HVAC Myths and Realities - ASHRAE

1 hvac Myths and Realities Trane Engineers Newsletter Live Series AGENDA Low delta T is unavoidable 55 F supply air temperature is adequate for today s loads ASHRAE Standard 15 has to be updated before new refrigerants can be used Single-zone VAV units do not need hot gas reheat VFDs and affinity laws Small changes in pressure can have a huge impact on airflow for flat fan curves and may cause the system to surge New chilled-water systems need to be variable-primary flow System airflow issues are the fans fault Claims to energy savings Low delta T is unavoidable. Myth Number 1 To n s= ( T GPM)/24 Solving for GPM= (Tons 24) / T Pumping Frictional Head Flow 2 Water HP (bhp)= (GPM head (ft))/3960 Water HP Flow 3 Delta T 3 Transport Energy is low delta T unavoidable?

2 V AHRI Certified Coil Air Flow (VAV) unloading Coil Delta T is low delta T unavoidable? 2015 Engineer s Newsletter Live Coil Selection and Optimization ASHRAE Chilled-Water Coil Selection Chilled-water cooling coils shall be selected to provide a 15 F or higher temperature difference between leaving and entering water temperatures and a minimum of 57 F leaving water temperature at design conditions. 1. 3-way control valves Reason 1: 3-Way Control Valves undesirable mixing in variable flow systems CHWS 42 F Bypass 42 F CHWR F Coil LVG 59 F 50% Coil Load CHWR = [(42 x 50) + (59 x 50)] / 100 = Eliminate them!

3 Coil Delta T = 17 F System Delta T = F 0 5 10 15 20 25 50 52 54 56 58 60 CHW Delta T leaving air setpoint 1. 3-way control valves 2. Control setpoint depression Reason 2: Supply Air Setpoint Depression overdriving coil capacity Avoid, limit and return 55 LAT = 16 DT 52 LAT = 11 DT 50 LAT = DT 0 5 10 15 20 25 35 40 45 50 CHW Delta T entering CHW temperature 1. 3-way control valves 2. LAT setpoint depression 3. Warmer chilled water Reason 3: Warmer Chilled Water Supply reduced heat transfer driving force LMTD Chilled water reset only at part load 42 CHWS = 16 DT 47 CHWS = DT 50 CHWS = 5 DT 1.

4 3-way control valves 2. LAT setpoint depression 3. Warmer chilled water 4. Deficient control valves Reason 4: Deficient Control Valves poor flow control at full and part loads Control Valve Issues 1. Improperly Selected / Oversized 2. Worn-out 3. Unstable control 4. $ (cheap) 5. 3-way valves 1. 3-way control valves 2. LAT setpoint depression 3. Warmer chilled water 4. Deficient control valves Reason 4: Deficient Control Valves poor flow control 8th floor control point 20 ft pd 2nd floor pressure 90 ft pd Specify quality valves specific to use 1. 3-way control valves 2. LAT setpoint depression 3. Warmer chilled water 4.

5 Deficient control valves Reason 4: Deficient Control Valves poor flow control Pressure independent valves? (PIV) 1. Mechanical 2. Electronic Pressure independent valves Not required May be beneficial 1. 3-way control valves 2. LAT setpoint depression 3. Warmer chilled water 4. Deficient control valves 5. Tertiary pumping / bridge tender circuits Reason 5: Tertiary Pumping undesirable mixing is hard to prevent CHWS CHWR Bypass Valve DP Don t mix to the return simply pressure boost 1. AHRI certified coil selections 2. AHU set point limits 3. Chilled water reset only at part load 4. Properly selected / high quality valves 5.

6 Pressure boosting no tertiary mixing Design Delta T and Greater is Achievable Low Delta T is unavoidable. Myth Number 1 55 F supply air temperature is adequate for today s loads. Myth Number 2 full load OA 96 F DB, 76 F WB MA 80 F DB RA 74 F DB, 52% RH 180 160 140 120 100 80 60 40 20 humidity ratio, grains/lb of dry air 110 30 40 50 60 70 80 100 90 dry-bulb temperature, F 80 70 50 40 30 60 full load OA MA RA SA SA 55 F DB (1,500 cfm) full load OA 96 F DB, 76 F WB MA 86 F DB RA 74 F DB, 57% RH 180 160 140 120 100 80 60 40 20 humidity ratio, grains/lb of dry air 110 30 40 50 60 70 80 100 90 dry-bulb temperature, F 80 70 50 40 30 60 full load OA MA RA SA SA 55 F DB (800 cfm)

7 RA' full load OA 96 F DB, 76 F WB MA 89 F DB RA 74 F DB, 52% RH 180 160 140 120 100 80 60 40 20 humidity ratio, grains/lb of dry air 110 30 40 50 60 70 80 100 90 dry-bulb temperature, F 80 70 50 40 30 60 full load OA MA RA SA SA 51 F DB (630 cfm) SA' Improving Dehumidification Cool and reheat Face-and-bypass dampers Reduce airflow Dual paths Desiccants full load OA 96 F DB, 76 F WB MA 86 F DB RA 74 F DB, 52% RH 180 160 140 120 100 80 60 40 20 humidity ratio, grains/lb of dry air 110 30 40 50 60 70 80 100 90 dry-bulb temperature, F 80 70 50 40 30 60 full load OA MA RA SA SA 52 (55) F DB (800 cfm) CA SA' Type III Series Desiccant (CDQ) MA SA 55 F DB 64% RH 42 gr/lb (43 F DP) 50 F DB 97% RH 52 gr/lb MA' 80 F DB 50% RH 77 gr/lb (60 F DP) (10,000 cfm) 75 F DB 67% RH 87 gr/lb (64 F DP) 8-12 rph CA C full load OA 96 F DB, 76 F WB MA 81 F DB RA 74 F DB, 52% RH 180 160 140 120 100 80 60 40 20 humidity ratio, grains/lb of dry air 110 30 40 50 60 70 80 100 90 dry-bulb temperature, F 80 70 50 40 30 60 full load OA MA RA SA SA 63 F DB (1,350 cfm) CA MA' SA' 55 F supply air temperature is adequate for today s loads.

8 Myth Number 2 Single-zone VAV units do not need hot gas reheat. Myth Number 3 outdoor condition sensible load latent load space SHR supply airflow outdoor airflow space temp supply air temp peak DB 96 F DB, 76 F WB 29,750 Btu/h 5,250 Btu/h 1,500 cfm 450 cfm 74 F F Classroom Example basic CV system 29,750 Btu/h (74 F Tsupply) 1,500 cfm = Jacksonville, Florida 180 160 140 120 100 80 60 40 20 humidity ratio, grains/lb of dry air 110 30 40 50 60 70 80 100 90 dry-bulb temperature, F 80 70 50 40 30 60 OA MA RA SA Jacksonville, FL 84 F DBT 76 F DPT (450 cfm) 77 F DBT 63 F DBT (1500 cfm) part load CV 96 F DBT 68 F DPT (450 cfm) 81 F DBT 74 F DBT 52% RH 55 F DBT (1500 cfm) ( tons)

9 Full load CV peak DPT OA MA SA peak DBT OA MA RA SA RA 74 F DBT 67% RH Example: K-12 Classroom OA MA RA SA 84 F DBT 76 F DPT (450 cfm) 77 F DBT 63 F DBT (1500 cfm) ( tons) 96 F DBT 68 F DPT (450 cfm) 81 F DBT 74 F DBT 52% RH 55 F DBT (1500 cfm) ( tons) 180 160 140 120 100 80 60 40 20 humidity ratio, grains/lb of dry air 110 30 40 50 60 70 80 100 90 dry-bulb temperature, F 80 70 50 40 30 60 peak DPT OA MA SA MA RA SA RA 74 F DBT 67% RH part load CV full load CV Jacksonville, FL peak DBT OA Example: K-12 Classroom peak DPT day zone humidity, %RH 67% cooling load, tons fan airflow, cfm 1500 mild/rainy day zone humidity, %RH 73% cooling load, tons fan airflow, cfm 1500 constant-speed fan Example: K-12 Classroom Hot Gas Reheat packaged DX units condenser MA reheat valve CA reheat coil evaporator constant-speed fan constant-speed fan with hot gas reheat 60% 1500 55% 1500 Space humidity is maintained Example.

10 K-12 Classroom peak DPT day zone humidity, %RH 67% cooling load, tons fan airflow, cfm 1500 mild/rainy day zone humidity, %RH 73% cooling load, tons fan airflow, cfm 1500 compressor energy SA OA 1500 cfm EA Improved Part-Load Dehumidification RA 450 cfm 55 F 96 F DBT 68 F DPT 1050 cfm zone 74 F OA MA RA SA 84 F DBT 76 F DPT (450 cfm) 77 F DBT 63 F DBT (1500 cfm) ( tons) 96 F DBT 68 F DPT (450 cfm) 81 F DBT 74 F DBT 52% RH 55 F DBT (1500 cfm) ( tons) 180 160 140 120 100 80 60 40 20 humidity ratio, grains/lb of dry air 110 30 40 50 60 70 80 100 90 dry-bulb temperature, F 80 70 50 40 30 60 peak DPT OA MA SA MA RA SA RA 74 F DBT 67% RH part load CV full load CV 84 F DBT 76 F DPT (450 cfm) 79 F DBT 55 F DBT (900 cfm) ( tons) 74 F DBT 57% RH part load SZVAV Jacksonville, FL Example.