condenser water pump with the respective condenser water valve. A sensor in the condenser water temperature to the chiller modulates the tower bypass valve to fully open to the tower. As the temperature rises the valve will modulate to provide flow to the tower. A signal is sent to the tower fan to maintain the setpoint temperature of 85 °F after the valve is open. A schematic of the condenser water loop in provided in the appendix. The secondary chilled water pump is energized and modulated to maintain a setpoint determined by the differential pressure at the end of the loop. Whenever the water in decoupler loop changes direction another chiller is energized similarly to the first. In the case when the chilled water temperature decreases, the chiller with the longer run time is deenergized. Since the animal holding areas are more critical than the office space, the chilled water valve to the air handling units that serve the office and cage wash spaces will be closed in the case of very high outdoor air temperatures.

Heating Hot Water Control

When the mixed air temperature falls below 31 °F, the preheat coils for AHU-G1-G6 are activated to supply 55 °F air to the animal holding areas. The setpoint temperature to activate the preheat coils varies for each air handling unit depending on the amount of return air or heat recovery. Steam is provide to the coils at 180 °F and returned at 160 °F. Air handling unit diagrams are attached in the appendix that illustrate the preheat temperature requirements.

Hot water is controlled by a temperature sensor in the supply water piping. The sensor modulates two normally closed steam valves in sequence to maintain the reset water temperature.

Reheat hot water is controlled by a heat exchanger valves. The water flowing through the pump is modulated by 1/3, 2/3 valves in sequence to maintain the programmed temperature setpoint.

AHU-G1 (Typical for AHU-G2 through AHU-G6)

The unit is 100% outside air and has a heat recovery coil, a hot water heating coil with circulating pump, supply air smoke dampers, precooling coil with 1/3 and 2/3 valves, post cooling coil, variable frequency drive fan and duct mounted humidifier. The unit runs continuously, but can be started and stopped by a DDC system.

When the unit is turned on the outside air and smoke dampers open. Then the supply and exhaust fans are energized. When the supply fan is turned off the outside air damper closes, the humidifier valve closes and the heating coil circulating pump and valve remain under control. When the dampers close the fan needs time delay for spindown.

The 46.5°F supply temperature is obtained by modulating the post cooling coil valve. If the temperature falls and the post cooling coil and precooling coil valves are closed, the heating coil will operate when the outdoor air falls below 38°F. In the event of heat recovery discharge air temperature rises in winter, the heat recovery bypass damper will modulate open. If the entering air temperature rises in winter, the heat recovery bypass damper modulates open. Controls modulate the humidifier valve to maintain 80% RH.

AHU-G7-Cage Wash

The unit is 100% outside air and has a steam preheat coil with integral face and bypass dampers, supply air smoke dampers, cooling coil, variable frequency drive fan and duct mounted humidifier. The unit is started and stopped by a DDC system.

The unit is energized similar to AHU-G1. The supply air temperature sensor through the DDC system modulates the unit cooling valve to maintain 57 °F. On a fall in temperature, the cooling coil valve is closed and bypass dampers modulate to maintain supply air temperature. A humidity sensor modifies the humidifier valve to maintain 80% relative humidity. When the outdoor air temperature falls below 45 °F, the preheat coil operates to maintain a supply temperature of 55 °F.

AHU-G8-Offices

The unit has supply and return air smoke dampers, hot water heating coil with circulating pump, cooling coil, variable frequency drive fan, and supply and return fans. The unit is started and stopped by a DDC system.

The unit is energized similar to AHU-G1. Since the air handling unit services office spaces, vav boxes and return air is utilized. The supply air temperature sensor through the DDC system is set to modulate the heating coil valve closed and stop the heating coil circulating pump when there is a rise in temperature. It then modulates the outdoor air damper open and return air damper to close and modulate the cooling coil valve to maintain the 55 °F supply air temperature. When the outside air is above 65 °F, the system operates at minimum air temperature. When the outdoor air temperature falls below 38 °F, the heating coil operates to maintain a supply temperature of 55 °F.

AHU-G9-Mechanical Equipment Room

The unit has supply and return air smoke dampers, hot water heating coil with circulating pump, cooling coil, constant volume fans, and supply and return fans. The unit runs continuously but can be started and stopped by a DDC system.

The unit is energized similar to AHU-G1. The return air temperature sensor through the DDC system is set to modulate the heating coil valve closed and stop the heating coil circulating pump when there is a rise in temperature. It then modulates the outdoor air damper open and return air damper to close and modulate the cooling coil valve to maintain the 55 °F supply air temperature. When the outside air is above 65 °F, the system operates at minimum air temperature. A unit supply temperature sensor prevents the supply air temperature from dropping below the setpoint. When the outdoor air temperature falls below 38 °F, the heating coil operates to maintain a supply temperature of 85 °F.

AHU-CH1-Chiller Room

The unit has supply hot water heating coil with circulating pump, cooling coil, constant volume supply fan and room exhaust fan with variable frequency drive indexed to normal or high speed exhaust. The unit runs continuously but can be started and stopped by a DDC system.

When the unit is energized to start, supply and exhaust are energized. The ceiling exhaust damper opens and the floor exhaust damper closes. Then the unit exhaust air damper opens and the end switch energizes the exhaust fans. The space air temperature sensor through the DDC system is set to modulate the heating coil valves closed and stop the heating coil circulating pump when there is a rise in temperature. It then modulates the cooling coil valve to maintain the 60 °F supply air temperature. When the outdoor air temperature falls below 38 °F, the heating coil operates to maintain a supply temperature of 90 °F.

Critique of System

The overall design of the system met the design requirements presented to the engineers. The owners were an integral part in the design process where they were presented with different design options and asked to select which best suited their needs. The building is currently under construction so there is no knowledge to how the systems operate in real life. Energy conservation and building sustainability were not strongly addressed. Since research laboratories are the most energy intensive buildings, more consideration needs to be addressed towards conservation.