Description

This paper presents field-monitored data from two supermarkets where the impact of implementing minor HVAC control improvements was evaluated. The control improvements were intended to increase the dehumidification capacity of the HVAC system and lower space humidity levels. Direct digital control (DDC) was installed at each store to monitor system performance and implement the control improvements. At the first test store, a 33,400 ft² (3,104 m² ) supermarket near Minneapolis, a conventional 50 ton (176 kW) split system conditioned the sales area. The HVAC system provided 0.6 cfm of supply air per ft² (3.0 L/s·m² ) of floor area, used mechanical suction pressure controls to modulate compressor capacity, and included a “face split” evaporator coil. The HVAC system was already configured and controlled to provide good dehumidification performance. Several simple control improvements were implemented with the DDC system but were found to have little impact on system operation, primarily because the system already worked well. The most significant control improvement was lowering the supply airflow below 0.5 cfm per ft² (2.5 L/s·m² ) of floor area at night when occupancy and loads were modest. This increased dehumidification capacity and saved fan energy with no detrimental impact on occupant comfort. Overall, the humidity in the store exceeded 45% RH for about 24 hours annually and exceeded 50% RH for only one hour.

At the second test store, a 50,000 ft² (4,647 m² ) supermarket near Indianapolis, three rooftop units (RTUs) with a total capacity of 98 tons (344 kW) conditioned the store. The largest RTU also was retrofit with a heat pipe heat exchanger to enhance dehumidification performance. The two largest RTUs used two-speed supply fans to vary airflow with loading. With the original controls, the heat pipe-assisted RTU was found to perform below expectations. During the first two-thirds of the summer season, the space humidity exceeded the 55% RH set point for more than 46 hours and simultaneous heating and cooling (i.e., reheating) was required for about 4 hours to maintain the humidity set point. Two control improvements were implemented and found to substantially improve dehumidification performance. The supply fan operated at low speed for first-stage cooling, and RTU staging was changed to base load the heat pipe-assisted unit. Implementing these control changes lowered space humidity levels by 10% to 15% RH.

The results from both supermarkets confirm the impact that supply airflow and part-load control of evaporator coil temperatures can have on dehumidification performance. Seemingly minor control adjustments can often have a big impact on the performance of supermarket HVAC systems. Even enhanced dehumidification technologies, such as heat pipe-assisted evaporator coils, can benefit from minor system tuning.

Units: Dual

Citation: Symposium, ASHRAE Transactions, vol. 105, pt. 1

Product Details

Published:

1999

Number of Pages:

12

File Size:

1 file , 380 KB

Product Code(s):

D-7566